Evaluating baculovirus mixtures against false codling moth Thaumatotibia leucotreta Meyrick. (Lepidoptera: Tortricidae)
- Authors: Tole, Siviwe
- Date: 2024-10-11
- Subjects: False codling moth Biological control , Baculoviruses , Integrated pest management , Natural pesticides , Granulovirus
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/463996 , vital:76464
- Description: False codling moth (FCM), Thaumatotibia leucotreta, is an important pest of citrus, stone fruit, avocados, peppers, and other important agricultural crops in southern Africa. Baculovirus-based biopesticides are components in an integrated pest management (IPM) programme to manage the pest in the field. Cryptogran™ and Cryptex™ which are CrleGV-SA based-biopesticides have been effective in the control of T. leucotreta for the past 15 years. Recently, CrpeNPV-based Multimax™ and Codlmax™ have been commercialised to control T. leucotreta and other important agricultural pests. Despite these viruses being relatively host-specific and safe to humans and animals in comparison to chemical insecticides, their application is hindered by their slow speed of kill, sensitivity to UV light, and the potential for insect resistance. Research investigating the effects of mixed baculoviral interactions against target pests has been a growing field of interest due to their potential to overcome such shortcomings. Previous studies using a combination of CrleGV-SA and CrpeNPV against T. leucotreta observed a reduction in lethal concentration in laboratory bioassays, indicating that such mixtures may have the potential for application in the field. This has led to the motivation to investigate further interactions between CrleGV-SA in combination with CrpeNPV, CpGV-M, and HearNPV-Au to understand better how these viruses interact and to determine whether synergistic, additive, or antagonistic interactions can occur against T. leucotreta. The outcome of these interactions will inform researchers and farmers about best practices concerning these viruses should they be combined against T. leucotreta in the future. Prior to performing mixed baculovirus infections in laboratory bioassays, oligonucleotides targeting unique regions in the viral genomes of CrleGV-SA, CrpeNPV, CpGV-M, and HearNPV-Au were designed using Primer-BLAST. The specificity of these oligonucleotides was further tested in silico using Geneious R11 software (11.1.5). The stocks of CrpeNPV, CpGV-M, and HearNPV-Au were purified using crude OB extraction from diseased C. peltastica, C. pomonella, and H. armigera larval cadavers provided by River Bioscience (Pty) Ltd (Gqeberha, South Africa). The stock of CrleGV-SA was purified using crude OB extraction from infected T. leucotreta cadavers. Subsequently, the unique oligonucleotides were used in PCR assays to detect if the samples contained the baculoviruses of interest. Amplicons of the expected sizes were generated indicating the presence of CrleGV-SA, CrpeNPV, CpGV-M, and HearNPV-Au in each of the samples. The OBs were counted using darkfield microscopy and a counting chamber before the single and mixed infections were initiated against T. leucotreta neonate larvae. Surface-dose biological assays were used to evaluate the relative virulence in terms of lethal concentration of CrleGV-SA, CrpeNPV, and CpGV-M, alone against T. leucotreta. After 7 days, the dose mortality data was analysed using “drc” in R studio and the LC50 and LC90 were compared amongst each virus. The CrleGV-SA treatment was estimated to be the most virulent in comparison to CrpeNPV and CpGV-M. A dose discriminate assay confirmed that HearNPV does not cause mortality in T. leucotreta. Similarly, the relative virulence in terms of lethal concentration of CrleGV-SA in various ratios in combination with CrpeNPV, CpGV-M, and HearNPV-Au was determined using 7-day surface dose biological assays. The CrleGV/CrpeNPV was the most virulent mixture with lower LC50 and LC90 values measured in comparison to CrleGV/CpGV and CrleGV/HearNPV, respectively. The Tammes Bakuniak graphic method confirmed the CrleGV/CrpeNPV, CrleGV/CpGV, and CrleGV/HearNPV mixtures to be antagonistic against T. leucotreta neonate larvae in terms of lethal concentration. The last aspect of the study was to determine the probable cause of larval death. A modified CTAB protocol was used to extract genomic DNA from neonate-sized T. leucotreta cadavers collected in single and mixed infection assays. The gDNA served as templates in PCR assays using the unique oligonucleotides. In single infections, the presence of CrleGV-SA in CrpeNPV and HearNPV inoculated larvae was observed. The results suggest possible covert infections of CrleGV-SA in the T. leucotreta colony which may be caused by virus infection or an unknown stress factor. The results from the mixed infections showed the presence of each virus in all replicates except for the CrleGV/CpGV and CrleGV/HearNPV mixtures. In the CrleGV/CpGV mixture, only CrleGV-SA was present in the last replicate, suggesting a possible competition for host resources. In the CrleGV/HearNPV mixture, only CrleGV-SA was detected in all 3 replicates, suggesting that HearNPV did not have any effect and the larvae died of the CrleGV-SA infection. This is the first study to report mixtures of CrleGV-SA in combination with CpGV-M and HearNPV-Au against T. leucotreta neonate larvae. Despite the antagonistic interactions observed in the evaluated mixtures, this study has laid a foundation to further investigate how these viruses interact in dual infections for the improved control of T. leucotreta. This may be done by evaluating different ratios and combinations of baculoviruses to those used in this study. , Thesis (MSc) -- Faculty of Science, Biochemistry, Microbiology & Bioinformatics, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Tole, Siviwe
- Date: 2024-10-11
- Subjects: False codling moth Biological control , Baculoviruses , Integrated pest management , Natural pesticides , Granulovirus
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/463996 , vital:76464
- Description: False codling moth (FCM), Thaumatotibia leucotreta, is an important pest of citrus, stone fruit, avocados, peppers, and other important agricultural crops in southern Africa. Baculovirus-based biopesticides are components in an integrated pest management (IPM) programme to manage the pest in the field. Cryptogran™ and Cryptex™ which are CrleGV-SA based-biopesticides have been effective in the control of T. leucotreta for the past 15 years. Recently, CrpeNPV-based Multimax™ and Codlmax™ have been commercialised to control T. leucotreta and other important agricultural pests. Despite these viruses being relatively host-specific and safe to humans and animals in comparison to chemical insecticides, their application is hindered by their slow speed of kill, sensitivity to UV light, and the potential for insect resistance. Research investigating the effects of mixed baculoviral interactions against target pests has been a growing field of interest due to their potential to overcome such shortcomings. Previous studies using a combination of CrleGV-SA and CrpeNPV against T. leucotreta observed a reduction in lethal concentration in laboratory bioassays, indicating that such mixtures may have the potential for application in the field. This has led to the motivation to investigate further interactions between CrleGV-SA in combination with CrpeNPV, CpGV-M, and HearNPV-Au to understand better how these viruses interact and to determine whether synergistic, additive, or antagonistic interactions can occur against T. leucotreta. The outcome of these interactions will inform researchers and farmers about best practices concerning these viruses should they be combined against T. leucotreta in the future. Prior to performing mixed baculovirus infections in laboratory bioassays, oligonucleotides targeting unique regions in the viral genomes of CrleGV-SA, CrpeNPV, CpGV-M, and HearNPV-Au were designed using Primer-BLAST. The specificity of these oligonucleotides was further tested in silico using Geneious R11 software (11.1.5). The stocks of CrpeNPV, CpGV-M, and HearNPV-Au were purified using crude OB extraction from diseased C. peltastica, C. pomonella, and H. armigera larval cadavers provided by River Bioscience (Pty) Ltd (Gqeberha, South Africa). The stock of CrleGV-SA was purified using crude OB extraction from infected T. leucotreta cadavers. Subsequently, the unique oligonucleotides were used in PCR assays to detect if the samples contained the baculoviruses of interest. Amplicons of the expected sizes were generated indicating the presence of CrleGV-SA, CrpeNPV, CpGV-M, and HearNPV-Au in each of the samples. The OBs were counted using darkfield microscopy and a counting chamber before the single and mixed infections were initiated against T. leucotreta neonate larvae. Surface-dose biological assays were used to evaluate the relative virulence in terms of lethal concentration of CrleGV-SA, CrpeNPV, and CpGV-M, alone against T. leucotreta. After 7 days, the dose mortality data was analysed using “drc” in R studio and the LC50 and LC90 were compared amongst each virus. The CrleGV-SA treatment was estimated to be the most virulent in comparison to CrpeNPV and CpGV-M. A dose discriminate assay confirmed that HearNPV does not cause mortality in T. leucotreta. Similarly, the relative virulence in terms of lethal concentration of CrleGV-SA in various ratios in combination with CrpeNPV, CpGV-M, and HearNPV-Au was determined using 7-day surface dose biological assays. The CrleGV/CrpeNPV was the most virulent mixture with lower LC50 and LC90 values measured in comparison to CrleGV/CpGV and CrleGV/HearNPV, respectively. The Tammes Bakuniak graphic method confirmed the CrleGV/CrpeNPV, CrleGV/CpGV, and CrleGV/HearNPV mixtures to be antagonistic against T. leucotreta neonate larvae in terms of lethal concentration. The last aspect of the study was to determine the probable cause of larval death. A modified CTAB protocol was used to extract genomic DNA from neonate-sized T. leucotreta cadavers collected in single and mixed infection assays. The gDNA served as templates in PCR assays using the unique oligonucleotides. In single infections, the presence of CrleGV-SA in CrpeNPV and HearNPV inoculated larvae was observed. The results suggest possible covert infections of CrleGV-SA in the T. leucotreta colony which may be caused by virus infection or an unknown stress factor. The results from the mixed infections showed the presence of each virus in all replicates except for the CrleGV/CpGV and CrleGV/HearNPV mixtures. In the CrleGV/CpGV mixture, only CrleGV-SA was present in the last replicate, suggesting a possible competition for host resources. In the CrleGV/HearNPV mixture, only CrleGV-SA was detected in all 3 replicates, suggesting that HearNPV did not have any effect and the larvae died of the CrleGV-SA infection. This is the first study to report mixtures of CrleGV-SA in combination with CpGV-M and HearNPV-Au against T. leucotreta neonate larvae. Despite the antagonistic interactions observed in the evaluated mixtures, this study has laid a foundation to further investigate how these viruses interact in dual infections for the improved control of T. leucotreta. This may be done by evaluating different ratios and combinations of baculoviruses to those used in this study. , Thesis (MSc) -- Faculty of Science, Biochemistry, Microbiology & Bioinformatics, 2024
- Full Text:
- Date Issued: 2024-10-11
The expression and evaluation of CrpeNPV gp37 as a formulation additive for enhanced infectivity with CrleGV-SA and improved Thaumatotibia leucotreta control
- Authors: Muleya, Naho
- Date: 2024-10-11
- Subjects: Cryptophlebia leucotreta Biological control , False Codling Moth , Cryptophlebia leucotreta granulovirus , Cryptophlebia peltastica nucleopolyhedrovirus , Citrus Diseases and pests South Africa , Baculoviruses
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/463919 , vital:76457
- Description: Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) is a significant pest native to Africa, causing damage to citrus and posing a threat to the export of fresh citrus in South Africa. Classified as a phytosanitary risk by several South African export markets, this pest necessitates effective control measures. Baculoviruses emerge as promising biological control agents against T. leucotreta due to their inherent safety and eco-friendly characteristics. Among these, Cryptophlebia leucotreta Granulovirus (CrleGV-SA) and Cryptophlebia peltastica Nucleopolyhedrovirus (CrpeNPV) stand out, both causing larval mortality upon infecting T. leucotreta. CrleGV-SA has been formulated into the products Cryptogran™, CryptoMax™ and Cryptex®, while CrpeNPV has been formulated into the product Multimax™. Both viruses are used in integrated pest management programmes to reduce fruit damage in agricultural fields, with CrleGV-SA having been employed against T. leucotreta for nearly 20 years in South Africa. However, these control options are limited by factors such as virulence and the slow speed of kill. This limitation can be addressed by exploiting potential synergistic relationships between baculoviruses infecting the same host. Previous studies have demonstrated that the truncated CpGV gp37 can enhance the infectivity of NPVs on other lepidopteran pests, such as Spodoptera exigua (Hübner). Although the mechanism behind this phenomenon remains unclear, it presents an opportunity to enhance the effectiveness of baculovirus-based management strategies. Notably, the genome of CrpeNPV encodes gp37, while CrleGV-SA lacks this gene. The potential interaction between CrleGV-SA and CrpeNPV gp37 remains unexplored. Therefore, investigating whether they exhibit synergistic or antagonistic effects is essential for optimising baculovirus-based management of T. leucotreta. This study aims to express CrpeNPV gp37 in a bacterial system and then evaluate its effect on larval mortality when combined with CrleGV-SA in laboratory bioassays. The initial step involved extracting genomic DNA (gDNA) from occlusion bodies (OBs) of CrpeNPV. A modified Quick DNA Miniprep plus kit was utilised, which entailed pre-treatment with Na2CO3 followed by neutralisation with Tris-HCI before gDNA extraction using the kit. Subsequently, the concentration of the gDNA was estimated using a Nanodrop spectrophotometer. Oligonucleotides targeting the CrpeNPV gp37 gene were designed for PCR amplification, with the gDNA serving as a template. The gp37 amplicon was identified through agarose gel electrophoresis and then gel purified in preparation for cloning. Secondly, the purified PCR product was cloned into the intermediate vector pJET1.2/blunt and then subcloned into the bacterial expression vector pCA528 through DNA ligation. The construction of recombinant plasmids (pJET-gp37 and pCA-gp37) was conducted and verified using Colony PCR, plasmid extraction, restriction enzyme analysis, and Sanger sequencing. Thirdly, the recombinant protein (6×His-SUMO-gp37) was expressed and purified using Nickel affinity chromatography and analysed through SDS-PAGE and Western blot techniques. The expression of 6×His-SUMO-gp37 was carried out at both 25 °C and 18 °C. A time course induction study was conducted, inducing transformed cells for 0-, 3-, 5-, and 24-hours post induction (hpi). SDS-PAGE and Western blotting of samples collected at various time points revealed that 6×His-SUMO-gp37, approximately 42 kDa in size, was visible from 3 hpi, with maximal expression at 24 hpi. Solubility analysis of 6×His-SUMO-gp37 was performed at both temperatures, showing solubility at 18 °C but predominantly present in the insoluble fraction. The soluble protein was purified under native conditions, while the insoluble protein was purified under denaturing conditions. Despite being unable to elute 6×His-SUMO-gp37 under native conditions, successful elution was achieved under denaturing conditions, confirmed via Western blot analysis. No further experiments were conducted on the eluted 6×His-SUMO-gp37 under denaturing conditions. Lastly, a preliminary surface dose bioassay was conducted to evaluate the efficacy of pelleted bacteria expressing 6×His-SUMO-gp37 in combination with CrleGV-SA against T. leucotreta neonates. Two lethal concentration doses of CrleGV-SA were prepared: a low concentration (2.96×104 OBs/mL) capable of killing 40 % of the T. leucotreta population, and a high concentration (2.96×105 OBs/mL) capable of killing 90 % of the population. The target protein, 6×His-SUMO-gp37, and the control, pCA528, were obtained by lysing the cells, centrifuging the samples, and collecting the insoluble fractions in pellet form. These fractions were then resuspended in PBS and used as treatments in combination with the prepared CrleGV-SA concentration doses. The concentration of the pellets was estimated using a Nanodrop spectrophotometer by measuring the absorbance at 280 nm. The bioassay results revealed that the combination of 100 μg/mL of pelleted bacteria expressing 6×His-SUMO-gp37 with CrleGV-SA had no effect on T. leucotreta larval mortality compared to CrleGV-SA alone. A one-way ANOVA was performed to assess differences among the virus treatment groups, concluding that no statistically significant differences were observed among the groups. The experiments in this study provided valuable insights for future research, particularly in exploring the use of a protein-virus combination as a novel method for pest control. , Thesis (MSc) -- Faculty of Science, Biochemistry, Microbiology & Bioinformatics, 2024
- Full Text:
- Date Issued: 2024-10-11
- Authors: Muleya, Naho
- Date: 2024-10-11
- Subjects: Cryptophlebia leucotreta Biological control , False Codling Moth , Cryptophlebia leucotreta granulovirus , Cryptophlebia peltastica nucleopolyhedrovirus , Citrus Diseases and pests South Africa , Baculoviruses
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/463919 , vital:76457
- Description: Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) is a significant pest native to Africa, causing damage to citrus and posing a threat to the export of fresh citrus in South Africa. Classified as a phytosanitary risk by several South African export markets, this pest necessitates effective control measures. Baculoviruses emerge as promising biological control agents against T. leucotreta due to their inherent safety and eco-friendly characteristics. Among these, Cryptophlebia leucotreta Granulovirus (CrleGV-SA) and Cryptophlebia peltastica Nucleopolyhedrovirus (CrpeNPV) stand out, both causing larval mortality upon infecting T. leucotreta. CrleGV-SA has been formulated into the products Cryptogran™, CryptoMax™ and Cryptex®, while CrpeNPV has been formulated into the product Multimax™. Both viruses are used in integrated pest management programmes to reduce fruit damage in agricultural fields, with CrleGV-SA having been employed against T. leucotreta for nearly 20 years in South Africa. However, these control options are limited by factors such as virulence and the slow speed of kill. This limitation can be addressed by exploiting potential synergistic relationships between baculoviruses infecting the same host. Previous studies have demonstrated that the truncated CpGV gp37 can enhance the infectivity of NPVs on other lepidopteran pests, such as Spodoptera exigua (Hübner). Although the mechanism behind this phenomenon remains unclear, it presents an opportunity to enhance the effectiveness of baculovirus-based management strategies. Notably, the genome of CrpeNPV encodes gp37, while CrleGV-SA lacks this gene. The potential interaction between CrleGV-SA and CrpeNPV gp37 remains unexplored. Therefore, investigating whether they exhibit synergistic or antagonistic effects is essential for optimising baculovirus-based management of T. leucotreta. This study aims to express CrpeNPV gp37 in a bacterial system and then evaluate its effect on larval mortality when combined with CrleGV-SA in laboratory bioassays. The initial step involved extracting genomic DNA (gDNA) from occlusion bodies (OBs) of CrpeNPV. A modified Quick DNA Miniprep plus kit was utilised, which entailed pre-treatment with Na2CO3 followed by neutralisation with Tris-HCI before gDNA extraction using the kit. Subsequently, the concentration of the gDNA was estimated using a Nanodrop spectrophotometer. Oligonucleotides targeting the CrpeNPV gp37 gene were designed for PCR amplification, with the gDNA serving as a template. The gp37 amplicon was identified through agarose gel electrophoresis and then gel purified in preparation for cloning. Secondly, the purified PCR product was cloned into the intermediate vector pJET1.2/blunt and then subcloned into the bacterial expression vector pCA528 through DNA ligation. The construction of recombinant plasmids (pJET-gp37 and pCA-gp37) was conducted and verified using Colony PCR, plasmid extraction, restriction enzyme analysis, and Sanger sequencing. Thirdly, the recombinant protein (6×His-SUMO-gp37) was expressed and purified using Nickel affinity chromatography and analysed through SDS-PAGE and Western blot techniques. The expression of 6×His-SUMO-gp37 was carried out at both 25 °C and 18 °C. A time course induction study was conducted, inducing transformed cells for 0-, 3-, 5-, and 24-hours post induction (hpi). SDS-PAGE and Western blotting of samples collected at various time points revealed that 6×His-SUMO-gp37, approximately 42 kDa in size, was visible from 3 hpi, with maximal expression at 24 hpi. Solubility analysis of 6×His-SUMO-gp37 was performed at both temperatures, showing solubility at 18 °C but predominantly present in the insoluble fraction. The soluble protein was purified under native conditions, while the insoluble protein was purified under denaturing conditions. Despite being unable to elute 6×His-SUMO-gp37 under native conditions, successful elution was achieved under denaturing conditions, confirmed via Western blot analysis. No further experiments were conducted on the eluted 6×His-SUMO-gp37 under denaturing conditions. Lastly, a preliminary surface dose bioassay was conducted to evaluate the efficacy of pelleted bacteria expressing 6×His-SUMO-gp37 in combination with CrleGV-SA against T. leucotreta neonates. Two lethal concentration doses of CrleGV-SA were prepared: a low concentration (2.96×104 OBs/mL) capable of killing 40 % of the T. leucotreta population, and a high concentration (2.96×105 OBs/mL) capable of killing 90 % of the population. The target protein, 6×His-SUMO-gp37, and the control, pCA528, were obtained by lysing the cells, centrifuging the samples, and collecting the insoluble fractions in pellet form. These fractions were then resuspended in PBS and used as treatments in combination with the prepared CrleGV-SA concentration doses. The concentration of the pellets was estimated using a Nanodrop spectrophotometer by measuring the absorbance at 280 nm. The bioassay results revealed that the combination of 100 μg/mL of pelleted bacteria expressing 6×His-SUMO-gp37 with CrleGV-SA had no effect on T. leucotreta larval mortality compared to CrleGV-SA alone. A one-way ANOVA was performed to assess differences among the virus treatment groups, concluding that no statistically significant differences were observed among the groups. The experiments in this study provided valuable insights for future research, particularly in exploring the use of a protein-virus combination as a novel method for pest control. , Thesis (MSc) -- Faculty of Science, Biochemistry, Microbiology & Bioinformatics, 2024
- Full Text:
- Date Issued: 2024-10-11
Augmentation of Aphytis melinus DeBach (Hymenoptera: Aphelinidae) for the control of California red scale Aonidiella aurantii Maskell (Hemiptera: Diaspididae) on citrus
- De Beer, Ernst Friedrich Ludwig
- Authors: De Beer, Ernst Friedrich Ludwig
- Date: 2024-04-05
- Subjects: Aphytis melinus , Aphytis , Aonidiella aurantii , Citrus Diseases and pests South Africa , Pests Biological control
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/435638 , vital:73175 , DOI 10.21504/10962/435638
- Description: Aphytis lingnanensis was reared and tested in South Africa in the early 2000s for augmentation against red scale on citrus and was found to be ineffective. Aphytis melinus is now commercially available and it is important that the efficacy of augmentation thereof on red scale is determined locally. Field trials, fitness assessments and molecular identification on A. melinus from two insectaries were done. Field trials was done in seven, five and six pairs of comparable release and control orchards across the Eastern and Western Cape during the seasons of 2019/2020, 2020/2021 and 2021/2022 respectively. Red scale infestation was monitored and a sample of 20 infested fruit from each orchard was randomly collected every four weeks. Aphytis spp. responsible for parasitism were identified and the percentage parasitism recorded. Results of this study of field trials suggest that the augmentation of A. melinus did not significantly increase the level of parasitism above that of the untreated control. Five repetitions with six replicates of flight and longevity tests were performed with wasps from each insectary. Wasps in the longevity test from two insectaries were kept at 23 °C and 65% RH with honey. Flight tests were performed in tubes of 16 by 30 cm, with a light above a clear, sticky ceiling at 23 °C and 65% RH. On average in five replicates, 65%, 33% and 17% A. melinus wasps were alive on day one, five and 10 respectively. The overall sex ratio was 1.58 for females to males, but 1.05, 2.19 and 2.66 for non-flyers, non-crawlers, crawlers, and flyers respectively. In flight tests for both insectaries combined, only 36.97% of wasps could initiate flight in 24 h while 56.96% remained on the tube floor, and 6.05% attempted to crawl upwards. No significant differences in flight performance were recorded between the two insectaries. Wasps from the local insectary lived significantly longer during the longevity tests but were shorter in transit than wasps from the overseas insectary. COI genes were sequenced and compared against Genbank sequences using BLAST. Molecular identifications did not confirm morphological identifications for all species, indicating unexpected genetic complexity. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2024
- Full Text:
- Date Issued: 2024-04-05
- Authors: De Beer, Ernst Friedrich Ludwig
- Date: 2024-04-05
- Subjects: Aphytis melinus , Aphytis , Aonidiella aurantii , Citrus Diseases and pests South Africa , Pests Biological control
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/435638 , vital:73175 , DOI 10.21504/10962/435638
- Description: Aphytis lingnanensis was reared and tested in South Africa in the early 2000s for augmentation against red scale on citrus and was found to be ineffective. Aphytis melinus is now commercially available and it is important that the efficacy of augmentation thereof on red scale is determined locally. Field trials, fitness assessments and molecular identification on A. melinus from two insectaries were done. Field trials was done in seven, five and six pairs of comparable release and control orchards across the Eastern and Western Cape during the seasons of 2019/2020, 2020/2021 and 2021/2022 respectively. Red scale infestation was monitored and a sample of 20 infested fruit from each orchard was randomly collected every four weeks. Aphytis spp. responsible for parasitism were identified and the percentage parasitism recorded. Results of this study of field trials suggest that the augmentation of A. melinus did not significantly increase the level of parasitism above that of the untreated control. Five repetitions with six replicates of flight and longevity tests were performed with wasps from each insectary. Wasps in the longevity test from two insectaries were kept at 23 °C and 65% RH with honey. Flight tests were performed in tubes of 16 by 30 cm, with a light above a clear, sticky ceiling at 23 °C and 65% RH. On average in five replicates, 65%, 33% and 17% A. melinus wasps were alive on day one, five and 10 respectively. The overall sex ratio was 1.58 for females to males, but 1.05, 2.19 and 2.66 for non-flyers, non-crawlers, crawlers, and flyers respectively. In flight tests for both insectaries combined, only 36.97% of wasps could initiate flight in 24 h while 56.96% remained on the tube floor, and 6.05% attempted to crawl upwards. No significant differences in flight performance were recorded between the two insectaries. Wasps from the local insectary lived significantly longer during the longevity tests but were shorter in transit than wasps from the overseas insectary. COI genes were sequenced and compared against Genbank sequences using BLAST. Molecular identifications did not confirm morphological identifications for all species, indicating unexpected genetic complexity. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2024
- Full Text:
- Date Issued: 2024-04-05
The enemy release hypothesis and beyond: Lagarosiphon major invasion dynamics and management options for New Zealand using native natural enemies from South Africa
- Authors: Baso, Nompumelelo Catherine
- Date: 2024-04-05
- Subjects: Enemy release hypothesis , Lagarosiphon major Biological control New Zealand , Hydrellia , Submerged aquatic vegetation , Invasion ecology
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/435627 , vital:73174 , DOI 10.21504/10962/435627
- Description: Numerous scientific investigations have demonstrated the destructive impact that exotic species can have on ecosystem services beyond a specific threshold. There are many explanations for why introduced plants are likely to be more successful outside their native range. One such explanation is offered by the Enemy Release Hypothesis (ERH), which states that plants automatically become superior competitors outside of their natural range due to release from top-down stressors (herbivory, parasites, and diseases) that is evident in the absence of their natural enemies. The underlying assumption of the ERH is that natural enemies are important regulators of plant species populations, and that the pressures from these natural enemies are felt more readily by native species compared to alien plants. Consequently, in the absence of such pressures, the ERH assumes that exotic plants can allocate more resources towards growth and reproduction, while effectively maintaining accumulated biomass. Classical biological control has previously been cited as evidence for the enemy release hypothesis. Therefore, the overarching aim and theme of this thesis was to investigate the role of ERH on the invasiveness of Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae) in New Zealand. Firstly, a literature search and a meta-analysis was used to synthesize existing studies in order to test for general applicability of this hypothesis to aquatic plant invasions. Furthermore, an empirical investigation was conducted in order to directly quantify enemy release in L. major populations invaded areas of New Zealand. To achieve this, various plant parameters of this plant, overall macrophyte and invertebrate diversity were measured and compared between sites in the native range in South Africa and the invaded areas in New Zealand. Although the meta-analysis showed variable evidence for this hypothesis depending on various modulating factors such as study type, plant growth form and measured parameters, for L. major, there was strong evidence of enemy release. The biogeographical comparisons showed that L. major exhibited increased fitness in most of the invaded sites, marked by elevated biomass accumulation, significantly higher shoot production, and the displacement of native plant species. The observed fitness advantages were directly correlated to a decrease in herbivory diversity and pressure upon the plant's introduction to New Zealand. Unlike the native populations, which contend with the presence of at least four co-occurring herbivores, including specialist herbivores, the invaded range had a substantially lower herbivore diversity, with only Hygraula nitens Butler (Lepidoptera: Crambidae) syn. Nymphula nitens, significantly damaging L. major. These findings emphasize the importance of understanding invasion ecology and theories such as ERH in order to advance aquatic plant management and also present valuable insights for developing effective strategies to mitigate the impact of invasive alien species on aquatic ecosystems. Specifically, results from the empirical investigation provide evidence in support of the ERH and highlight the suitability of implementing biological control strategies to manage the L. major invasion in New Zealand. Previous studies have shown the suitability of two specialist herbivores, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), and Polypedilum tuburcinatum Andersen (Diptera: Chironomidae), as potential biological control agents. This control strategy presents a sustainable and ecologically responsible approach, promoting coexistence between exotic plants and native species rather than displacement through competitive exclusion. With the apparent dominance of L. major at various New Zealand localities, the subsequent objective of this thesis was to investigate the competitive interactions between L. major and another invasive Hydrocharitaceae, Egeria densa Planchon, as driven by herbivory. Combinations of two host specific Ephydrid flies, H. lagarosiphon and H. egeriae, were used at eight different factorial combination of planting densities. The analysis of plant parameters and the application of inverse linear models revealed that L. major often exhibits relatively higher fitness, especially in low monoculture treatments when the two insects were isolated. However, multiple inverse linear models revealed that actual competitive outcomes are dependent on factors such as initial plant density and herbivory regime, with competitive interactions generally being mild. Nevertheless, the presence of H. lagarosiphon resulted in facilitation of E. densa growth. Thus, even at lower densities, these insects still had an impact on the observed interactions, further emphasizing suitability as damaging biological control agents. Lastly, focusing on the abiotic component of L. major invasion, Species Distribution Models (SDMs) were employed to map potential suitable habitat for this species, as well as predict the consequences of climate change on this. Correlative and mechanistic modelling was also used to simulate suitable habitat for potential biological control agents, thus addressing the potential for mismatches between host plant distribution and insect suitable range. The Maximum Entropy Species Distribution Modelling (MaxEnt) algorithm revealed that more than 90% of all freshwater ecosystems in New Zealand are susceptible to L. major invasion, with suitability projected to expand further under future climate scenarios. Moreover, correlative modelling using this method suggests limited suitable habitat for both herbivores. However, degree-day modelling, which also takes into account the physiological requirements, showed that H. lagarosiphon has the potential to produce viable populations in several parts of New Zealand. Overall, this thesis explored the intricate web of biotic and abiotic factors influencing the success of L. major outside its native range. The results emphasize the potential impacts of climate change on the invasion potential and management strategies for L. major. The findings also advocate for the implementation of sustainable and ecologically sound management solutions, such as biological control, to manage this species. , Thesis (PhD) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-04-05
- Authors: Baso, Nompumelelo Catherine
- Date: 2024-04-05
- Subjects: Enemy release hypothesis , Lagarosiphon major Biological control New Zealand , Hydrellia , Submerged aquatic vegetation , Invasion ecology
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/435627 , vital:73174 , DOI 10.21504/10962/435627
- Description: Numerous scientific investigations have demonstrated the destructive impact that exotic species can have on ecosystem services beyond a specific threshold. There are many explanations for why introduced plants are likely to be more successful outside their native range. One such explanation is offered by the Enemy Release Hypothesis (ERH), which states that plants automatically become superior competitors outside of their natural range due to release from top-down stressors (herbivory, parasites, and diseases) that is evident in the absence of their natural enemies. The underlying assumption of the ERH is that natural enemies are important regulators of plant species populations, and that the pressures from these natural enemies are felt more readily by native species compared to alien plants. Consequently, in the absence of such pressures, the ERH assumes that exotic plants can allocate more resources towards growth and reproduction, while effectively maintaining accumulated biomass. Classical biological control has previously been cited as evidence for the enemy release hypothesis. Therefore, the overarching aim and theme of this thesis was to investigate the role of ERH on the invasiveness of Lagarosiphon major (Ridl.) Moss ex Wager (Hydrocharitaceae) in New Zealand. Firstly, a literature search and a meta-analysis was used to synthesize existing studies in order to test for general applicability of this hypothesis to aquatic plant invasions. Furthermore, an empirical investigation was conducted in order to directly quantify enemy release in L. major populations invaded areas of New Zealand. To achieve this, various plant parameters of this plant, overall macrophyte and invertebrate diversity were measured and compared between sites in the native range in South Africa and the invaded areas in New Zealand. Although the meta-analysis showed variable evidence for this hypothesis depending on various modulating factors such as study type, plant growth form and measured parameters, for L. major, there was strong evidence of enemy release. The biogeographical comparisons showed that L. major exhibited increased fitness in most of the invaded sites, marked by elevated biomass accumulation, significantly higher shoot production, and the displacement of native plant species. The observed fitness advantages were directly correlated to a decrease in herbivory diversity and pressure upon the plant's introduction to New Zealand. Unlike the native populations, which contend with the presence of at least four co-occurring herbivores, including specialist herbivores, the invaded range had a substantially lower herbivore diversity, with only Hygraula nitens Butler (Lepidoptera: Crambidae) syn. Nymphula nitens, significantly damaging L. major. These findings emphasize the importance of understanding invasion ecology and theories such as ERH in order to advance aquatic plant management and also present valuable insights for developing effective strategies to mitigate the impact of invasive alien species on aquatic ecosystems. Specifically, results from the empirical investigation provide evidence in support of the ERH and highlight the suitability of implementing biological control strategies to manage the L. major invasion in New Zealand. Previous studies have shown the suitability of two specialist herbivores, Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), and Polypedilum tuburcinatum Andersen (Diptera: Chironomidae), as potential biological control agents. This control strategy presents a sustainable and ecologically responsible approach, promoting coexistence between exotic plants and native species rather than displacement through competitive exclusion. With the apparent dominance of L. major at various New Zealand localities, the subsequent objective of this thesis was to investigate the competitive interactions between L. major and another invasive Hydrocharitaceae, Egeria densa Planchon, as driven by herbivory. Combinations of two host specific Ephydrid flies, H. lagarosiphon and H. egeriae, were used at eight different factorial combination of planting densities. The analysis of plant parameters and the application of inverse linear models revealed that L. major often exhibits relatively higher fitness, especially in low monoculture treatments when the two insects were isolated. However, multiple inverse linear models revealed that actual competitive outcomes are dependent on factors such as initial plant density and herbivory regime, with competitive interactions generally being mild. Nevertheless, the presence of H. lagarosiphon resulted in facilitation of E. densa growth. Thus, even at lower densities, these insects still had an impact on the observed interactions, further emphasizing suitability as damaging biological control agents. Lastly, focusing on the abiotic component of L. major invasion, Species Distribution Models (SDMs) were employed to map potential suitable habitat for this species, as well as predict the consequences of climate change on this. Correlative and mechanistic modelling was also used to simulate suitable habitat for potential biological control agents, thus addressing the potential for mismatches between host plant distribution and insect suitable range. The Maximum Entropy Species Distribution Modelling (MaxEnt) algorithm revealed that more than 90% of all freshwater ecosystems in New Zealand are susceptible to L. major invasion, with suitability projected to expand further under future climate scenarios. Moreover, correlative modelling using this method suggests limited suitable habitat for both herbivores. However, degree-day modelling, which also takes into account the physiological requirements, showed that H. lagarosiphon has the potential to produce viable populations in several parts of New Zealand. Overall, this thesis explored the intricate web of biotic and abiotic factors influencing the success of L. major outside its native range. The results emphasize the potential impacts of climate change on the invasion potential and management strategies for L. major. The findings also advocate for the implementation of sustainable and ecologically sound management solutions, such as biological control, to manage this species. , Thesis (PhD) -- Faculty of Science, Botany, 2024
- Full Text:
- Date Issued: 2024-04-05
Evaluation of potential oviposition deterrents for false codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae)
- Authors: Dambuza, Khalipha
- Date: 2023-10-13
- Subjects: Cryptophlebia leucotreta , Pests Integrated control , Semiochemicals , Agricultural pests Control , Oviposition , Essences and essential oils
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424479 , vital:72157
- Description: There has been extensive research on the use of semiochemicals as deterrents or true repellents in insect pest management, particularly in push-pull strategies. Much of this research has focused on pests of medical and veterinary importance and has been limited for agricultural pests. This means there is an opportunity to study use of deterrents to manage pests of agricultural importance. No study has been conducted on deterrents for false codling moth (FCM), Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), a key phytosanitary pest in citrus orchards across South Africa. This study assessed FCM oviposition deterrence in botanicals (plants (n = 11) and essential oils (n = 15)), and some commercial pesticides (n = 7) used for FCM control in South Africa. All tested botanicals were selected based on an extensive literature review of plant compounds that have been reported to deter or repel lepidopteran pests. Choice and no-choice oviposition bioassays were conducted in complete darkness in a controlled environment room. Oranges treated with solutions/suspensions of potential oviposition deterrents were placed into a cage with gravid FCM females for four hours, with oviposition being recorded every hour. Of the 33 tested compounds, only eight significantly reduced FCM oviposition (P < 0.05) compared to the control in oviposition bioassays i.e. two essential oils (lavender and peppermint), two plant crude extracts (garlic and marigold), one fruit (Mango), and three commercial FCM insecticides (Delegate, Coragen, and Warlock). All identified oviposition deterrents, except for Mango, were further investigated for their ovicidal properties in concentration response bioassays, where all botanicals were identified to have dual action (both deterrent and ovicidal properties), as they significantly (P < 0.05) reduced FCM oviposition and egg hatch. Garlic was the most efficacious botanical whilst Warlock was the only commercial insecticide that did not show ovicidal activity (F = 41.17, P = 0.0622). Larval penetration of the host fruit was less than egg hatch for all tested compounds in concentration response bioassays. Oviposition, egg hatch, and larval penetration were all affected by concentration, with the higher concentrations being the most effective. The efficacy of these deterrent compounds should be further tested in semi-field and/or field trials, and they may have potential in FCM management as allomone dispensers or sprays. They can also be implemented in push-pull strategies where they can be used in conjunction with FCM attractants. Lastly, repellence studies should be conducted in absentia of the host fruit to determine whether oviposition deterrence was a result of true repellence or odour masking. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Dambuza, Khalipha
- Date: 2023-10-13
- Subjects: Cryptophlebia leucotreta , Pests Integrated control , Semiochemicals , Agricultural pests Control , Oviposition , Essences and essential oils
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424479 , vital:72157
- Description: There has been extensive research on the use of semiochemicals as deterrents or true repellents in insect pest management, particularly in push-pull strategies. Much of this research has focused on pests of medical and veterinary importance and has been limited for agricultural pests. This means there is an opportunity to study use of deterrents to manage pests of agricultural importance. No study has been conducted on deterrents for false codling moth (FCM), Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), a key phytosanitary pest in citrus orchards across South Africa. This study assessed FCM oviposition deterrence in botanicals (plants (n = 11) and essential oils (n = 15)), and some commercial pesticides (n = 7) used for FCM control in South Africa. All tested botanicals were selected based on an extensive literature review of plant compounds that have been reported to deter or repel lepidopteran pests. Choice and no-choice oviposition bioassays were conducted in complete darkness in a controlled environment room. Oranges treated with solutions/suspensions of potential oviposition deterrents were placed into a cage with gravid FCM females for four hours, with oviposition being recorded every hour. Of the 33 tested compounds, only eight significantly reduced FCM oviposition (P < 0.05) compared to the control in oviposition bioassays i.e. two essential oils (lavender and peppermint), two plant crude extracts (garlic and marigold), one fruit (Mango), and three commercial FCM insecticides (Delegate, Coragen, and Warlock). All identified oviposition deterrents, except for Mango, were further investigated for their ovicidal properties in concentration response bioassays, where all botanicals were identified to have dual action (both deterrent and ovicidal properties), as they significantly (P < 0.05) reduced FCM oviposition and egg hatch. Garlic was the most efficacious botanical whilst Warlock was the only commercial insecticide that did not show ovicidal activity (F = 41.17, P = 0.0622). Larval penetration of the host fruit was less than egg hatch for all tested compounds in concentration response bioassays. Oviposition, egg hatch, and larval penetration were all affected by concentration, with the higher concentrations being the most effective. The efficacy of these deterrent compounds should be further tested in semi-field and/or field trials, and they may have potential in FCM management as allomone dispensers or sprays. They can also be implemented in push-pull strategies where they can be used in conjunction with FCM attractants. Lastly, repellence studies should be conducted in absentia of the host fruit to determine whether oviposition deterrence was a result of true repellence or odour masking. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Date Issued: 2023-10-13
The possible effect of insecticide drift from citrus orchards, and acute toxicity of insecticides on the biocontrol agents of Pontederia crassipes (Mart.) Solms-Laub (Pontederiaceae) established along citrus orchards in the Lowveld region of Mpumalanga Province, South Africa
- Authors: Mabuza, Mefika Michael
- Date: 2023-10-13
- Subjects: Acute toxicity , Pontederia crassipes , Biocontrol , Water hyacinth Biological control , Insecticides Toxicology , Nutrient , Citrus Diseases and pests
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424468 , vital:72156
- Description: This study investigated the possible effect of insecticide drift on naturalized biological control agents of Pontederia crassipes (Mart.) Solms-Laub (Pontederiaceae), in the Lowveld region of Mpumalanga Province of South Africa. Occurrence and abundance of biocontrol agents were recorded at three sites on the Crocodile River and at three dams adjacent to citrus orchards. Leaves of P. crassipes and water samples were collected for insecticide residues and also nutrient status of the water and plants. Eccritotarsus catarinensis Carvalho (Hemiptera: Miridae), Neochetina spp. (combined) (Coleoptera: Curculionidae), and Orthogalumna terebrantis Wallwork (Sarcoptiformes: Galumnidae) were recorded with notable variation in abundance between the river and dams across regions. Insecticide residues were not detected on all leaves sampled across study regions, however, nutrients were detected with nitrate ranging between oligotrophic and mesotrophic. Phosphorus was also detected, but, neither of the nutrients correlated with the occurrence and abundance of naturalized biological control agents of P. crassipes. Bioassays were conducted to measure the effect of commonly used insecticides (viz. Methomyl and Chlorpyrifos) on the survival and feeding damage of biological control agents of P. crassipes. Survival of individual insects was recorded between 0.5 and 120 hours for Megamelus scutellaris and Neochetina eichhorniae Warner (Coleoptera: Curculionidae) adults for treatments where insecticides were topically applied onto the insects or leaves were dipped into the pesticides. Concentrations below field rates, recommended and above field rates of Methomyl and Chlorpyrifos on either exposure techniques significantly reduced survival and feeding of biocontrol agents. Methomyl was more toxic compared to Chlorpyrifos and it significantly reduced the survival of M. scutellaris and N. eichhorniae. In conclusion, in this study, population abundance of biocontrol agents of P. crassipes at the Lowveld region of Mpumalanga was not influenced by pesticide drift, but, insecticides commonly used in the citrus orchards has the potential to negatively impact naturalized biological control of P. crassipes as demonstrated by the bioassays. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Mabuza, Mefika Michael
- Date: 2023-10-13
- Subjects: Acute toxicity , Pontederia crassipes , Biocontrol , Water hyacinth Biological control , Insecticides Toxicology , Nutrient , Citrus Diseases and pests
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424468 , vital:72156
- Description: This study investigated the possible effect of insecticide drift on naturalized biological control agents of Pontederia crassipes (Mart.) Solms-Laub (Pontederiaceae), in the Lowveld region of Mpumalanga Province of South Africa. Occurrence and abundance of biocontrol agents were recorded at three sites on the Crocodile River and at three dams adjacent to citrus orchards. Leaves of P. crassipes and water samples were collected for insecticide residues and also nutrient status of the water and plants. Eccritotarsus catarinensis Carvalho (Hemiptera: Miridae), Neochetina spp. (combined) (Coleoptera: Curculionidae), and Orthogalumna terebrantis Wallwork (Sarcoptiformes: Galumnidae) were recorded with notable variation in abundance between the river and dams across regions. Insecticide residues were not detected on all leaves sampled across study regions, however, nutrients were detected with nitrate ranging between oligotrophic and mesotrophic. Phosphorus was also detected, but, neither of the nutrients correlated with the occurrence and abundance of naturalized biological control agents of P. crassipes. Bioassays were conducted to measure the effect of commonly used insecticides (viz. Methomyl and Chlorpyrifos) on the survival and feeding damage of biological control agents of P. crassipes. Survival of individual insects was recorded between 0.5 and 120 hours for Megamelus scutellaris and Neochetina eichhorniae Warner (Coleoptera: Curculionidae) adults for treatments where insecticides were topically applied onto the insects or leaves were dipped into the pesticides. Concentrations below field rates, recommended and above field rates of Methomyl and Chlorpyrifos on either exposure techniques significantly reduced survival and feeding of biocontrol agents. Methomyl was more toxic compared to Chlorpyrifos and it significantly reduced the survival of M. scutellaris and N. eichhorniae. In conclusion, in this study, population abundance of biocontrol agents of P. crassipes at the Lowveld region of Mpumalanga was not influenced by pesticide drift, but, insecticides commonly used in the citrus orchards has the potential to negatively impact naturalized biological control of P. crassipes as demonstrated by the bioassays. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Date Issued: 2023-10-13
A native weevil and an exotic planthopper: investigating potential biological control agents for nymphaea mexicana zuccarini (nymphaeaceae) and its hybrids in South Africa
- Authors: Reid, Megan Kim
- Date: 2023-03-31
- Subjects: Nymphaeaceae South Africa , Water lilies Biological control South Africa , Host specificity , Genetic variation , Bagous longulus
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/422554 , vital:71957 , DOI 10.21504/10962/422554
- Description: Nymphaea mexicana Zuccarini (Nymphaeaceae) is an invasive plant originating from southern USA and Mexico that has become problematic in South Africa, invading several water bodies around the country. Manual removal of this plant is very labour intensive and is not cost efficient or effective for long term control, while the use of herbicides is damaging to the environment and expensive. Consequently, this plant is a desirable candidate for biological control, which takes advantage of enemy release of the target weed and aims to re-establish population suppression induced by host specific natural enemies. Initiating biological control requires that several steps are followed to maximise the success of the programme, and the first few of these, including overseas surveys in the native range of the plant, have already been completed. This thesis aimed to continue biological control research for this species to take further steps at effectively managing the plant. Firstly, pre-release surveys in the invaded range are necessary to: determine what factors (including enemy release) contribute to the invasiveness of the target weed; establish a baseline of information to allow for comparison after biological control agents have been released; and identify any insect herbivores that may already be present in the country. The pre-release surveys conducted in this study revealed useful information about N. mexicana invasions in South Africa and provided evidence that enemy release is applicable to this case. However, these studies determined that a native weevil, Bagous longulus Gyllenhal (Coleoptera: Curculionidae), has expanded its host range to include the exotic N. mexicana at three sites, and may thus have potential for management of the species through augmentative releases. The invasion of N. mexicana in South Africa is further complicated by the presence of several Nymphaea hybrids originating from a complex history of horticultural trade. Although previous research has shown that several hybrid groups are present in South Africa, their parentage is not known. As biological control requires the use of host specific insects adapted to overcome the unique chemical and morphological defences utilised by plant species, hybrids are notoriously difficult to manage because they possess intermediate characters inherited from parent species to which natural enemies may not have adapted. Although biological control of hybrids is challenging, other case studies have demonstrated that it is possible to find suitable agents, but the chances of success are increased if putative parents of the hybrids are known so that they can be surveyed for natural enemies. Further molecular studies including possible parents of the Nymphaea hybrids in South Africa were thus carried out in this thesis to focus future surveying efforts. Two main hybrid groups were identified with genetic similarity to two tested putative Nymphaea parents, and this will allow further investigations of these species to improve the chances of successfully managing these hybrid groups. Some of the tested hybrids showed genetic contributions from multiple groups, some of which were unidentified, so it is necessary to prioritise the most problematic hybrids for biological control. With more insight into the genetic makeup of the Nymphaea hybrids in South Africa, investigations into the host specificity of potential biological control agents can be conducted. The ideal biological control agent should have a broad enough host range to impact and survive on both N. mexicana and its hybrids, but without a host range so broad that it would pose risk to native South African species. Host specificity trials are thus necessary to determine the suitability of potential agents. The identification of B. longulus feeding on N. mexicana during pre-release surveys motivated further investigations to determine the natural distribution, field host range, and host specificity of B. longulus in experimentally controlled conditions. Further surveys were therefore conducted at native Nymphaea sites around South Africa in addition to host specificity trials using the native Nymphaea nouchali Burm. f. (Nymphaeaceae), two populations of N. mexicana, and a cultivated hybrid. Results from the surveys and host specificity tests suggest that B. longulus is widely distributed across South Africa, is specific to Nymphaea with no observed preference between N. mexicana and the native N. nouchali, and does not perform well on Nymphaea hybrids. Hence, B. longulus is promising for use in new association biological control through augmentative releases but is not suitable for management of hybrids. In addition to the potential use of the South African B. longulus, it is necessary to conduct host specificity trials for natural enemies from the native range of N. mexicana that were prioritised in previous studies. Megamelus toddi Beamer (Hemiptera: Delphacidae) is one such species that was imported into quarantined laboratory conditions from Florida, USA. Host specificity trials were conducted using the same test plants as described for the studies on B. longulus, in addition to multigeneration trials to determine how long M. toddi could survive on non-target host plants. As with the B. longulus studies, no statistically significant differences in preference were observed between N. mexicana and N. nouchali, but M. toddi could not complete development on the test hybrid, indicating that this species is also unsuitable for the management of Nymphaea hybrids. Despite suboptimal plant health, M. toddi completed development for three generations on the native N. nouchali. This lack of host specificity deems M. toddi unsafe for release in South Africa but highlights the importance of following predefined steps to develop a biological control programme. The concluding chapter of this thesis discusses the aforementioned findings in a broader context by considering the driving forces of plant invasions in general and specifically for N. mexicana in South Africa. Case studies are also consulted to provide insight into how to proceed with managing Nymphaea hybrids in South Africa, while the factors governing host specificity and host range expansion are also discussed and considered in the context of B. longulus and M. toddi. Finally, after a consideration of the limitations of these studies, recommendations are made to continue the development of biological control for N. mexicana in South Africa. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Date Issued: 2023-03-31
- Authors: Reid, Megan Kim
- Date: 2023-03-31
- Subjects: Nymphaeaceae South Africa , Water lilies Biological control South Africa , Host specificity , Genetic variation , Bagous longulus
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/422554 , vital:71957 , DOI 10.21504/10962/422554
- Description: Nymphaea mexicana Zuccarini (Nymphaeaceae) is an invasive plant originating from southern USA and Mexico that has become problematic in South Africa, invading several water bodies around the country. Manual removal of this plant is very labour intensive and is not cost efficient or effective for long term control, while the use of herbicides is damaging to the environment and expensive. Consequently, this plant is a desirable candidate for biological control, which takes advantage of enemy release of the target weed and aims to re-establish population suppression induced by host specific natural enemies. Initiating biological control requires that several steps are followed to maximise the success of the programme, and the first few of these, including overseas surveys in the native range of the plant, have already been completed. This thesis aimed to continue biological control research for this species to take further steps at effectively managing the plant. Firstly, pre-release surveys in the invaded range are necessary to: determine what factors (including enemy release) contribute to the invasiveness of the target weed; establish a baseline of information to allow for comparison after biological control agents have been released; and identify any insect herbivores that may already be present in the country. The pre-release surveys conducted in this study revealed useful information about N. mexicana invasions in South Africa and provided evidence that enemy release is applicable to this case. However, these studies determined that a native weevil, Bagous longulus Gyllenhal (Coleoptera: Curculionidae), has expanded its host range to include the exotic N. mexicana at three sites, and may thus have potential for management of the species through augmentative releases. The invasion of N. mexicana in South Africa is further complicated by the presence of several Nymphaea hybrids originating from a complex history of horticultural trade. Although previous research has shown that several hybrid groups are present in South Africa, their parentage is not known. As biological control requires the use of host specific insects adapted to overcome the unique chemical and morphological defences utilised by plant species, hybrids are notoriously difficult to manage because they possess intermediate characters inherited from parent species to which natural enemies may not have adapted. Although biological control of hybrids is challenging, other case studies have demonstrated that it is possible to find suitable agents, but the chances of success are increased if putative parents of the hybrids are known so that they can be surveyed for natural enemies. Further molecular studies including possible parents of the Nymphaea hybrids in South Africa were thus carried out in this thesis to focus future surveying efforts. Two main hybrid groups were identified with genetic similarity to two tested putative Nymphaea parents, and this will allow further investigations of these species to improve the chances of successfully managing these hybrid groups. Some of the tested hybrids showed genetic contributions from multiple groups, some of which were unidentified, so it is necessary to prioritise the most problematic hybrids for biological control. With more insight into the genetic makeup of the Nymphaea hybrids in South Africa, investigations into the host specificity of potential biological control agents can be conducted. The ideal biological control agent should have a broad enough host range to impact and survive on both N. mexicana and its hybrids, but without a host range so broad that it would pose risk to native South African species. Host specificity trials are thus necessary to determine the suitability of potential agents. The identification of B. longulus feeding on N. mexicana during pre-release surveys motivated further investigations to determine the natural distribution, field host range, and host specificity of B. longulus in experimentally controlled conditions. Further surveys were therefore conducted at native Nymphaea sites around South Africa in addition to host specificity trials using the native Nymphaea nouchali Burm. f. (Nymphaeaceae), two populations of N. mexicana, and a cultivated hybrid. Results from the surveys and host specificity tests suggest that B. longulus is widely distributed across South Africa, is specific to Nymphaea with no observed preference between N. mexicana and the native N. nouchali, and does not perform well on Nymphaea hybrids. Hence, B. longulus is promising for use in new association biological control through augmentative releases but is not suitable for management of hybrids. In addition to the potential use of the South African B. longulus, it is necessary to conduct host specificity trials for natural enemies from the native range of N. mexicana that were prioritised in previous studies. Megamelus toddi Beamer (Hemiptera: Delphacidae) is one such species that was imported into quarantined laboratory conditions from Florida, USA. Host specificity trials were conducted using the same test plants as described for the studies on B. longulus, in addition to multigeneration trials to determine how long M. toddi could survive on non-target host plants. As with the B. longulus studies, no statistically significant differences in preference were observed between N. mexicana and N. nouchali, but M. toddi could not complete development on the test hybrid, indicating that this species is also unsuitable for the management of Nymphaea hybrids. Despite suboptimal plant health, M. toddi completed development for three generations on the native N. nouchali. This lack of host specificity deems M. toddi unsafe for release in South Africa but highlights the importance of following predefined steps to develop a biological control programme. The concluding chapter of this thesis discusses the aforementioned findings in a broader context by considering the driving forces of plant invasions in general and specifically for N. mexicana in South Africa. Case studies are also consulted to provide insight into how to proceed with managing Nymphaea hybrids in South Africa, while the factors governing host specificity and host range expansion are also discussed and considered in the context of B. longulus and M. toddi. Finally, after a consideration of the limitations of these studies, recommendations are made to continue the development of biological control for N. mexicana in South Africa. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2023
- Full Text:
- Date Issued: 2023-03-31
Development and optimisation of a qPCR assay for the enumeration of Cryptophlebia leucotreta granulovirus (CrleGV) used for commercial applications
- Authors: Mela, Thuthula
- Date: 2022-10-14
- Subjects: Cryptophlebia leucotreta granulovirus , Cryptophlebia leucotreta , Late expression factor 8 (LEF-8) , Late expression factor 9 , Dark field microscopy , Genomic DNA , Polymerase chain reaction , Plasmids
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362949 , vital:65377
- Description: The citrus industry contributes significantly to the South African agricultural sector. Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) is highly important to the South African citrus industry as it is classified as a phytosanitary pest by most international markets. Thaumatotibia leucotreta has caused an estimated annual loss of up to R100 million to the industry. In order to control T. leucotreta in South Africa, an integrated pest management (IPM) programme has been used. One of the components of this programme is Cryptophlebia leucotreta granulovirus (CrleGV), which has been formulated to a registered biopesticide namely Cryptogran and has been successfully applied in the field for over 15 years. To use CrleGV as biopesticides, quantification of the viral particles is required to perform bioassays for field trials and formulation, among other applications. Darkfield microscopy is a traditional method used for the quantification of CrleGV; however, the method is characterised as being subjective, tedious, labour intensive, and time-consuming. This study aims to develop and optimise a qPCR technique to accurately quantify CrleGV-SA OBs using plasmid DNA for downstream applications. Firstly, lef-8, lef-9, and granulin conserved genes from CrleGV-SA and CrleGV-CV3 genome sequences were analysed by performing multiple alignments to evaluate the degree of identity between these genes. This was done to design two sets of oligonucleotides (internal and external) from regions with the highest identity. Subsequently, in silico testing was done to evaluate the designed oligonucleotides to determine whether they specifically bind to the selected target regions. Secondly, three sets of DNA plasmids (pJET1.2-Gran, pJET1.2-lef-9, and pJET1.2-lef-8) were constructed, each containing a target region for either granulin, lef-9, and lef-8 genes for use as standards in a downstream qPCR assay. This was achieved by first extracting gDNA from CrleGV-SA OBs and using the gDNA as a template to PCR amplify the target regions of the selected gene regions with the designed oligonucleotides. Subsequently, the PCR amplified regions were then directly ligated into the pJET1.2/blunt vector, and the plasmids were confirmed by colony PCR, restriction enzyme digestion, and Sanger sequencing. Thirdly, two different methods of CrleGV-SA gDNA extraction were compared to determine which method has the best yields in terms of concentration. The extraction methods compared were the Quick-DNA Miniprep Plus kit according to manufacturer’s instructions (Method 1a), pre-treatment with Na2CO3 prior to using the Quick-DNA Miniprep Plus kit (Method 1b), pre- treatment with Na2CO3, and neutralisation with Tris-HCl prior to gDNA extraction using the Quick-DNA Miniprep Plus kit (Method 1c) and the CTAB method (Method 2). The gDNA concentration and purity for all samples were determined using a Nanodrop spectrophotometer. Method 1c (Na2CO3 and Tris-HCl pre-treated plus Quick-DNA Miniprep Plus kit) was the most efficient at extracting genomic DNA compared with the other methods, resulting in the highest DNA concentration in short processing time. Fourthly, plasmid standards were evaluated for use in the qPCR assay. This was done as it was important to consider the efficacy of the oligonucleotides; including the ability of the oligonucleotides to anneal to the appropriate segment of DNA without extensive formation of oligonucleotides dimers, non-specific annealing, or formation of secondary structure. In addition, it was done to ensure that highly accurate standard curves were generated. The standard curves were to be utilised in the downstream qPCR assay to determine the quantity of test samples by interpolation, reading from the values within the standard curve. Lastly, darkfield microscopy and qPCR methods of enumeration were compared to verify their accuracy and determine the most consistent and comparable method. This was achieved by quantifying the purified, crude-purified, and viral formulated CrleGV-SA suspensions using these methods. Subsequently, a statistical analysis was conducted to compare the results produced by the two enumeration methods. The obtained results showed that the granulin, lef- 8 and lef-9 qPCR values did not significantly differ from the darkfield microscopy results. The findings of this study revealed that the two assays, lef-8 qPCR and lef-9 qPCR, were more robust, sensitive, and efficient for the quantification of CrleGV-SA. Thus, this study has successfully developed a qPCR assay that is comparable with the traditional darkfield microscopy counting technique. This is the first study to use the qPCR technique to enumerate CrleGV-SA using plasmid standards. The developed qPCR assay is reliable, rapid, and cost- effective and has a great potential to be used as an alternative method to darkfield microscopy in the laboratory and commercial settings. , Thesis (MSc) -- Faculty of Science, Biochemistry and Microbiology, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Mela, Thuthula
- Date: 2022-10-14
- Subjects: Cryptophlebia leucotreta granulovirus , Cryptophlebia leucotreta , Late expression factor 8 (LEF-8) , Late expression factor 9 , Dark field microscopy , Genomic DNA , Polymerase chain reaction , Plasmids
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362949 , vital:65377
- Description: The citrus industry contributes significantly to the South African agricultural sector. Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) is highly important to the South African citrus industry as it is classified as a phytosanitary pest by most international markets. Thaumatotibia leucotreta has caused an estimated annual loss of up to R100 million to the industry. In order to control T. leucotreta in South Africa, an integrated pest management (IPM) programme has been used. One of the components of this programme is Cryptophlebia leucotreta granulovirus (CrleGV), which has been formulated to a registered biopesticide namely Cryptogran and has been successfully applied in the field for over 15 years. To use CrleGV as biopesticides, quantification of the viral particles is required to perform bioassays for field trials and formulation, among other applications. Darkfield microscopy is a traditional method used for the quantification of CrleGV; however, the method is characterised as being subjective, tedious, labour intensive, and time-consuming. This study aims to develop and optimise a qPCR technique to accurately quantify CrleGV-SA OBs using plasmid DNA for downstream applications. Firstly, lef-8, lef-9, and granulin conserved genes from CrleGV-SA and CrleGV-CV3 genome sequences were analysed by performing multiple alignments to evaluate the degree of identity between these genes. This was done to design two sets of oligonucleotides (internal and external) from regions with the highest identity. Subsequently, in silico testing was done to evaluate the designed oligonucleotides to determine whether they specifically bind to the selected target regions. Secondly, three sets of DNA plasmids (pJET1.2-Gran, pJET1.2-lef-9, and pJET1.2-lef-8) were constructed, each containing a target region for either granulin, lef-9, and lef-8 genes for use as standards in a downstream qPCR assay. This was achieved by first extracting gDNA from CrleGV-SA OBs and using the gDNA as a template to PCR amplify the target regions of the selected gene regions with the designed oligonucleotides. Subsequently, the PCR amplified regions were then directly ligated into the pJET1.2/blunt vector, and the plasmids were confirmed by colony PCR, restriction enzyme digestion, and Sanger sequencing. Thirdly, two different methods of CrleGV-SA gDNA extraction were compared to determine which method has the best yields in terms of concentration. The extraction methods compared were the Quick-DNA Miniprep Plus kit according to manufacturer’s instructions (Method 1a), pre-treatment with Na2CO3 prior to using the Quick-DNA Miniprep Plus kit (Method 1b), pre- treatment with Na2CO3, and neutralisation with Tris-HCl prior to gDNA extraction using the Quick-DNA Miniprep Plus kit (Method 1c) and the CTAB method (Method 2). The gDNA concentration and purity for all samples were determined using a Nanodrop spectrophotometer. Method 1c (Na2CO3 and Tris-HCl pre-treated plus Quick-DNA Miniprep Plus kit) was the most efficient at extracting genomic DNA compared with the other methods, resulting in the highest DNA concentration in short processing time. Fourthly, plasmid standards were evaluated for use in the qPCR assay. This was done as it was important to consider the efficacy of the oligonucleotides; including the ability of the oligonucleotides to anneal to the appropriate segment of DNA without extensive formation of oligonucleotides dimers, non-specific annealing, or formation of secondary structure. In addition, it was done to ensure that highly accurate standard curves were generated. The standard curves were to be utilised in the downstream qPCR assay to determine the quantity of test samples by interpolation, reading from the values within the standard curve. Lastly, darkfield microscopy and qPCR methods of enumeration were compared to verify their accuracy and determine the most consistent and comparable method. This was achieved by quantifying the purified, crude-purified, and viral formulated CrleGV-SA suspensions using these methods. Subsequently, a statistical analysis was conducted to compare the results produced by the two enumeration methods. The obtained results showed that the granulin, lef- 8 and lef-9 qPCR values did not significantly differ from the darkfield microscopy results. The findings of this study revealed that the two assays, lef-8 qPCR and lef-9 qPCR, were more robust, sensitive, and efficient for the quantification of CrleGV-SA. Thus, this study has successfully developed a qPCR assay that is comparable with the traditional darkfield microscopy counting technique. This is the first study to use the qPCR technique to enumerate CrleGV-SA using plasmid standards. The developed qPCR assay is reliable, rapid, and cost- effective and has a great potential to be used as an alternative method to darkfield microscopy in the laboratory and commercial settings. , Thesis (MSc) -- Faculty of Science, Biochemistry and Microbiology, 2022
- Full Text:
- Date Issued: 2022-10-14
Effect of Helicosporidium sp. (Chlorophyta; Trebouxiophyceae) infection on Cyrtobagous salviniae Calder and Sands (Coleoptera: Curculionidae), a biological control agent for the invasive Salvinia molesta D.S. Mitchell (Salviniaceae) in South
- Authors: Mphephu, Tshililo Emmanuel
- Date: 2022-10-14
- Subjects: Salvinia molesta South Africa , Weeds Biological control , Cyrtobagous salviniae , Ketoconazole
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/365815 , vital:65792 , DOI https://doi.org/10.21504/10962/365815
- Description: The effectiveness of established biological control agents depends on biotic and abiotic interactions in the introduced range. The weevil, Cyrtobagous salviniae Calder and Sands (Coleoptera: Curculionidae), was released as a biological control against Salvinia molesta D.S. Mitchell (Salviniaceae) in South Africa in 1985. This agent has been highly successful against S. molesta and has significantly reduced the weed’s populations around the country. However, in 2007, the parasitic alga, Helicosporidium sp. (an undescribed species), was detected in field-collected C. salviniae adults in South Africa. The distribution and impacts of this disease on the weevil and its efficacy as a control agent were not known. In this thesis, the prevalence, infection load, and impact of Helicosporidium sp. on C. salviniae was determined. In 2019, adult weevils were collected from 10 sites across the Eastern Cape, KwaZulu-Natal, Limpopo, and Western Cape provinces and screened to determine the occurrence, infection load, and geographic distribution of Helicosporidium sp. Transmission mechanisms of this disease in C. salviniae were then evaluated. The possible impact of Helicosporidium sp. was assessed by comparing the feeding rates and the reproductive output of the diseased and healthy adults of C. salviniae. An attempt was then made to eliminate the disease in C. salviniae through the application of the antibiotic, ketoconazole. Further, the role of temperature on infection load in C. salviniae was also assessed. Finally, recommendations for the long-term biological control programme against S. molesta in South Africa were made. The disease covers the entire distribution range of C. salviniae in South Africa, with the disease occurrence rate ranging from 92.15% to 100% insects infected per site. Helicosporidium sp. was found to transmit vertically within the populations of C. salviniae. Infection by the Helicosporidium sp. disease reduced the reproductive output of C. salviniae as well its impact on biomass reduction of S. molesta when a diseased culture was compared to a healthy culture from the USA. 98.44 to 98.55% of Helicosporidium sp. loads were reduced through multiple applications of ketoconazole concentrations under in vitro trials. In vivo treatments resulted in 70% control of Helicosporidium sp. in the adults of C. salviniae that were fed ketoconazole three times over a 21 day period. Adult C. salviniae feeding and survival performances were similar when fed fronds of S. molesta inoculated with ketoconazole and water. The lowest and highest disease loads of Helicosporidium sp. were recorded when the weevils were reared at 30°C and 14°C, respectively. As expected, the highest impact and reproductive output of C. salviniae were at 30°C. The evaluations discussed in this thesis highlight the role of diseases in biological control agents, and gaps in both the pre-release and post-release monitoring that should integrate screening of diseases in these studies. Although the combined application of the antibiotic and temperature will reduce Helicosporidium sp. loads and impact, this technology is most likely only applicable where the weevils are reared in small numbers in a rearing facility and not really applicable to the field situation. It is important to release healthy agents that will cause efficient control of the target weed plant species, therefore, when introducing new biological control agents, the health status of such agents needs to be understood. Therefore, long-term field monitoring and assessment of the impact of C. salviniae on S. molesta should be conducted to track all the changes that may result due to the presence of Helicosporidium sp. This long-term monitoring and assessment will give a more informative role of Helicosporidium sp. in field populations of C. salviniae. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Mphephu, Tshililo Emmanuel
- Date: 2022-10-14
- Subjects: Salvinia molesta South Africa , Weeds Biological control , Cyrtobagous salviniae , Ketoconazole
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/365815 , vital:65792 , DOI https://doi.org/10.21504/10962/365815
- Description: The effectiveness of established biological control agents depends on biotic and abiotic interactions in the introduced range. The weevil, Cyrtobagous salviniae Calder and Sands (Coleoptera: Curculionidae), was released as a biological control against Salvinia molesta D.S. Mitchell (Salviniaceae) in South Africa in 1985. This agent has been highly successful against S. molesta and has significantly reduced the weed’s populations around the country. However, in 2007, the parasitic alga, Helicosporidium sp. (an undescribed species), was detected in field-collected C. salviniae adults in South Africa. The distribution and impacts of this disease on the weevil and its efficacy as a control agent were not known. In this thesis, the prevalence, infection load, and impact of Helicosporidium sp. on C. salviniae was determined. In 2019, adult weevils were collected from 10 sites across the Eastern Cape, KwaZulu-Natal, Limpopo, and Western Cape provinces and screened to determine the occurrence, infection load, and geographic distribution of Helicosporidium sp. Transmission mechanisms of this disease in C. salviniae were then evaluated. The possible impact of Helicosporidium sp. was assessed by comparing the feeding rates and the reproductive output of the diseased and healthy adults of C. salviniae. An attempt was then made to eliminate the disease in C. salviniae through the application of the antibiotic, ketoconazole. Further, the role of temperature on infection load in C. salviniae was also assessed. Finally, recommendations for the long-term biological control programme against S. molesta in South Africa were made. The disease covers the entire distribution range of C. salviniae in South Africa, with the disease occurrence rate ranging from 92.15% to 100% insects infected per site. Helicosporidium sp. was found to transmit vertically within the populations of C. salviniae. Infection by the Helicosporidium sp. disease reduced the reproductive output of C. salviniae as well its impact on biomass reduction of S. molesta when a diseased culture was compared to a healthy culture from the USA. 98.44 to 98.55% of Helicosporidium sp. loads were reduced through multiple applications of ketoconazole concentrations under in vitro trials. In vivo treatments resulted in 70% control of Helicosporidium sp. in the adults of C. salviniae that were fed ketoconazole three times over a 21 day period. Adult C. salviniae feeding and survival performances were similar when fed fronds of S. molesta inoculated with ketoconazole and water. The lowest and highest disease loads of Helicosporidium sp. were recorded when the weevils were reared at 30°C and 14°C, respectively. As expected, the highest impact and reproductive output of C. salviniae were at 30°C. The evaluations discussed in this thesis highlight the role of diseases in biological control agents, and gaps in both the pre-release and post-release monitoring that should integrate screening of diseases in these studies. Although the combined application of the antibiotic and temperature will reduce Helicosporidium sp. loads and impact, this technology is most likely only applicable where the weevils are reared in small numbers in a rearing facility and not really applicable to the field situation. It is important to release healthy agents that will cause efficient control of the target weed plant species, therefore, when introducing new biological control agents, the health status of such agents needs to be understood. Therefore, long-term field monitoring and assessment of the impact of C. salviniae on S. molesta should be conducted to track all the changes that may result due to the presence of Helicosporidium sp. This long-term monitoring and assessment will give a more informative role of Helicosporidium sp. in field populations of C. salviniae. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-10-14
Genetic analysis and field application of a UV-tolerant strain of CrleGV for improved control of Thaumatotibia leucotreta
- Authors: Bennett, Tahnee Tashia
- Date: 2022-10-14
- Subjects: Cryptophlebia leucotreta Biological control , Pests Integrated control , Biological pest control agents , Ultraviolet radiation , Oligonucleotides
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362741 , vital:65358
- Description: Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), also known as false codling moth (FCM), is indigenous to sub-Saharan Africa. Thaumatotibia leucotreta has been controlled through an integrated pest management (IPM) programme, which includes chemical control, sterile insect technique (SIT), cultural and biological control. As part of the biological control, a key component is the use of Cryptophlebia leucotreta granulovirus (CrleGV-SA). Currently, CryptogranTM, a commercial formulation of CrleGV, is the preferred product to use in South Africa for the control of T. leucotreta. The registration of the biopesticide Cryptogran (River bioscience, South Africa) was established after conducting extensive field trials with CrleGV-SA. One of the major factors affecting the baculovirus efficacy in the field is UV irradiation. A UV-tolerant Cryptophlebia leucotreta granulovirus (CrleGV-SA-C5) isolate was isolated after consecutive cycles of UV exposure. This UV-tolerant isolate is genetically distinct from the CrleGV-SA isolate. The CrleGV-SA-C5 isolate has the potential as a biological control agent. The control of T. leucotreta in South Africa could be improved by the development of novel isolates into new biopesticide formulations. To date, there has not been any field trials conducted on the CrleGV-SA-C5 isolate. Therefore, it is important to determine the biological and genetic stability of this isolate and to conduct field trials with CrleGV-SA- C5 to test the efficacy of the isolate before possible production into a biopesticide. A de novo assembly was conducted to reassemble the genome of CrleGV-SA-C5 which was followed by a sequence comparison with the CrleGV-SA genome. The identification of SNPs, led to the design of oligonucleotides flanking the regions where the SNPs were detected. Polymerase chain reaction amplification of the target regions was conducted using the oligonucleotides. After sequence comparison, seven SNPs were detected and PCR amplification was successful using the three oligonucleotides, Pif-2, HypoP and Lef-8/HP. To differentiate between CrleGV-SA-C5 and CrleGV-SA genomes and confirm the presence of the SNPs, two methods of screening were conducted. The first was the construction of six plasmids, the plasmids contained the targeted pif-2, HypoP, and the Lef-8/HP insert regions from both the CrleGV-SA-C5 and CrleGV-SA genome region where the SNPs were identified, followed by sequencing. The Five recombinant plasmids, pC5_Pif-2, pSA_Pif-2, pC5_HypoP, pSA_HypoP, and pC5_Lef-8/HP were successfully sequenced. No amplicon was obtained for one of the plasmids used as template (pSA_Lef-8/HP) and therefore the PCR product used for cloning was sequenced instead. Sequence alignment confirmed the presence of four of the five targeted SNPs in the genome of the CrleGV-SA-C5 isolate. However, of these only one SNP (UV_7) rendered a suitable marker for the differentiation between the CrleGV-SA-C5 and CrleGV-SA isolates as the SNPs, UV_2, UV_3 and UV_5, were also present in the CrleGV- SA sequences. The second screening method was a quantitative polymerase chain reaction (qPCR) melt curve analysis to differentiate between the CrleGV-SA-C5 and CrleGV-SA isolates. qPCR melt curve analysis was done using the CrleGV-SA-C5 and CrleGV-SA HypoP PCR products. This technique was unable to differentiate between the CrleGV-SA-C5 and CrleGV-SA isolates. However, this may be as a result of sequence data confirming that SNP UV_5 originally identified in the CrleGV-SA-C5 HypoP region was identical to the SNP at the same position in the CrleGV-SA HypoP region. Following the differentiation of the CrleGV-SA-C5 and CrleGV-SA isolates through two screening methods, the genetic integrity of the CrleGV-SA-C5 isolate after two virus bulk-ups was determined by PCR amplification of the target regions in the bulk-up virus followed by sequencing. Prior to virus bulk-up, surface dose bioassays were conducted on 4th instar larvae and LC50 and LC90 values of 4.01 x 106 OBs/ml and 8.75 x 109 OBs/ml respectively were obtained. The CrleGV-SA-C5 isolate was then bulked up in fourth instar T. leucotreta larvae using the LC90 value that was determined. Sequencing of the target regions from the CrleGV- SA-C5_BU2 (bulk-up 2) was conducted. Sequencing results confirmed the presence of the target SNPs in the CrleGV-SA-C5_BU2 genome. The UV-tolerance of the CrleGV-SA-C5 isolate in comparison to the CrleGV-SA isolate was evaluated by detached fruit bioassays under natural UV irradiation. Two detached fruit bioassays were set-up, a UV exposure and a non-UV exposure bioassay set-up. Three treatments were used for each bioassay set-up which were the viruses CrleGV-SA-C5 and CrleGV-SA and a ddH2O control. Statistical analysis indicated that there was no significant difference between the virus treatments in both the UV exposed detached fruit bioassay and the non-UV exposed detached fruit bioassay. This study is the second study to report on the de novo assembly of the CrleGV-SA-C5 and sequence comparison with the CrleGV-SA genome, and the first to report on the UV-tolerance of the CrleGV-SA-C5 isolate by detached fruit bioassays. Future work could involve further evaluation of intraspecific genetic variability in the CrleGV-SA-C5 isolate and to identify any additional SNPs present within the genome that can be used as suitable markers for differentiation between the CrleGV-SA-C5 and CrleGV-SA isolates. It was recognised that it is required to conduct further detached fruit bioassays and field trials, but with improved protocols, for the efficacy and UV-tolerance of the CrleGV-SA-C5 isolate to be conclusively determined. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Bennett, Tahnee Tashia
- Date: 2022-10-14
- Subjects: Cryptophlebia leucotreta Biological control , Pests Integrated control , Biological pest control agents , Ultraviolet radiation , Oligonucleotides
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362741 , vital:65358
- Description: Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), also known as false codling moth (FCM), is indigenous to sub-Saharan Africa. Thaumatotibia leucotreta has been controlled through an integrated pest management (IPM) programme, which includes chemical control, sterile insect technique (SIT), cultural and biological control. As part of the biological control, a key component is the use of Cryptophlebia leucotreta granulovirus (CrleGV-SA). Currently, CryptogranTM, a commercial formulation of CrleGV, is the preferred product to use in South Africa for the control of T. leucotreta. The registration of the biopesticide Cryptogran (River bioscience, South Africa) was established after conducting extensive field trials with CrleGV-SA. One of the major factors affecting the baculovirus efficacy in the field is UV irradiation. A UV-tolerant Cryptophlebia leucotreta granulovirus (CrleGV-SA-C5) isolate was isolated after consecutive cycles of UV exposure. This UV-tolerant isolate is genetically distinct from the CrleGV-SA isolate. The CrleGV-SA-C5 isolate has the potential as a biological control agent. The control of T. leucotreta in South Africa could be improved by the development of novel isolates into new biopesticide formulations. To date, there has not been any field trials conducted on the CrleGV-SA-C5 isolate. Therefore, it is important to determine the biological and genetic stability of this isolate and to conduct field trials with CrleGV-SA- C5 to test the efficacy of the isolate before possible production into a biopesticide. A de novo assembly was conducted to reassemble the genome of CrleGV-SA-C5 which was followed by a sequence comparison with the CrleGV-SA genome. The identification of SNPs, led to the design of oligonucleotides flanking the regions where the SNPs were detected. Polymerase chain reaction amplification of the target regions was conducted using the oligonucleotides. After sequence comparison, seven SNPs were detected and PCR amplification was successful using the three oligonucleotides, Pif-2, HypoP and Lef-8/HP. To differentiate between CrleGV-SA-C5 and CrleGV-SA genomes and confirm the presence of the SNPs, two methods of screening were conducted. The first was the construction of six plasmids, the plasmids contained the targeted pif-2, HypoP, and the Lef-8/HP insert regions from both the CrleGV-SA-C5 and CrleGV-SA genome region where the SNPs were identified, followed by sequencing. The Five recombinant plasmids, pC5_Pif-2, pSA_Pif-2, pC5_HypoP, pSA_HypoP, and pC5_Lef-8/HP were successfully sequenced. No amplicon was obtained for one of the plasmids used as template (pSA_Lef-8/HP) and therefore the PCR product used for cloning was sequenced instead. Sequence alignment confirmed the presence of four of the five targeted SNPs in the genome of the CrleGV-SA-C5 isolate. However, of these only one SNP (UV_7) rendered a suitable marker for the differentiation between the CrleGV-SA-C5 and CrleGV-SA isolates as the SNPs, UV_2, UV_3 and UV_5, were also present in the CrleGV- SA sequences. The second screening method was a quantitative polymerase chain reaction (qPCR) melt curve analysis to differentiate between the CrleGV-SA-C5 and CrleGV-SA isolates. qPCR melt curve analysis was done using the CrleGV-SA-C5 and CrleGV-SA HypoP PCR products. This technique was unable to differentiate between the CrleGV-SA-C5 and CrleGV-SA isolates. However, this may be as a result of sequence data confirming that SNP UV_5 originally identified in the CrleGV-SA-C5 HypoP region was identical to the SNP at the same position in the CrleGV-SA HypoP region. Following the differentiation of the CrleGV-SA-C5 and CrleGV-SA isolates through two screening methods, the genetic integrity of the CrleGV-SA-C5 isolate after two virus bulk-ups was determined by PCR amplification of the target regions in the bulk-up virus followed by sequencing. Prior to virus bulk-up, surface dose bioassays were conducted on 4th instar larvae and LC50 and LC90 values of 4.01 x 106 OBs/ml and 8.75 x 109 OBs/ml respectively were obtained. The CrleGV-SA-C5 isolate was then bulked up in fourth instar T. leucotreta larvae using the LC90 value that was determined. Sequencing of the target regions from the CrleGV- SA-C5_BU2 (bulk-up 2) was conducted. Sequencing results confirmed the presence of the target SNPs in the CrleGV-SA-C5_BU2 genome. The UV-tolerance of the CrleGV-SA-C5 isolate in comparison to the CrleGV-SA isolate was evaluated by detached fruit bioassays under natural UV irradiation. Two detached fruit bioassays were set-up, a UV exposure and a non-UV exposure bioassay set-up. Three treatments were used for each bioassay set-up which were the viruses CrleGV-SA-C5 and CrleGV-SA and a ddH2O control. Statistical analysis indicated that there was no significant difference between the virus treatments in both the UV exposed detached fruit bioassay and the non-UV exposed detached fruit bioassay. This study is the second study to report on the de novo assembly of the CrleGV-SA-C5 and sequence comparison with the CrleGV-SA genome, and the first to report on the UV-tolerance of the CrleGV-SA-C5 isolate by detached fruit bioassays. Future work could involve further evaluation of intraspecific genetic variability in the CrleGV-SA-C5 isolate and to identify any additional SNPs present within the genome that can be used as suitable markers for differentiation between the CrleGV-SA-C5 and CrleGV-SA isolates. It was recognised that it is required to conduct further detached fruit bioassays and field trials, but with improved protocols, for the efficacy and UV-tolerance of the CrleGV-SA-C5 isolate to be conclusively determined. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-10-14
The effect of orchard sanitation and predatory ants on the eclosion of the internal feeding pests and Oriental fruit fly, in South Africa
- Authors: Makitla, Tshepang
- Date: 2022-10-14
- Subjects: Orchards South Africa , Phytosanitation , Citrus Diseases and pests Biological control , Ants , Insects as biological pest control agents
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362927 , vital:65375
- Description: There are several pests of phytosanitary concern in the citrus industry in South Africa. Orchard sanitation can play an important role in suppressing the populations of these pests, however there are little data on the efficacy of sanitation techniques. Therefore, the current study investigated the effect of fruit disposal techniques and burying depths on the eclosion of the most important pests of citrus in South Africa, false codling moth Thaumatotibia leucotreta Meyrick (Lepidoptera: Tortricidae), Mediterranean fruit fly or Medfly Ceratitis capitata Wiedemann (Diptera: Tephritidae), Natal fruit fly Ceratitis rosa Karsh (Diptera: Tephritidae), and Oriental fruit fly Bactrocera dorsalis Hendel (Diptera: Tephritidae). Abscised C. sinensis fruits were inoculated with larvae of T. leucotreta, and eggs of C. capitata, C. rosa, and B. dorsalis, before being disposed as pulped, or whole, and buried at different depths (0 cm, 5 cm, 25 cm, and 50 cm). Abundance and richness of predatory ants were monitored using pitfall traps to ascertain their effect on the mortality of the immature stages of these pests. Ceratitis capitata and C. rosa failed to eclose from the inoculated fruits disposed at different depths, however, T. leucotreta and B. dorsalis adults did eclosed. Significantly fewer B. dorsalis eclosed from fruits that were pulped in comparison to eclosion where the fruit were left whole (F (3, 16) = 11.45, P < 0.01). Furthermore, depth of burial had a significant effect on the number of eclosed adults of Drosophila sp (F (3, 112) = 3.43, P < 0.01). Burying fruits at 50 cm suppressed the eclosion of all the internal feeding pests tested. Twenty-seven thousand seventy-three individual ants (Hymenoptera: Formicidae) were sampled from the same plots as used above, with at least 47% and 53% sampled from plots where pulped and whole C. sinensis fruits were disposed of, respectively. The ants were identified to morphospecies which included Pheidole1, Pheidole2, Formicinae1, Formicinae2, Formicinae3, and Myrmicinae1. The disposal of the inoculated C. sinensis fruits either as pulped or whole and burying at different depths significantly suppressed and/or delayed the eclosion of either of the tested internal feeding pests of citrus. Although, predacious ants were sampled from the same treatment plots they did not affect the survival or eclosion of the tested pests, and this could be attributed to the application of the slow toxic ant bait. Therefore, based on the observed results B. dorsalis adults showed the ability to eclose from 50 cm depth where fruit was either disposed as pulped or whole, thus, citrus farmers are advised to use hammer mill that will finely crush sanitised fruit, and/or bury fruit beyond 50 cm depth to prevent the adult od this pest from eclosing. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-10-14
- Authors: Makitla, Tshepang
- Date: 2022-10-14
- Subjects: Orchards South Africa , Phytosanitation , Citrus Diseases and pests Biological control , Ants , Insects as biological pest control agents
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/362927 , vital:65375
- Description: There are several pests of phytosanitary concern in the citrus industry in South Africa. Orchard sanitation can play an important role in suppressing the populations of these pests, however there are little data on the efficacy of sanitation techniques. Therefore, the current study investigated the effect of fruit disposal techniques and burying depths on the eclosion of the most important pests of citrus in South Africa, false codling moth Thaumatotibia leucotreta Meyrick (Lepidoptera: Tortricidae), Mediterranean fruit fly or Medfly Ceratitis capitata Wiedemann (Diptera: Tephritidae), Natal fruit fly Ceratitis rosa Karsh (Diptera: Tephritidae), and Oriental fruit fly Bactrocera dorsalis Hendel (Diptera: Tephritidae). Abscised C. sinensis fruits were inoculated with larvae of T. leucotreta, and eggs of C. capitata, C. rosa, and B. dorsalis, before being disposed as pulped, or whole, and buried at different depths (0 cm, 5 cm, 25 cm, and 50 cm). Abundance and richness of predatory ants were monitored using pitfall traps to ascertain their effect on the mortality of the immature stages of these pests. Ceratitis capitata and C. rosa failed to eclose from the inoculated fruits disposed at different depths, however, T. leucotreta and B. dorsalis adults did eclosed. Significantly fewer B. dorsalis eclosed from fruits that were pulped in comparison to eclosion where the fruit were left whole (F (3, 16) = 11.45, P < 0.01). Furthermore, depth of burial had a significant effect on the number of eclosed adults of Drosophila sp (F (3, 112) = 3.43, P < 0.01). Burying fruits at 50 cm suppressed the eclosion of all the internal feeding pests tested. Twenty-seven thousand seventy-three individual ants (Hymenoptera: Formicidae) were sampled from the same plots as used above, with at least 47% and 53% sampled from plots where pulped and whole C. sinensis fruits were disposed of, respectively. The ants were identified to morphospecies which included Pheidole1, Pheidole2, Formicinae1, Formicinae2, Formicinae3, and Myrmicinae1. The disposal of the inoculated C. sinensis fruits either as pulped or whole and burying at different depths significantly suppressed and/or delayed the eclosion of either of the tested internal feeding pests of citrus. Although, predacious ants were sampled from the same treatment plots they did not affect the survival or eclosion of the tested pests, and this could be attributed to the application of the slow toxic ant bait. Therefore, based on the observed results B. dorsalis adults showed the ability to eclose from 50 cm depth where fruit was either disposed as pulped or whole, thus, citrus farmers are advised to use hammer mill that will finely crush sanitised fruit, and/or bury fruit beyond 50 cm depth to prevent the adult od this pest from eclosing. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-10-14
The effects of elevated CO2 on feeding guild responses of biological control agents of Pontederia crassipes Mart. (Pontederiaceae)
- Authors: Paper, Matthew Keenan
- Date: 2022-04-06
- Subjects: Carbon dioxide , Pontederia crassipes , Biological pest control agents , Invasive plants Biological control , Pontederiaceae Climatic factors
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/455338 , vital:75422
- Description: Elevated CO2 (eCO2) and rising global temperatures have the potential to alter plant-insect interactions with important implications for plant community structure and function. Previous studies on plant-insect interactions have shown that eCO2 will affect insect feeding guilds differently, impacting negatively, positively or having very little effect. The implications of this on the global invasive plant biological control programme is largely unknown. This study investigates the response of one of the world’s most invasive aquatic plants, Pontederia ( = Eichhornia) crassipes Mart. (Pontederiaceae), to predicted eCO2 conditions of 800 ppm and how this may affect the feeding response of two biological control agents representing different feeding guilds; the leaf chewing Cornops aquaticum Brüner (Orthoptera: Acrididae) and the phloem-feeding Megamelus scutellaris Berg (Hemiptera: Delphacidae). A factorial eCO2 x feeding impact study was conducted at the Rhodes University Elevated CO2 Facility in the Eastern Cape Province of South Africa over 13 weeks in the growing season of 2019. The effect of insect herbivory by C. aquaticum and M. scutellaris at two atmospheric CO2 concentrations, representing current and future predicted concentrations (400 ppm and 800 ppm) on P. crassipes was examined through both biomass and ecophysiological measures. Assimilation rates, C:N ratio, total dry weight and relative growth rate of P. crassipes were unaffected by eCO2 conditions, and plants experienced no CO2 fertilization in eutrophic water conditions representative of South African waterways. Effects of eCO2 on insect herbivory varied depending on the feeding guild. Pontederia crassipes showed compensatory growth responses when exposed to C. aquaticum herbivory regardless of CO2 treatment, but chewing herbivory damage remained constant, and the agent maintained efficacy. Pontederia crassipes showed down-regulation of photosynthesis when exposed to M. scutellaris due to eCO2-related feeding responses by M. scutellaris increasing substantially through a significant (30%) increase in adult population density under eCO2 conditions. These results indicate that the plant-insect interactions that underpin biological control programmes for P. crassipes should remain successful under future CO2 conditions. Phloem-feeding insect damage (M. scutellaris) was significantly greater than chewing damage in this study, suggesting that invasive plant biological control programmes will need to shift focus away from the charismatic chewing insect herbivores and onto the often-neglected phloem-feeding biological control agents due to their overwhelmingly positive response to eCO2. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-04-06
- Authors: Paper, Matthew Keenan
- Date: 2022-04-06
- Subjects: Carbon dioxide , Pontederia crassipes , Biological pest control agents , Invasive plants Biological control , Pontederiaceae Climatic factors
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/455338 , vital:75422
- Description: Elevated CO2 (eCO2) and rising global temperatures have the potential to alter plant-insect interactions with important implications for plant community structure and function. Previous studies on plant-insect interactions have shown that eCO2 will affect insect feeding guilds differently, impacting negatively, positively or having very little effect. The implications of this on the global invasive plant biological control programme is largely unknown. This study investigates the response of one of the world’s most invasive aquatic plants, Pontederia ( = Eichhornia) crassipes Mart. (Pontederiaceae), to predicted eCO2 conditions of 800 ppm and how this may affect the feeding response of two biological control agents representing different feeding guilds; the leaf chewing Cornops aquaticum Brüner (Orthoptera: Acrididae) and the phloem-feeding Megamelus scutellaris Berg (Hemiptera: Delphacidae). A factorial eCO2 x feeding impact study was conducted at the Rhodes University Elevated CO2 Facility in the Eastern Cape Province of South Africa over 13 weeks in the growing season of 2019. The effect of insect herbivory by C. aquaticum and M. scutellaris at two atmospheric CO2 concentrations, representing current and future predicted concentrations (400 ppm and 800 ppm) on P. crassipes was examined through both biomass and ecophysiological measures. Assimilation rates, C:N ratio, total dry weight and relative growth rate of P. crassipes were unaffected by eCO2 conditions, and plants experienced no CO2 fertilization in eutrophic water conditions representative of South African waterways. Effects of eCO2 on insect herbivory varied depending on the feeding guild. Pontederia crassipes showed compensatory growth responses when exposed to C. aquaticum herbivory regardless of CO2 treatment, but chewing herbivory damage remained constant, and the agent maintained efficacy. Pontederia crassipes showed down-regulation of photosynthesis when exposed to M. scutellaris due to eCO2-related feeding responses by M. scutellaris increasing substantially through a significant (30%) increase in adult population density under eCO2 conditions. These results indicate that the plant-insect interactions that underpin biological control programmes for P. crassipes should remain successful under future CO2 conditions. Phloem-feeding insect damage (M. scutellaris) was significantly greater than chewing damage in this study, suggesting that invasive plant biological control programmes will need to shift focus away from the charismatic chewing insect herbivores and onto the often-neglected phloem-feeding biological control agents due to their overwhelmingly positive response to eCO2. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2022
- Full Text:
- Date Issued: 2022-04-06
An investigation into yeast-baculovirus synergism for the improved control of Thaumatotibia leucotreta, an economically important pest of citrus
- Authors: Van der Merwe, Marcél
- Date: 2021-10-29
- Subjects: Baculoviruses , Cryptophlebia leucotreta , Yeast , Natural pesticides , Citrus Diseases and pests , Biological pest control agents , Pests Integrated control , Thaumatotibia leucotreta
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191236 , vital:45073
- Description: A mutualistic association between Cydia pomonella and yeasts belonging to the genus Metschnikowia has previously been demonstrated. Larval feeding galleries inoculated with M. andauensis, reduced larval mortality and enhanced larval development. Additionally, adult C. pomonella female oviposition preference was also shown to be influenced by the volatiles produced by M. andauensis. This mutualistic relationship was manipulated for biological control purposes, by combining M. pulcherrima with the baculovirus Cydia pomonella granulovirus. The combination of M. pulcherrima with brown cane sugar and CpGV in laboratory assays and field trials resulted in a significant increase in larval mortality. A similar observation was made when M. pulcherrima was substituted for Saccharomyces cerevisiae. This indicates that yeasts harbour the potential for use in biological control, especially when combined with other well-established biocontrol methods. Thaumatotibia leucotreta is a phytophagous insect endemic to southern Africa. It is highly significant to the South African citrus industry due to its classification as a phytosanitary pest by most international markets. An integrated pest management programme has been implemented to control T. leucotreta. The baculovirus Cryptophlebia leucotreta granulovirus forms one component of this programme and is highly effective. In this study, we proposed to determine which yeast species occur naturally in the gut of T. leucotreta larvae and to examine whether any of the isolated yeast species, when combined with the CrleGV-SA, enhance its effectiveness. Firstly, Navel oranges infested with T. leucotreta larvae were collected from geographically distinct citrus-producing regions across South Africa. This led to the isolation and identification of six yeast species from the gut of T. leucotreta larvae via PCR amplification and sequencing of the internal transcribed spacer region and D1/D2 domain of the large subunit. Six yeast species were identified, viz. Meyerozyma guilliermondii, Hanseniaspora uvarum, Clavispora lusitaniae, Kluyveromyces marxianus, Pichia kudriavzevii and Pichia kluyveri. Additionally, Saccharomyces cerevisiae was included as a control in all trials due to its commercial availability and use in the artificial diet used to rear T. leucotreta. Secondly, larval development and attraction assays were conducted with the isolated yeast species. Thaumatotibia leucotreta larvae that fed on Navel oranges inoculated with M. guilliermondii, P. kluyveri, H. uvarum, and S. cerevisiae had accelerated developmental periods and reduced mortality rates. Additionally, it was demonstrated that T. leucotreta neonates were attracted to YPD broth cultures inoculated with P. kluyveri, H. uvarum, P. kudriavzevii and K. marxianus for feeding. Thirdly, oviposition preference assays were conducted with adult T. leucotreta females to determine whether the isolated yeast species influence their egg-laying in two-choice and multiple-choice tests. Navel oranges were inoculated with a specific yeast isolate, and mated adult females were left to oviposit. Meyerozyma guilliermondii, P. kudriavzevii and H. uvarum were shown to influence adult T. leucotreta female oviposition preference in two-choice tests. However, multiple-choice tests using the aforementioned yeast species did not mimic these results. Lastly, a series of detached fruit bioassays were performed to determine the optimal yeast:virus ratio, test all isolated yeast species in combination with CrleGV-SA and to further enhance yeast/virus formulation through the addition of an adjuvant and surfactant. CrleGV-SA was applied at a lethal concentration that would kill 50 % of T. leucotreta larvae. The optimal yeast concentration to use alongside CrleGV-SA was determined. Pichia kluyveri, P. kudriavzevii, K. marxianus and S. cerevisiae in combination with CrleGV-SA increased larval mortality compared to CrleGV-SA alone. The inclusion of molasses and BREAK-THRU® S 240 to P. kudriavzevii and S. cerevisiae plus CrleGV-SA formulations greatly enhanced their efficacy. Additionally, semi-field trials were initiated using P. kudriavzevii and S. cerevisiae, with promising preliminary results being obtained, although more replicates need to be performed. The experiments performed in this study provide a platform for further research into the application of a yeast/virus combination as a novel control and monitoring option for T. leucotreta in the field. , Thesis (PhD) -- Faculty of Science, Biochemistry and Microbiology, 2021
- Full Text:
- Date Issued: 2021-10-29
- Authors: Van der Merwe, Marcél
- Date: 2021-10-29
- Subjects: Baculoviruses , Cryptophlebia leucotreta , Yeast , Natural pesticides , Citrus Diseases and pests , Biological pest control agents , Pests Integrated control , Thaumatotibia leucotreta
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191236 , vital:45073
- Description: A mutualistic association between Cydia pomonella and yeasts belonging to the genus Metschnikowia has previously been demonstrated. Larval feeding galleries inoculated with M. andauensis, reduced larval mortality and enhanced larval development. Additionally, adult C. pomonella female oviposition preference was also shown to be influenced by the volatiles produced by M. andauensis. This mutualistic relationship was manipulated for biological control purposes, by combining M. pulcherrima with the baculovirus Cydia pomonella granulovirus. The combination of M. pulcherrima with brown cane sugar and CpGV in laboratory assays and field trials resulted in a significant increase in larval mortality. A similar observation was made when M. pulcherrima was substituted for Saccharomyces cerevisiae. This indicates that yeasts harbour the potential for use in biological control, especially when combined with other well-established biocontrol methods. Thaumatotibia leucotreta is a phytophagous insect endemic to southern Africa. It is highly significant to the South African citrus industry due to its classification as a phytosanitary pest by most international markets. An integrated pest management programme has been implemented to control T. leucotreta. The baculovirus Cryptophlebia leucotreta granulovirus forms one component of this programme and is highly effective. In this study, we proposed to determine which yeast species occur naturally in the gut of T. leucotreta larvae and to examine whether any of the isolated yeast species, when combined with the CrleGV-SA, enhance its effectiveness. Firstly, Navel oranges infested with T. leucotreta larvae were collected from geographically distinct citrus-producing regions across South Africa. This led to the isolation and identification of six yeast species from the gut of T. leucotreta larvae via PCR amplification and sequencing of the internal transcribed spacer region and D1/D2 domain of the large subunit. Six yeast species were identified, viz. Meyerozyma guilliermondii, Hanseniaspora uvarum, Clavispora lusitaniae, Kluyveromyces marxianus, Pichia kudriavzevii and Pichia kluyveri. Additionally, Saccharomyces cerevisiae was included as a control in all trials due to its commercial availability and use in the artificial diet used to rear T. leucotreta. Secondly, larval development and attraction assays were conducted with the isolated yeast species. Thaumatotibia leucotreta larvae that fed on Navel oranges inoculated with M. guilliermondii, P. kluyveri, H. uvarum, and S. cerevisiae had accelerated developmental periods and reduced mortality rates. Additionally, it was demonstrated that T. leucotreta neonates were attracted to YPD broth cultures inoculated with P. kluyveri, H. uvarum, P. kudriavzevii and K. marxianus for feeding. Thirdly, oviposition preference assays were conducted with adult T. leucotreta females to determine whether the isolated yeast species influence their egg-laying in two-choice and multiple-choice tests. Navel oranges were inoculated with a specific yeast isolate, and mated adult females were left to oviposit. Meyerozyma guilliermondii, P. kudriavzevii and H. uvarum were shown to influence adult T. leucotreta female oviposition preference in two-choice tests. However, multiple-choice tests using the aforementioned yeast species did not mimic these results. Lastly, a series of detached fruit bioassays were performed to determine the optimal yeast:virus ratio, test all isolated yeast species in combination with CrleGV-SA and to further enhance yeast/virus formulation through the addition of an adjuvant and surfactant. CrleGV-SA was applied at a lethal concentration that would kill 50 % of T. leucotreta larvae. The optimal yeast concentration to use alongside CrleGV-SA was determined. Pichia kluyveri, P. kudriavzevii, K. marxianus and S. cerevisiae in combination with CrleGV-SA increased larval mortality compared to CrleGV-SA alone. The inclusion of molasses and BREAK-THRU® S 240 to P. kudriavzevii and S. cerevisiae plus CrleGV-SA formulations greatly enhanced their efficacy. Additionally, semi-field trials were initiated using P. kudriavzevii and S. cerevisiae, with promising preliminary results being obtained, although more replicates need to be performed. The experiments performed in this study provide a platform for further research into the application of a yeast/virus combination as a novel control and monitoring option for T. leucotreta in the field. , Thesis (PhD) -- Faculty of Science, Biochemistry and Microbiology, 2021
- Full Text:
- Date Issued: 2021-10-29
Distribution, ecological and economic impacts and competition of the invasive alien aquatic weeds (Pontederia crassipes Mart., Pistia stratiotes L., Salvinia molesta D.S. Mitch. and Azolla filiculoides Lam.) in Madagascar
- Authors: Lehavana, Adolphe
- Date: 2021-10-29
- Subjects: Pontederiaceae Madagascar , Water lettuce Madagascar , Salvinia molesta Madagascar , Azolla filiculoides Madagascar , Introduced aquatic organisms , Aquatic weeds Economic aspects , Aquatic weeds Social aspects , Aquatic weeds Geographical distribution
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191225 , vital:45072
- Description: In Madagascar, as in several countries in the world, the invasion by four aquatic weeds (Pontederia crassipes Mart. (Pontederiaceae), Pistia stratiotes L. (Araceae), Salvinia molesta D.S. Mitch Salviniaceae) and Azolla filiculoides Lam. (Azollaceae) are among the drivers of environmental and socio-economic deterioration in aquatic ecosystems. Pistia stratiotes was first recorded on the island in the 19th century, and P. crassipes from the beginning of the 20th century, while S. molesta and A. filiculoides were only documented during in the 21st century. From the 1920s, botanists such as Henri Perrier de la Bathie and Raymond Decary were already aware of the dangers caused, in particular by P. crassipes in other countries, and raised the alarm, but little attention has been paid to these species. The aim of the research conducted for this thesis was to determine the distribution, socio-economic and ecological impacts of these four invasive alien aquatic weeds in Madagascar and to make recommendations for their control. First, the distributions of these four aquatic weeds were mapped. This mapping exercise compiled data from different sources including herbarium records, online data and field visits across Madagascar. The mapping study was undertaken from August 2015 to June 2020. Except for mountainous areas above 1800 m (Tsaratanana Massif, Ankaratra Massif and Andringitra Massif) where no data were available, all of Madagascar's bioclimates were invaded by at least one of the four aquatic weeds. In total, at least one species was recorded in 18 of the 22 Regions. Pontederia crassipes was recorded in 13 Regions, S. molesta in 14 Regions, P. stratiotes in 12 Regions, and A. filiculoides in 13 Regions. Herbarium records revealed the oldest record for P. stratiotes to be 1847, 1931 for P. crassipes, 1995 for S. molesta and there were no herbarium specimens for A. filiculoides prior to the start of the current study in 2015. We now know where these four weeds occur and how abundant they are. An objective of this research was to assess the impacts of the four invasive aquatic plants on the socio-economy of the island, mainly on rice production and fishing. Between 2016 and 2019, 102 households in three regions, Soanierana Ivongo, Foulpointe and Antananarivo, were randomly selected and questioned on the impact of these weeds in their aquatic ecosystems and their livelihoods such as fishing and rice growing. Surveys revealed that the four aquatic weeds significantly threatened household activities. On the east coast of Madagascar, the invasions of these four invasive species decreased fish and freshwater shrimp production by 82%. On the high plateau of Madagascar, they reduced rice yield by 30% despite requiring an additional expense of US$ 1,107/ha for control. Although farmers surveyed only used manual control to manage these weeds, they were receptive to other control methods, including integrated control using herbicides and biological control. Another objective of this research was to determine the ecological impacts of the four weeds and specifically if freshwater ecosystem functioning would return after control. To assess the ecological impact, between February 2017 to August 2019, on Lake Antsokafina, the following abiotic and biotic factors were considered: physico-chemistry of water, succession of macrophyte community and animal diversity. With the exception of turbidity, the values of the physico-chemical parameters of the water (pH, electrical conductivity, water temperature and turbidity), were similar between the infested zone and cleared zone. A study on the invasion process of aquatic weeds showed that the plant community succession of the lake changed over time in the areas that had been cleared. The submerged species Ceratophyllum demersum was the pioneer, followed by creeping species such as Echinochloa colona and Ipomoea aquatica, before the area was recolonized by aquatic weeds. Among the aquatic weeds, S. molesta was the most aggressive, covering 92% of the area one year after the start of the experiment. For animal diversity, bird, shrimp and fish community were assessed. The cleaning of the plots in the lake allowed the resumption of fishing activity providing 50 to 200g/catch for shrimp and from 0.25 to 0.5kg/catch for fish per person per day, while no catch was obtained in the areas infested by aquatic weeds were fishermen still attempting to harvest fish/shrimp from the aquatic weed infested areas. Three species of birds, Humblot’s Heron (Ardea humbloti), the white-faced whistling duck (Dendrocygna viduata) and red-billed teal (Anas erythrorhyncha) returned once the areas had been cleared. A manipulated outdoor as descriptor for laboratory experiment was conducted to determine the level and nature of competition of four aquatic weeds species against the indigenous floating fern, Salvinia hastata Desv. (Salviniaceae), using an additive series density model. It was shown that all four invasive species outcompeted S. hastata, with P. crassipes being 24 times more dominant, followed by P. stratiotes at 12 times, S. molesta at 8 times, and finally A. filiculoides at 1.2 times more dominant. This study provided direct evidence of the biodiversity impact of these four species and thus also provided an environmental argument for their control. Based on the findings of this study, a series of recommendations was formulated to manage the invasions of alien species in Madagascar with particular attention to invasive aquatic weeds. These recommendations mainly concern the establishment of management structures and legal instruments such as the creation of a lead government agency at national level and a cross-sectorial invasive species advisory committee, which should review legislation and regulations related to invasive species. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10-29
- Authors: Lehavana, Adolphe
- Date: 2021-10-29
- Subjects: Pontederiaceae Madagascar , Water lettuce Madagascar , Salvinia molesta Madagascar , Azolla filiculoides Madagascar , Introduced aquatic organisms , Aquatic weeds Economic aspects , Aquatic weeds Social aspects , Aquatic weeds Geographical distribution
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191225 , vital:45072
- Description: In Madagascar, as in several countries in the world, the invasion by four aquatic weeds (Pontederia crassipes Mart. (Pontederiaceae), Pistia stratiotes L. (Araceae), Salvinia molesta D.S. Mitch Salviniaceae) and Azolla filiculoides Lam. (Azollaceae) are among the drivers of environmental and socio-economic deterioration in aquatic ecosystems. Pistia stratiotes was first recorded on the island in the 19th century, and P. crassipes from the beginning of the 20th century, while S. molesta and A. filiculoides were only documented during in the 21st century. From the 1920s, botanists such as Henri Perrier de la Bathie and Raymond Decary were already aware of the dangers caused, in particular by P. crassipes in other countries, and raised the alarm, but little attention has been paid to these species. The aim of the research conducted for this thesis was to determine the distribution, socio-economic and ecological impacts of these four invasive alien aquatic weeds in Madagascar and to make recommendations for their control. First, the distributions of these four aquatic weeds were mapped. This mapping exercise compiled data from different sources including herbarium records, online data and field visits across Madagascar. The mapping study was undertaken from August 2015 to June 2020. Except for mountainous areas above 1800 m (Tsaratanana Massif, Ankaratra Massif and Andringitra Massif) where no data were available, all of Madagascar's bioclimates were invaded by at least one of the four aquatic weeds. In total, at least one species was recorded in 18 of the 22 Regions. Pontederia crassipes was recorded in 13 Regions, S. molesta in 14 Regions, P. stratiotes in 12 Regions, and A. filiculoides in 13 Regions. Herbarium records revealed the oldest record for P. stratiotes to be 1847, 1931 for P. crassipes, 1995 for S. molesta and there were no herbarium specimens for A. filiculoides prior to the start of the current study in 2015. We now know where these four weeds occur and how abundant they are. An objective of this research was to assess the impacts of the four invasive aquatic plants on the socio-economy of the island, mainly on rice production and fishing. Between 2016 and 2019, 102 households in three regions, Soanierana Ivongo, Foulpointe and Antananarivo, were randomly selected and questioned on the impact of these weeds in their aquatic ecosystems and their livelihoods such as fishing and rice growing. Surveys revealed that the four aquatic weeds significantly threatened household activities. On the east coast of Madagascar, the invasions of these four invasive species decreased fish and freshwater shrimp production by 82%. On the high plateau of Madagascar, they reduced rice yield by 30% despite requiring an additional expense of US$ 1,107/ha for control. Although farmers surveyed only used manual control to manage these weeds, they were receptive to other control methods, including integrated control using herbicides and biological control. Another objective of this research was to determine the ecological impacts of the four weeds and specifically if freshwater ecosystem functioning would return after control. To assess the ecological impact, between February 2017 to August 2019, on Lake Antsokafina, the following abiotic and biotic factors were considered: physico-chemistry of water, succession of macrophyte community and animal diversity. With the exception of turbidity, the values of the physico-chemical parameters of the water (pH, electrical conductivity, water temperature and turbidity), were similar between the infested zone and cleared zone. A study on the invasion process of aquatic weeds showed that the plant community succession of the lake changed over time in the areas that had been cleared. The submerged species Ceratophyllum demersum was the pioneer, followed by creeping species such as Echinochloa colona and Ipomoea aquatica, before the area was recolonized by aquatic weeds. Among the aquatic weeds, S. molesta was the most aggressive, covering 92% of the area one year after the start of the experiment. For animal diversity, bird, shrimp and fish community were assessed. The cleaning of the plots in the lake allowed the resumption of fishing activity providing 50 to 200g/catch for shrimp and from 0.25 to 0.5kg/catch for fish per person per day, while no catch was obtained in the areas infested by aquatic weeds were fishermen still attempting to harvest fish/shrimp from the aquatic weed infested areas. Three species of birds, Humblot’s Heron (Ardea humbloti), the white-faced whistling duck (Dendrocygna viduata) and red-billed teal (Anas erythrorhyncha) returned once the areas had been cleared. A manipulated outdoor as descriptor for laboratory experiment was conducted to determine the level and nature of competition of four aquatic weeds species against the indigenous floating fern, Salvinia hastata Desv. (Salviniaceae), using an additive series density model. It was shown that all four invasive species outcompeted S. hastata, with P. crassipes being 24 times more dominant, followed by P. stratiotes at 12 times, S. molesta at 8 times, and finally A. filiculoides at 1.2 times more dominant. This study provided direct evidence of the biodiversity impact of these four species and thus also provided an environmental argument for their control. Based on the findings of this study, a series of recommendations was formulated to manage the invasions of alien species in Madagascar with particular attention to invasive aquatic weeds. These recommendations mainly concern the establishment of management structures and legal instruments such as the creation of a lead government agency at national level and a cross-sectorial invasive species advisory committee, which should review legislation and regulations related to invasive species. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10-29
The biological control of Egeria densa Planch. (Hydrocharitaceae) in South Africa
- Authors: Smith, Rosali
- Date: 2021-10-29
- Subjects: Egeria (Plant genus) Biological control South Africa , Hydrocharitaceae Biological control South Africa , Aquatic weeds Biological control South Africa , Leafminers South Africa , Plant invasions South Africa , Resilience (Ecology) South Africa , Freshwater ecology South Africa , Hydrellia South Africa , Submerged macrophyte
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191102 , vital:45060 , 10.21504/10962/191102
- Description: Over the last thirty years, biological control, the use of host-specific natural enemies, has been a huge asset in the management exotic aquatic macrophytes such as Pistia stratiotes L. (Araceae), Pontederia crassipes Mart. (Solms) (Pontederiaceae), Azolla filiculoides Lam. (Azollaceae), Salvinia molesta D.S. Mitch (Salviniaceae) and Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae), also known as the “Big Bad Five” in South Africa. Despite these successes, freshwater ecosystems in South Africa have been harder to restore to an invasive macrophyte-free space, due to chronic disturbances such eutrophication, propagule dispersal and hydrological alterations. In the Anthropocene, where human activities have profound effects on their environment, these disturbances weakens ecological resilience and drive aquatic plant invasions. Due to long periods of invasions and the presence of a new suite of exotic aquatic plant propagules, native vegetation recolonization has been slow or even absent. Instead, the release of resources, such as sunlight, nutrient and space through aquatic weed management acts as a catalyst for secondary biological invasion. New invasive aquatic weeds include submerged and rooted emergent growth types, with Egeria densa Planch. (Hydrocharitaceae) the most widely distributed submerged aquatic weed in South Africa. It can quickly form dense monoculture stands that have ecological, economic and social impacts. Because of its ability to regenerate from plant fragments with double nodes, mechanical control is inappropriate. Additionally, mechanical and chemical control not only affects E. densa but have significant non-target effects. In response to its rapid spread over the last 20 years, especially following floating invasive aquatic management, a biological control programme was initiated, and in 2018, the leaf-mining fly, Hydrellia egeriae Rodrigues (Diptera: Ephydridae) was released. This was the first release of a biological control agent against E. densa in the world, and the first agent released against a submerged aquatic weed in South Africa. This thesis comprises the subsequent step of a biological control program when permission for the release of an agent have been obtained. A brief history of macrophyte invasions in South Africa’s unique freshwater systems are given in the literature review. Contributing factors to secondary invasions within the context of ecological resilience are introduced. An argument for the benefit of biological control as nuisance control is given, especially because E. densa and its natural enemy, H. egeriae is the focus species of this thesis. The main goal after permission for the release of an agent have been obtained, is to establish and build-up field populations. Research questions in this thesis aimed to investigate factors that contribute to or negate this goal. Through laboratory and field experiments we investigated the thermal physiology of the agent, and its climatic suitability to its novel range; different release strategies on field establishment and biotic resistance through the acquisition of novel parasitoids. Considering the longevity of this biological control program, we investigated the effects of elevated CO2 on the interaction between E. densa and H. egeriae through open top chamber experiments. Laboratory thermal physiology results showed that the agent is able to survive, develop and proliferate at all E. densa sites throughout the year. This is confirmed with the establishment of the agent at two release sites, the Nahoon River in the Eastern Cape Province and the Midmar Dam in KwaZulu-Natal. Post-release surveys showed that H. egeriae requires augmentative releases to sustain field populations. Without augmentative releases, H. egeriae herbivory levels were almost negligent. However, a contributing factor to low field-populations was parasitism. The biological control agent acquired three parasitoids, which have previously been described from Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), a specific herbivore to Lagarosiphon major (Ridl.) Moss (Hydrocharitaceae). These results provide information on the immediate establishment and effectiveness of the H. egeriae. Results from the elevated CO2 study suggest that E. densa will become less nutritious through a shift in leaf C/N ratio, when ambient 800ppm is bubbled into experimental growth chambers. Hydrellia egeriae feeding was affected by ambient CO2 levels and plant nutrient availability. The set levels of ambient CO2 levels used in this experiment produced dissolved inorganic carbon levels that were lower than dissolved inorganic carbon levels in E. densa invaded sites. This suggests that, submerged aquatic plant-insect interactions may be harder to predict from only laboratory experiments. Further investigations are necessary to establish system-specific characteristics i.e. dissolved inorganic carbon and target plant nutritional quality. The biological control of E. densa in South Africa is still in its infancy. This study presents results from post-release surveys up until two years after the agent was released. From this study, Hydrellia egeriae exhibits the potential to be an effective biological control agent, but release strategies should be adapted to sustain field populations and to limit field parasitism effects. Continued post-release surveys will provide a more comprehensive idea of the seasonal fluctuations of field-populations and parasitism. Surveys at multiple sites will provide information on potential site specific characteristics that contribute to or negate biological effort. Considering the high nutrient status of South African freshwater systems, a more holistic approach to E. densa management is necessary. This will require the strengthening of ecological resilience to prevent systems from shifting into an alternate invasive stable state. In addition, aquatic weed management needs to be addressed by a resilient social network, which ultimately calls for the strengthening of socio-ecological resilience. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10-29
- Authors: Smith, Rosali
- Date: 2021-10-29
- Subjects: Egeria (Plant genus) Biological control South Africa , Hydrocharitaceae Biological control South Africa , Aquatic weeds Biological control South Africa , Leafminers South Africa , Plant invasions South Africa , Resilience (Ecology) South Africa , Freshwater ecology South Africa , Hydrellia South Africa , Submerged macrophyte
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/191102 , vital:45060 , 10.21504/10962/191102
- Description: Over the last thirty years, biological control, the use of host-specific natural enemies, has been a huge asset in the management exotic aquatic macrophytes such as Pistia stratiotes L. (Araceae), Pontederia crassipes Mart. (Solms) (Pontederiaceae), Azolla filiculoides Lam. (Azollaceae), Salvinia molesta D.S. Mitch (Salviniaceae) and Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae), also known as the “Big Bad Five” in South Africa. Despite these successes, freshwater ecosystems in South Africa have been harder to restore to an invasive macrophyte-free space, due to chronic disturbances such eutrophication, propagule dispersal and hydrological alterations. In the Anthropocene, where human activities have profound effects on their environment, these disturbances weakens ecological resilience and drive aquatic plant invasions. Due to long periods of invasions and the presence of a new suite of exotic aquatic plant propagules, native vegetation recolonization has been slow or even absent. Instead, the release of resources, such as sunlight, nutrient and space through aquatic weed management acts as a catalyst for secondary biological invasion. New invasive aquatic weeds include submerged and rooted emergent growth types, with Egeria densa Planch. (Hydrocharitaceae) the most widely distributed submerged aquatic weed in South Africa. It can quickly form dense monoculture stands that have ecological, economic and social impacts. Because of its ability to regenerate from plant fragments with double nodes, mechanical control is inappropriate. Additionally, mechanical and chemical control not only affects E. densa but have significant non-target effects. In response to its rapid spread over the last 20 years, especially following floating invasive aquatic management, a biological control programme was initiated, and in 2018, the leaf-mining fly, Hydrellia egeriae Rodrigues (Diptera: Ephydridae) was released. This was the first release of a biological control agent against E. densa in the world, and the first agent released against a submerged aquatic weed in South Africa. This thesis comprises the subsequent step of a biological control program when permission for the release of an agent have been obtained. A brief history of macrophyte invasions in South Africa’s unique freshwater systems are given in the literature review. Contributing factors to secondary invasions within the context of ecological resilience are introduced. An argument for the benefit of biological control as nuisance control is given, especially because E. densa and its natural enemy, H. egeriae is the focus species of this thesis. The main goal after permission for the release of an agent have been obtained, is to establish and build-up field populations. Research questions in this thesis aimed to investigate factors that contribute to or negate this goal. Through laboratory and field experiments we investigated the thermal physiology of the agent, and its climatic suitability to its novel range; different release strategies on field establishment and biotic resistance through the acquisition of novel parasitoids. Considering the longevity of this biological control program, we investigated the effects of elevated CO2 on the interaction between E. densa and H. egeriae through open top chamber experiments. Laboratory thermal physiology results showed that the agent is able to survive, develop and proliferate at all E. densa sites throughout the year. This is confirmed with the establishment of the agent at two release sites, the Nahoon River in the Eastern Cape Province and the Midmar Dam in KwaZulu-Natal. Post-release surveys showed that H. egeriae requires augmentative releases to sustain field populations. Without augmentative releases, H. egeriae herbivory levels were almost negligent. However, a contributing factor to low field-populations was parasitism. The biological control agent acquired three parasitoids, which have previously been described from Hydrellia lagarosiphon Deeming (Diptera: Ephydridae), a specific herbivore to Lagarosiphon major (Ridl.) Moss (Hydrocharitaceae). These results provide information on the immediate establishment and effectiveness of the H. egeriae. Results from the elevated CO2 study suggest that E. densa will become less nutritious through a shift in leaf C/N ratio, when ambient 800ppm is bubbled into experimental growth chambers. Hydrellia egeriae feeding was affected by ambient CO2 levels and plant nutrient availability. The set levels of ambient CO2 levels used in this experiment produced dissolved inorganic carbon levels that were lower than dissolved inorganic carbon levels in E. densa invaded sites. This suggests that, submerged aquatic plant-insect interactions may be harder to predict from only laboratory experiments. Further investigations are necessary to establish system-specific characteristics i.e. dissolved inorganic carbon and target plant nutritional quality. The biological control of E. densa in South Africa is still in its infancy. This study presents results from post-release surveys up until two years after the agent was released. From this study, Hydrellia egeriae exhibits the potential to be an effective biological control agent, but release strategies should be adapted to sustain field populations and to limit field parasitism effects. Continued post-release surveys will provide a more comprehensive idea of the seasonal fluctuations of field-populations and parasitism. Surveys at multiple sites will provide information on potential site specific characteristics that contribute to or negate biological effort. Considering the high nutrient status of South African freshwater systems, a more holistic approach to E. densa management is necessary. This will require the strengthening of ecological resilience to prevent systems from shifting into an alternate invasive stable state. In addition, aquatic weed management needs to be addressed by a resilient social network, which ultimately calls for the strengthening of socio-ecological resilience. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10-29
Evaluation of Megabruchidius tonkineus (Coleoptera: Chrysomelidae: Bruchinae), a candidate biological control agent for Gleditsia triacanthos L. (Fabaceae) in South Africa
- Salgado Astudillo, Sara Elizabeth
- Authors: Salgado Astudillo, Sara Elizabeth
- Date: 2021-10
- Subjects: Honey locust South Africa , Honey locust Biological control South Africa , Invasive plants Biological control South Africa , Biogeography South Africa , Biogeography Climatic factors South Africa , Megabruchidius tonkineus South Africa
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10962/188386 , vital:44749
- Description: Gleditsia triacanthos L. (Fabaceae) (honey locust) is a fast-growing, deciduous tree indigenous to the United States of America. Introduced around the world as an ornamental tree, it has become invasive in a number of countries. Where it is invasive, G. triacanthos competes and replaces indigenous species; it creates dense stands along watercourses, posing a significant environmental threat. In South Africa, G. triacanthos is regarded as one of the country’s fastest spreading weeds. Gleditsia triacanthos produces numerous seeds contained in large hanging pods. Once dislodged from the pods, the seeds are dispersed by birds and mammals, including livestock, which eat the pods. It has been suggested that the seeds should be the target for biological control programme. Some invasive alien plant species are characterised by their ability to spread and establish in new ecosystems because they tolerate a wide range of environmental conditions. In order to predict areas of likely invasion, species distribution models (SDMs) are used to identify areas climatically suitable for their invasion, so enabling better targeted control of the plant species. Gleditsia triacanthos adapts to a wide range of climates and soil types, and tolerates salinity, drought and frost. Currently primarily restricted to the Grassland Biome of South Africa, G. triacanthos has doubled its distribution area in the past 15 years, and it is not known how far the species will spread. In this study we used two different modelling programmes, CLIMEX and MaxEnt, to predict areas where G. triacanthos could find favourable growing conditions; both SDMs showed that most of the country is suitable for G. triacanthos and that it will probably continue to spread, if left unmanaged, into new bioregions, such as the Karoo. In South Africa, the Asian seed-feeding bruchid, Megabruchidius tonkineus (Pic, 1914) (Coleoptera: Chrysomelidae: Bruchinae) has been recorded in the plant’s seed pods and has been considered as a biological control agent. The insect was not released as part of a formal biological control programme and neither host-specificity nor impact studies were conducted on the species prior to its discovery. In 2017 a decision was made to re-consider its status as a Abstract biological control agent until further details of its biology, host specificity, and impact on the seeds of G. triacanthos in South Africa were available. This study shows that Megabruchidius tonkineus has established across the entire G. triacanthos population in South Africa damaging approximately 9% of seeds. Laboratory studies show that, Megabruchidius tonkineus completes its larval development in the seeds of G. triacanthos in about 66.80 ± 0.6880 SE days before eclosing. In addition, the adult females oviposit on the following Fabaceae species: Arachis hypogaea, Albizia, julibrissin, Cicer arietinum, Pisum sativum, Dipogon lignosus, Peltophorum africanum, Podalyria buxifolia Senegalia burkei, Umtiza listerina and Vachellia sieberiana. However, larval development was limited to G. triacanthos. It is concluded that the seed-feeding beetle is not a threat to native Fabaceae species in South Africa, however, it does not damage enough G. triacanthos seeds to be considered a valuable biological control agent at this stage, and additional seed-feeding biological control agents should be considered to reduce the number of G. triacanthos seeds entering the environment. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10
- Authors: Salgado Astudillo, Sara Elizabeth
- Date: 2021-10
- Subjects: Honey locust South Africa , Honey locust Biological control South Africa , Invasive plants Biological control South Africa , Biogeography South Africa , Biogeography Climatic factors South Africa , Megabruchidius tonkineus South Africa
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10962/188386 , vital:44749
- Description: Gleditsia triacanthos L. (Fabaceae) (honey locust) is a fast-growing, deciduous tree indigenous to the United States of America. Introduced around the world as an ornamental tree, it has become invasive in a number of countries. Where it is invasive, G. triacanthos competes and replaces indigenous species; it creates dense stands along watercourses, posing a significant environmental threat. In South Africa, G. triacanthos is regarded as one of the country’s fastest spreading weeds. Gleditsia triacanthos produces numerous seeds contained in large hanging pods. Once dislodged from the pods, the seeds are dispersed by birds and mammals, including livestock, which eat the pods. It has been suggested that the seeds should be the target for biological control programme. Some invasive alien plant species are characterised by their ability to spread and establish in new ecosystems because they tolerate a wide range of environmental conditions. In order to predict areas of likely invasion, species distribution models (SDMs) are used to identify areas climatically suitable for their invasion, so enabling better targeted control of the plant species. Gleditsia triacanthos adapts to a wide range of climates and soil types, and tolerates salinity, drought and frost. Currently primarily restricted to the Grassland Biome of South Africa, G. triacanthos has doubled its distribution area in the past 15 years, and it is not known how far the species will spread. In this study we used two different modelling programmes, CLIMEX and MaxEnt, to predict areas where G. triacanthos could find favourable growing conditions; both SDMs showed that most of the country is suitable for G. triacanthos and that it will probably continue to spread, if left unmanaged, into new bioregions, such as the Karoo. In South Africa, the Asian seed-feeding bruchid, Megabruchidius tonkineus (Pic, 1914) (Coleoptera: Chrysomelidae: Bruchinae) has been recorded in the plant’s seed pods and has been considered as a biological control agent. The insect was not released as part of a formal biological control programme and neither host-specificity nor impact studies were conducted on the species prior to its discovery. In 2017 a decision was made to re-consider its status as a Abstract biological control agent until further details of its biology, host specificity, and impact on the seeds of G. triacanthos in South Africa were available. This study shows that Megabruchidius tonkineus has established across the entire G. triacanthos population in South Africa damaging approximately 9% of seeds. Laboratory studies show that, Megabruchidius tonkineus completes its larval development in the seeds of G. triacanthos in about 66.80 ± 0.6880 SE days before eclosing. In addition, the adult females oviposit on the following Fabaceae species: Arachis hypogaea, Albizia, julibrissin, Cicer arietinum, Pisum sativum, Dipogon lignosus, Peltophorum africanum, Podalyria buxifolia Senegalia burkei, Umtiza listerina and Vachellia sieberiana. However, larval development was limited to G. triacanthos. It is concluded that the seed-feeding beetle is not a threat to native Fabaceae species in South Africa, however, it does not damage enough G. triacanthos seeds to be considered a valuable biological control agent at this stage, and additional seed-feeding biological control agents should be considered to reduce the number of G. triacanthos seeds entering the environment. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10
Potential Synergism between Entomopathogenic Fungi and Entomopathogenic Nematodes for the control of false codling moth (Thaumatotibia leucotreta)
- Authors: Prinsloo, Samantha Lee
- Date: 2021-10
- Subjects: Cryptophlebia leucotreta , Entomopathogenic fungi , Insect nematodes , Citrus Diseases and pests , Cryptophlebia leucotreta Biological control , Pests Integrated control , Biological pest control agents
- Language: English
- Type: Masters theses , text
- Identifier: http://hdl.handle.net/10962/188832 , vital:44790
- Description: False codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) (FCM), is a major phytosanitary pest of citrus in South Africa. Sufficient control measures for the soil-dwelling life stages of FCM have yet to be identified and owing to restrictions on the use of insecticides, non-chemical control options have been investigated including the use of entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN). Laboratory and field trials on an indigenous EPF, Metarhizium anisopliae FCM Ar 23 B3, have shown that this isolate is capable of inducing mortality in FCM soil-dwelling life stages. Other agents that have been highlighted as potential controls for soil-dwelling FCM life stages are the EPN species Steinernema yirgalemense 157-C, S. jeffreyense J194 and H. noenieputensis 158-C. This study conducted laboratory bioassays to assess the virulence of these four control agents on fifth instar FCM, in 24-well plates. These results reaffirmed the virulence of the four microbial control agents at their recommended doses of 50 IJs (EPN) and 1×107 conidia/ml (EPF) against fifth instar FCM with 80 to 96% larval mortality recorded. The EPF isolate exhibited the lowest mortality whilst S. yirgalemense induced the greatest mortality. In addition, the lethal concentration (LC) values for each isolate were determined using dose response bioassays. These values were previously unknown for all EPN species and for the EPF isolate based on the methodology used in this study. The LC50 results in order from lowest to highest EPN IJ concentration requirements were 4.38 IJs (S. yirgalemense), 4.47 IJs (S. jeffreyense) and 7.11 IJs (H. noenieputensis). The EPF isolate exhibited an LC50 of 3.42×105 conidia/ml. Lastly, research has shown that the combination of two control agents may increase control of late instar lepidopteran and coleopteran larvae, through synergistic interactions. Thus, the interactions that occurred between the combination of these EPN species with the EPF isolate were determined. This study found that when all three EPN species were combined simultaneously and sequentially with the EPF isolate M. anisopliae FCM AR 23 B3, additive interactions took place with exception of the simultaneous application of S. yirgalemense and H. noenieputensis, with the EPF and S. jeffreyense applied 24 h post EPF application. For the former, a synergistic interaction was found, whilst for the latter two, an antagonistic interaction. Although no strongly synergistic interactions were observed, additive interactions have been shown to reach a synergistic level when certain parameters are changed. Moving forward, a uniform methodology for conducting EPF/EPN interaction experiments has been suggested. It has also been recommended that due to the additive interactions observed in this study, laboratory soil-bioassays and field trials should be carried out for all three EPN species in combination with the EPF isolate. This research will inevitably facilitate the constant knowledge into management strategies for the phytosanitary pest, FCM in South African citrus. , Thesis (MSc) -- Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10
- Authors: Prinsloo, Samantha Lee
- Date: 2021-10
- Subjects: Cryptophlebia leucotreta , Entomopathogenic fungi , Insect nematodes , Citrus Diseases and pests , Cryptophlebia leucotreta Biological control , Pests Integrated control , Biological pest control agents
- Language: English
- Type: Masters theses , text
- Identifier: http://hdl.handle.net/10962/188832 , vital:44790
- Description: False codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) (FCM), is a major phytosanitary pest of citrus in South Africa. Sufficient control measures for the soil-dwelling life stages of FCM have yet to be identified and owing to restrictions on the use of insecticides, non-chemical control options have been investigated including the use of entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN). Laboratory and field trials on an indigenous EPF, Metarhizium anisopliae FCM Ar 23 B3, have shown that this isolate is capable of inducing mortality in FCM soil-dwelling life stages. Other agents that have been highlighted as potential controls for soil-dwelling FCM life stages are the EPN species Steinernema yirgalemense 157-C, S. jeffreyense J194 and H. noenieputensis 158-C. This study conducted laboratory bioassays to assess the virulence of these four control agents on fifth instar FCM, in 24-well plates. These results reaffirmed the virulence of the four microbial control agents at their recommended doses of 50 IJs (EPN) and 1×107 conidia/ml (EPF) against fifth instar FCM with 80 to 96% larval mortality recorded. The EPF isolate exhibited the lowest mortality whilst S. yirgalemense induced the greatest mortality. In addition, the lethal concentration (LC) values for each isolate were determined using dose response bioassays. These values were previously unknown for all EPN species and for the EPF isolate based on the methodology used in this study. The LC50 results in order from lowest to highest EPN IJ concentration requirements were 4.38 IJs (S. yirgalemense), 4.47 IJs (S. jeffreyense) and 7.11 IJs (H. noenieputensis). The EPF isolate exhibited an LC50 of 3.42×105 conidia/ml. Lastly, research has shown that the combination of two control agents may increase control of late instar lepidopteran and coleopteran larvae, through synergistic interactions. Thus, the interactions that occurred between the combination of these EPN species with the EPF isolate were determined. This study found that when all three EPN species were combined simultaneously and sequentially with the EPF isolate M. anisopliae FCM AR 23 B3, additive interactions took place with exception of the simultaneous application of S. yirgalemense and H. noenieputensis, with the EPF and S. jeffreyense applied 24 h post EPF application. For the former, a synergistic interaction was found, whilst for the latter two, an antagonistic interaction. Although no strongly synergistic interactions were observed, additive interactions have been shown to reach a synergistic level when certain parameters are changed. Moving forward, a uniform methodology for conducting EPF/EPN interaction experiments has been suggested. It has also been recommended that due to the additive interactions observed in this study, laboratory soil-bioassays and field trials should be carried out for all three EPN species in combination with the EPF isolate. This research will inevitably facilitate the constant knowledge into management strategies for the phytosanitary pest, FCM in South African citrus. , Thesis (MSc) -- Science, Zoology and Entomology, 2021
- Full Text:
- Date Issued: 2021-10
Economic evaluation of chemical and biological control methods on four aquatic weeds in South Africa
- Authors: Maluleke, Mary
- Date: 2020
- Subjects: Invasive plants -- Biological control -- Economic aspects -- South Africa , Introduced organisms -- Biological control -- Economic aspects -- South Africa , Aquatic weeds -- Biological control -- Economic aspects -- South Africa , Aquatic weeds -- Control -- Economic aspects -- South Africa , Aquatic resources -- Management , Cost effectiveness , Net present value , Herbicides -- Cost effectiveness , Working for Water Programme , Water conservation -- South Africa
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: http://hdl.handle.net/10962/145953 , vital:38481
- Description: Invasive alien plants (IAPs) of various kinds pose a threat to ecosystems, biodiversity, conservation and overall economy. In a world experiencing exponential increase in IAPs – this issue has become endemic, especially for developing countries such as South Africa. South Africa is a water scarce country and IAPs increase water stress. Thus, South Africa must invest in a more realistic, environmentally and economically inclusive policy outlook on the management of IAPs including aquatic weeds. This is especially urgent when considering the changing global climate, which is predicted to further reduce the quantity and quality of potable water. The Working for Water Programme (WfW) in South Africa aimed at addressing the issue of IAPs in a way that protects the environment as well as produces maximum return to society through poverty alleviation. As such, the aquatic weeds management strategy put in place for four of South Africa’s aquatic weeds Pista stratiotes, Salvinia molesta, Azolla filiculoides and Myriophyllum aquaticum - should be one that is cost-effective, efficient and sustainable; yielding the best possible return on investment. Since these four weeds are already under complete biological control, in the absence of biological agents, the WfW programme would have used herbicides to control these weeds. As such, this thesis conducted a retrospective analysis of the relative herbicide cost-saving associated with the use of biological control. To do this, due to existing limitations, E. crassipes was used as a surrogate weed and its herbicide control costs were used as proxy for the herbicide control cost estimates of the four selected weeds; with reasonable conversion factors applied to cater for the biological difference of the five weeds. Using the cost benefit analysis (CBA) framework, the net present cost (NPC) of each control method was calculated to which the relative cost-saving was considered to represent the avoided cost of using biological control instead of chemical control on these weeds. The avoided cost was used as the main benefit component when deriving the relative benefit cost ratios (BCR). Two scenarios were used, one assuming no follow-up requirement and the other assuming one follow-up requirement for chemical control. Using an 8% discount rate, the study found that the estimated cost of the biological control method on all four aquatic weeds was about R7,843,205 while for chemical control the estimated costs would have costed R149,580,142, R268,264,838 and R881,711,738 for application by means of a boat, bakkie and knapsack. Chemical control cost estimates would have increased to about R164,538,052, R295,216,120 and R1,008,761,000 for boat, bakkie and knapsack approach respectively when including a possible follow-up programme. These would have led to positive BCRs of 90.24:1, 164.97:1 and 557.99:1 across the three chemical control approaches without a follow-up (with BCR of about 99.67:1, 182.00:1 and 631.56:1 for the boat, bakkie and knapsack approach respectively with the accepted follow-up programme). When running a sensitivity test with varying discount rates of 5% and 10%, these results remained robust. As such, failing to reject the dominant hypothesis in literature, the main conclusion of the study is that biological control is indeed the more cost-effective management option compared to chemical control with respect to herbicide cost-saving. Further, biological control is most-likely to produce more environmental cost-saving and water-saving over chemical control. The study recommends the continued use of the biological control investment on the four aquatic weeds under study as well as on emerging aquatic weeds such as Iris pseudacorus, Nymphaea mexicana and Sagittaria platyphylla in South Africa.
- Full Text:
- Date Issued: 2020
Economic evaluation of chemical and biological control methods on four aquatic weeds in South Africa
- Authors: Maluleke, Mary
- Date: 2020
- Subjects: Invasive plants -- Biological control -- Economic aspects -- South Africa , Introduced organisms -- Biological control -- Economic aspects -- South Africa , Aquatic weeds -- Biological control -- Economic aspects -- South Africa , Aquatic weeds -- Control -- Economic aspects -- South Africa , Aquatic resources -- Management , Cost effectiveness , Net present value , Herbicides -- Cost effectiveness , Working for Water Programme , Water conservation -- South Africa
- Language: English
- Type: Thesis , Masters , MCom
- Identifier: http://hdl.handle.net/10962/145953 , vital:38481
- Description: Invasive alien plants (IAPs) of various kinds pose a threat to ecosystems, biodiversity, conservation and overall economy. In a world experiencing exponential increase in IAPs – this issue has become endemic, especially for developing countries such as South Africa. South Africa is a water scarce country and IAPs increase water stress. Thus, South Africa must invest in a more realistic, environmentally and economically inclusive policy outlook on the management of IAPs including aquatic weeds. This is especially urgent when considering the changing global climate, which is predicted to further reduce the quantity and quality of potable water. The Working for Water Programme (WfW) in South Africa aimed at addressing the issue of IAPs in a way that protects the environment as well as produces maximum return to society through poverty alleviation. As such, the aquatic weeds management strategy put in place for four of South Africa’s aquatic weeds Pista stratiotes, Salvinia molesta, Azolla filiculoides and Myriophyllum aquaticum - should be one that is cost-effective, efficient and sustainable; yielding the best possible return on investment. Since these four weeds are already under complete biological control, in the absence of biological agents, the WfW programme would have used herbicides to control these weeds. As such, this thesis conducted a retrospective analysis of the relative herbicide cost-saving associated with the use of biological control. To do this, due to existing limitations, E. crassipes was used as a surrogate weed and its herbicide control costs were used as proxy for the herbicide control cost estimates of the four selected weeds; with reasonable conversion factors applied to cater for the biological difference of the five weeds. Using the cost benefit analysis (CBA) framework, the net present cost (NPC) of each control method was calculated to which the relative cost-saving was considered to represent the avoided cost of using biological control instead of chemical control on these weeds. The avoided cost was used as the main benefit component when deriving the relative benefit cost ratios (BCR). Two scenarios were used, one assuming no follow-up requirement and the other assuming one follow-up requirement for chemical control. Using an 8% discount rate, the study found that the estimated cost of the biological control method on all four aquatic weeds was about R7,843,205 while for chemical control the estimated costs would have costed R149,580,142, R268,264,838 and R881,711,738 for application by means of a boat, bakkie and knapsack. Chemical control cost estimates would have increased to about R164,538,052, R295,216,120 and R1,008,761,000 for boat, bakkie and knapsack approach respectively when including a possible follow-up programme. These would have led to positive BCRs of 90.24:1, 164.97:1 and 557.99:1 across the three chemical control approaches without a follow-up (with BCR of about 99.67:1, 182.00:1 and 631.56:1 for the boat, bakkie and knapsack approach respectively with the accepted follow-up programme). When running a sensitivity test with varying discount rates of 5% and 10%, these results remained robust. As such, failing to reject the dominant hypothesis in literature, the main conclusion of the study is that biological control is indeed the more cost-effective management option compared to chemical control with respect to herbicide cost-saving. Further, biological control is most-likely to produce more environmental cost-saving and water-saving over chemical control. The study recommends the continued use of the biological control investment on the four aquatic weeds under study as well as on emerging aquatic weeds such as Iris pseudacorus, Nymphaea mexicana and Sagittaria platyphylla in South Africa.
- Full Text:
- Date Issued: 2020
Investigations into biological control options for Lycium ferocissimum Miers, African Boxthorn (Solanaceae) for Australia
- Authors: Mauda, Evans Vusani
- Date: 2020
- Subjects: Lycium ferocissimum , Solanaceae -- Biological control -- Australia , Weeds -- Control -- Australia , Invasive plants -- Biological control -- Australia , Insects as biological pest control agents -- Australia , Insect-plant relationships
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167142 , vital:41441
- Description: Lycium ferocissimum Miers (Solanaceae) (African boxthorn or boxthorn) is a shrub native to South Africa,and has become naturalised and invasive in Australia and New Zealand. The plant is listed on the Noxious Weed List for Australian States and territories. Although other control methods are available, biological control presents a potentially sustainable intervention for reducing populations of this weed in Australia. In South Africa, the plant has been recorded from two allopatric populations, one in the Eastern Cape Province, the other in the Western Cape Provinces, however, there taxonomic and morphological uncertainties are reported in the literature. Therefore, before native range surveys for potential biological control agents could be considered, the taxonomic uncertainty needed to be resolved. The two geographically distinct areas, as well as the Australia population were sampled to assess morphological and genetic variation. All samples collected in Australia were confirmed as L.ferocissimum, with no evidence of hybridisation with any other Lycium species. Nuclear and chloroplast genetic diversity within L.ferocissimum across South Africa was high, and Australia was low, with no evidence of genetic seperation. One ehaplotypes found across Australia was found at only two sites in South Africa, both in the Western Cape, suggesting that the Australian lineage may have originated from this region. Ten samples from South Africa, putatively identified in the field as L.ferocissimum, were genetically characterised as different (unidentified) Lycium species. The majority of plants sampled were confirmed as L.ferocissimum, sharing a common haplotype (haplotype 5) with sampled specimens from Australia. Morphological analyses across different Lycium species in South Africa did not identify any leaf or floral characteristics unique to L.ferocissimum, and thus morphological identification in the native range remains problematic. Surveys for phytophagous in sects on L.ferocissimum were carried out regularly over a two-year period in the two regions. The number of insect species found in the Eastern Cape Province (55) was higher than in the Western Cape Province (41), but insect diversity based on Shannon indices was highest in the Western Cape Province. Indicator species analysis revealed eight insect herbivore species driving the differences in the herbivore communities between the two provinces. Based on insect distribution, abundance, feeding preference and available literature, three species were prioritised as potential biological control agents. These include the leaf-chewing beetles, Cassida distinguenda Spaeth (Chrysomelidae) and Cleta eckloni Mulsant (Coccinellidae), and the leaf-mining weevil, Neoplatygaster serietuberculata Gyllenhal (Curculionidae). Native range studies such as this are perhaps the most technically difficult and logistically time-consuming part of the biological control programme. Yet, the entire outcome of a programme depends on the suite of potential agents feeding on the weed. The information gained during this stage significantly contributed to the prioritization of agents for further host-range testing and possible release. Here we showed how molecular and genetic characterisations of the target weed can be us ed to accurately define the identity and phylogeny of the target species. In addition, the study also highlighted the importance of considering plant morphology and how phenotypic plasticity may influence infield plant identifications while conducting native range surveys. By gaining further information during long-term and wide spread native range surveys we were not just able to provide a list of herbivorous insect fauna and fungi associated with the plant, but were able to prioritise the phytophagous species that held the most potential as biological control agents.
- Full Text:
- Date Issued: 2020
- Authors: Mauda, Evans Vusani
- Date: 2020
- Subjects: Lycium ferocissimum , Solanaceae -- Biological control -- Australia , Weeds -- Control -- Australia , Invasive plants -- Biological control -- Australia , Insects as biological pest control agents -- Australia , Insect-plant relationships
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167142 , vital:41441
- Description: Lycium ferocissimum Miers (Solanaceae) (African boxthorn or boxthorn) is a shrub native to South Africa,and has become naturalised and invasive in Australia and New Zealand. The plant is listed on the Noxious Weed List for Australian States and territories. Although other control methods are available, biological control presents a potentially sustainable intervention for reducing populations of this weed in Australia. In South Africa, the plant has been recorded from two allopatric populations, one in the Eastern Cape Province, the other in the Western Cape Provinces, however, there taxonomic and morphological uncertainties are reported in the literature. Therefore, before native range surveys for potential biological control agents could be considered, the taxonomic uncertainty needed to be resolved. The two geographically distinct areas, as well as the Australia population were sampled to assess morphological and genetic variation. All samples collected in Australia were confirmed as L.ferocissimum, with no evidence of hybridisation with any other Lycium species. Nuclear and chloroplast genetic diversity within L.ferocissimum across South Africa was high, and Australia was low, with no evidence of genetic seperation. One ehaplotypes found across Australia was found at only two sites in South Africa, both in the Western Cape, suggesting that the Australian lineage may have originated from this region. Ten samples from South Africa, putatively identified in the field as L.ferocissimum, were genetically characterised as different (unidentified) Lycium species. The majority of plants sampled were confirmed as L.ferocissimum, sharing a common haplotype (haplotype 5) with sampled specimens from Australia. Morphological analyses across different Lycium species in South Africa did not identify any leaf or floral characteristics unique to L.ferocissimum, and thus morphological identification in the native range remains problematic. Surveys for phytophagous in sects on L.ferocissimum were carried out regularly over a two-year period in the two regions. The number of insect species found in the Eastern Cape Province (55) was higher than in the Western Cape Province (41), but insect diversity based on Shannon indices was highest in the Western Cape Province. Indicator species analysis revealed eight insect herbivore species driving the differences in the herbivore communities between the two provinces. Based on insect distribution, abundance, feeding preference and available literature, three species were prioritised as potential biological control agents. These include the leaf-chewing beetles, Cassida distinguenda Spaeth (Chrysomelidae) and Cleta eckloni Mulsant (Coccinellidae), and the leaf-mining weevil, Neoplatygaster serietuberculata Gyllenhal (Curculionidae). Native range studies such as this are perhaps the most technically difficult and logistically time-consuming part of the biological control programme. Yet, the entire outcome of a programme depends on the suite of potential agents feeding on the weed. The information gained during this stage significantly contributed to the prioritization of agents for further host-range testing and possible release. Here we showed how molecular and genetic characterisations of the target weed can be us ed to accurately define the identity and phylogeny of the target species. In addition, the study also highlighted the importance of considering plant morphology and how phenotypic plasticity may influence infield plant identifications while conducting native range surveys. By gaining further information during long-term and wide spread native range surveys we were not just able to provide a list of herbivorous insect fauna and fungi associated with the plant, but were able to prioritise the phytophagous species that held the most potential as biological control agents.
- Full Text:
- Date Issued: 2020
Quantifying ecosystem restoration recovery and restoration practice following the biological control of invasive alien macrophytes in Southern Africa
- Authors: Motitsoe, Samuel Nkopane
- Date: 2020
- Subjects: Salvinia molesta , Ceratophyllum demersum , Nymphaea mexicana , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Restoration monitoring (Ecology) -- South Africa , Biolotical invasions -- Environmental aspects
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167519 , vital:41488
- Description: Invasive alien aquatic plants (IAAP) species are known to have deleterious effects on the freshwater ecosystems they invade. This includes both socio-economic and ecologically important ecosystem goods and services. Thus, IAAP species are declared a serious threat, second only to habitat modification for causing a loss of aquatic biodiversity. Three control methods have been widely applied to control IAAP species invasion globally; mechanical, chemical and biological control. Both mechanical and chemical control methods are considered short-term and expensive, whereas biological control methods are regarded an effective and long-term solution for IAAP species control at the landscape level. But, little is known of the ecological recovery following the biological control of IAAP species, with mechanical control known to have had mixed success and chemical control to have non-targeted effects on aquatic ecosystems, causing harm to wildlife and human well-being. Biological control practitioners measure the success of biological control based on: (1) the biological control agents’ establishment and the negative impacts they impose on the targeted weed; and (2) the weeds biomass reduction and an increase in native macrophytes species. Arguably, measures of biological control success have been subjective and variable across the globe. Although some field studies have demonstrated biological control success to have positive socio-economic returns, there is little literature on ecological benefits. Furthermore, there is limited understanding on ecosystem recovery and possible restoration efforts following the biological control IAAP species, as compared to alien weeds in terrestrial and riparian ecosystems. Thus, this thesis aimed to quantify the ecological recovery i.e. aquatic biodiversity, ecosystem processes and trophic interactions following the management of Salvinia molesta in freshwater ecosystems. The research employed a suite of Before-After Control-Impact mesocosm and field studies to investigate the response of aquatic microalgae, macroinvertebrates and their interactions (food web structure and function) during S. molesta infestation and after mechanical and biological control. The mesocosm experiment (Before invasion, During invasion & After control) showed that both aquatic microalgae and macroinvertebrate diversity indices were reliable biological indicators of S. molesta ecological impacts and recovery following control. The restored treatment (100% S. molesta cover + biological control agents), demonstrated complete aquatic microalgae and macroinvertebrate recovery following biological control, similar to the control treatment (open water), where the degraded/impacted treatment (100% S. molesta cover with no biological control agents) showed a drastic decline in aquatic biodiversity and a complete shift in aquatic biota assemblage structure. Thus, the biological control effort by Cyrtobagous salviniae, the biological control agent for S. molesta, assisted in the recovery of aquatic biota following successful biological control. The field study (four field sites, two sites controlled mechanically and two biologically) investigated water quality, aquatic biodiversity and community trophic interactions (aquatic food web) “before and after” S. molesta control. The study showed a drastic decline in aquatic biodiversity (with three sites showing no record of aquatic macroinvertebrates, thus no biotic interactions during infestation) and poor water quality due to the shade-effect (light barrier due to floating S. molesta mats on the water surface) during the “before” S. molesta control phase. However, following both mechanical and biological control (“after” S. molesta control phase), there was a significant shift in abiotic and biotic ecosystem characteristics as compared to the “before” S. molesta control phase. Thus, rapid ecosystem recovery was apparent as a result of aquatic microalgae and macroinvertebrates recolonisation. Sites showed a normal functioning ecosystem where improved water quality, increased biodiversity, productivity and trophic interactions, was indicative of the return of biologically and functionally important species which were lost during the “before” S. molesta phase. Although the clear water state showed positive outcomes at Westlake River, these were short lived when the system was dominated by a cosmopolitan submerged Ceratophyllum demersum, and later replaced by a floating-leaved emergent IAAP Nymphaea mexicana. Each state was responsible for a significant shift in both biotic and abiotic characteristics, affirming macrophyte abilities to influence aquatic environments structure and functions. Furthermore, this event showed a clear example of a secondary invasion. Thus, a holistic IAAP species management strategy is necessary to restore previously invaded ecosystems and prevent subsequent secondary invasion and ecosystem degradation. In conclusion, the S. molesta shade-effect like any other free-floating IAAP species, was identified as the main degrading factor and responsible for water quality reduction, loss of aquatic diversity and shift in aquatic biota assemblage structure. Following S. molesta removal (or shade-effect elimination), there was a positive response to aquatic ecosystem species abundance, richness, diversity and community structure. Therefore, in combination, aquatic biota recolonisation rate and increases in biological and functional diversity were instrumental in the recovery of ecosystem structure and functions, following the control of S. molesta. Echoing existing literature, this thesis recommends: (1) IAAP species management programmes (mechanical and/or biological control) should not only aim to control the weed but also focus on ecosystems recovery and possible restoration goals; (2) biological control should be used where appropriate to combat free-floating IAAP species in freshwater ecosystems, followed by active introduction of native macrophyte propagules since they are limited by anthropogenic activities; and (3) more freshwater case studies are needed to add to our understanding of IAAP species management and restoration effort incorporating long-term monitoring.
- Full Text:
- Date Issued: 2020
- Authors: Motitsoe, Samuel Nkopane
- Date: 2020
- Subjects: Salvinia molesta , Ceratophyllum demersum , Nymphaea mexicana , Invasive plants -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Restoration monitoring (Ecology) -- South Africa , Biolotical invasions -- Environmental aspects
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/167519 , vital:41488
- Description: Invasive alien aquatic plants (IAAP) species are known to have deleterious effects on the freshwater ecosystems they invade. This includes both socio-economic and ecologically important ecosystem goods and services. Thus, IAAP species are declared a serious threat, second only to habitat modification for causing a loss of aquatic biodiversity. Three control methods have been widely applied to control IAAP species invasion globally; mechanical, chemical and biological control. Both mechanical and chemical control methods are considered short-term and expensive, whereas biological control methods are regarded an effective and long-term solution for IAAP species control at the landscape level. But, little is known of the ecological recovery following the biological control of IAAP species, with mechanical control known to have had mixed success and chemical control to have non-targeted effects on aquatic ecosystems, causing harm to wildlife and human well-being. Biological control practitioners measure the success of biological control based on: (1) the biological control agents’ establishment and the negative impacts they impose on the targeted weed; and (2) the weeds biomass reduction and an increase in native macrophytes species. Arguably, measures of biological control success have been subjective and variable across the globe. Although some field studies have demonstrated biological control success to have positive socio-economic returns, there is little literature on ecological benefits. Furthermore, there is limited understanding on ecosystem recovery and possible restoration efforts following the biological control IAAP species, as compared to alien weeds in terrestrial and riparian ecosystems. Thus, this thesis aimed to quantify the ecological recovery i.e. aquatic biodiversity, ecosystem processes and trophic interactions following the management of Salvinia molesta in freshwater ecosystems. The research employed a suite of Before-After Control-Impact mesocosm and field studies to investigate the response of aquatic microalgae, macroinvertebrates and their interactions (food web structure and function) during S. molesta infestation and after mechanical and biological control. The mesocosm experiment (Before invasion, During invasion & After control) showed that both aquatic microalgae and macroinvertebrate diversity indices were reliable biological indicators of S. molesta ecological impacts and recovery following control. The restored treatment (100% S. molesta cover + biological control agents), demonstrated complete aquatic microalgae and macroinvertebrate recovery following biological control, similar to the control treatment (open water), where the degraded/impacted treatment (100% S. molesta cover with no biological control agents) showed a drastic decline in aquatic biodiversity and a complete shift in aquatic biota assemblage structure. Thus, the biological control effort by Cyrtobagous salviniae, the biological control agent for S. molesta, assisted in the recovery of aquatic biota following successful biological control. The field study (four field sites, two sites controlled mechanically and two biologically) investigated water quality, aquatic biodiversity and community trophic interactions (aquatic food web) “before and after” S. molesta control. The study showed a drastic decline in aquatic biodiversity (with three sites showing no record of aquatic macroinvertebrates, thus no biotic interactions during infestation) and poor water quality due to the shade-effect (light barrier due to floating S. molesta mats on the water surface) during the “before” S. molesta control phase. However, following both mechanical and biological control (“after” S. molesta control phase), there was a significant shift in abiotic and biotic ecosystem characteristics as compared to the “before” S. molesta control phase. Thus, rapid ecosystem recovery was apparent as a result of aquatic microalgae and macroinvertebrates recolonisation. Sites showed a normal functioning ecosystem where improved water quality, increased biodiversity, productivity and trophic interactions, was indicative of the return of biologically and functionally important species which were lost during the “before” S. molesta phase. Although the clear water state showed positive outcomes at Westlake River, these were short lived when the system was dominated by a cosmopolitan submerged Ceratophyllum demersum, and later replaced by a floating-leaved emergent IAAP Nymphaea mexicana. Each state was responsible for a significant shift in both biotic and abiotic characteristics, affirming macrophyte abilities to influence aquatic environments structure and functions. Furthermore, this event showed a clear example of a secondary invasion. Thus, a holistic IAAP species management strategy is necessary to restore previously invaded ecosystems and prevent subsequent secondary invasion and ecosystem degradation. In conclusion, the S. molesta shade-effect like any other free-floating IAAP species, was identified as the main degrading factor and responsible for water quality reduction, loss of aquatic diversity and shift in aquatic biota assemblage structure. Following S. molesta removal (or shade-effect elimination), there was a positive response to aquatic ecosystem species abundance, richness, diversity and community structure. Therefore, in combination, aquatic biota recolonisation rate and increases in biological and functional diversity were instrumental in the recovery of ecosystem structure and functions, following the control of S. molesta. Echoing existing literature, this thesis recommends: (1) IAAP species management programmes (mechanical and/or biological control) should not only aim to control the weed but also focus on ecosystems recovery and possible restoration goals; (2) biological control should be used where appropriate to combat free-floating IAAP species in freshwater ecosystems, followed by active introduction of native macrophyte propagules since they are limited by anthropogenic activities; and (3) more freshwater case studies are needed to add to our understanding of IAAP species management and restoration effort incorporating long-term monitoring.
- Full Text:
- Date Issued: 2020