The effect of biological control on the population dynamics of Pontederia crassipes Mart. (Pontederiaceae) and Salvinia minima Baker (Salviniales: Salviniaceae)
- Authors: Chikodza, Tressia
- Date: 2024-10-11
- Subjects: Pontederia crassipes , Salvinia minima , Pontederiaceae Biological control South Africa , Salviniaceae Biological control South Africa , Population dynamics , Secondary invasion
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464462 , vital:76513
- Description: Pontederia crassipes is widely regarded as the most damaging floating aquatic weed in terms of its invasive traits, and its impact on aquatic ecosystems. Biological control using host-specific natural enemies is widely used for its control, with the most recent agent released being a planthopper, Megamelus scutellaris, in South Africa and the USA. In South Africa, inundative releases of M. scutellaris have been shown to control the weed even at eutrophic, and high elevation cold sites, such as Hartbeespoort Dam, arguably Africa’s most hypertrophic impoundment. However, subsequent to the control of P. crassipes on Hartbeespoort Dam, in 2021, a secondary invader, Salvinia minima, dominated the water system. Salvinia minima is only known from a handful sites near Hartbeespoort Dam and there is currently no approved biocontrol agent for it. The population dynamics of P. crassipes and S. minima at the Dam were overall interchanging due to insect feeding on M. scutellaris reducing P. crassipes populations thus allowing S. minima to become dominant. Intense feeding from M. scutellaris resulted in the reduction of P. crassipes populations. During late autumn and winter, remaining P. crassipes plants experienced frost damage from cold temperatures. The decrease in P. crassipes allowed S. minima to successfully invade, potentially as the result of less competition from P. crassipes, and therefore available resources, such as space, nutrients, and light. However, as spring approached, P. crassipes regenerated from its seed bank, and S. minima populations diminished. The changes in dominance were observed in 2021 and 2022 but in 2023, this trend was not evident as S. minima did not dominate as in previous years. This thesis investigated the combination of competition and herbivory by M. scutellaris on the vigour of P. crassipes and S. minima to understand the dynamics of these two highly invasive species. Competitive abilities of P. crassipes and S. minima were determined using an inverse linear model with plant weight as the yield variable. In the absence of herbivory, P. crassipes, was 4 times more competitive than S. minima, but as competitive when exposed to M. scutellaris feeding. Salvinia minima was 1.2 times as aggressive as P. crassipes in the absence of herbivory, but 2.6 times as competitive when M. scutellaris was established on P. crassipes. In the presence of herbivory on P. crassipes, interspecific competition coefficients from P. crassipes on S. minima were no longer statistically significant. These results indicate that the competitive ability of P. crassipes was reduced through herbivory when grown with S. minima, explaining the temporal dominance between the two species at Hartbeespoort Dam. Some studies have reported that P. crassipes releases allelochemicals as a competitive strategy to algae and phytoplankton. Previously identified allelochemicals include N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine and linoleic acid. This thesis investigated the presence or absence of key chemical compounds released by P. crassipes during allelopathy with microbes and phytoplankton to determine its potential to inhibit S. minima growth. Methanol extracts from P. crassipes roots, leaves, and field samples underwent LC-ESI-MS/MS analysis, creating a molecular network to match chemical profiles. Of the investigated compounds, N-phenyl-1-naphthylamine and N-phenyl-2-naphthylamine were absent in P. crassipes and field samples, while linoleic acid was consistently found. Its presence suggests its potential defensive role against S. minima. Future research should explore allelochemical production in the presence of S. minima and algae to confirm if S. minima elicits an allelochemical response by P. crassipes, or whether the methods used here were insufficient to detect allelochemical production. Understanding whether P. crassipes employs allelochemicals, especially in the presence of S. minima, could shed light on its competitive advantage beyond its invasive nature at the Dam. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
- Authors: Chikodza, Tressia
- Date: 2024-10-11
- Subjects: Pontederia crassipes , Salvinia minima , Pontederiaceae Biological control South Africa , Salviniaceae Biological control South Africa , Population dynamics , Secondary invasion
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/464462 , vital:76513
- Description: Pontederia crassipes is widely regarded as the most damaging floating aquatic weed in terms of its invasive traits, and its impact on aquatic ecosystems. Biological control using host-specific natural enemies is widely used for its control, with the most recent agent released being a planthopper, Megamelus scutellaris, in South Africa and the USA. In South Africa, inundative releases of M. scutellaris have been shown to control the weed even at eutrophic, and high elevation cold sites, such as Hartbeespoort Dam, arguably Africa’s most hypertrophic impoundment. However, subsequent to the control of P. crassipes on Hartbeespoort Dam, in 2021, a secondary invader, Salvinia minima, dominated the water system. Salvinia minima is only known from a handful sites near Hartbeespoort Dam and there is currently no approved biocontrol agent for it. The population dynamics of P. crassipes and S. minima at the Dam were overall interchanging due to insect feeding on M. scutellaris reducing P. crassipes populations thus allowing S. minima to become dominant. Intense feeding from M. scutellaris resulted in the reduction of P. crassipes populations. During late autumn and winter, remaining P. crassipes plants experienced frost damage from cold temperatures. The decrease in P. crassipes allowed S. minima to successfully invade, potentially as the result of less competition from P. crassipes, and therefore available resources, such as space, nutrients, and light. However, as spring approached, P. crassipes regenerated from its seed bank, and S. minima populations diminished. The changes in dominance were observed in 2021 and 2022 but in 2023, this trend was not evident as S. minima did not dominate as in previous years. This thesis investigated the combination of competition and herbivory by M. scutellaris on the vigour of P. crassipes and S. minima to understand the dynamics of these two highly invasive species. Competitive abilities of P. crassipes and S. minima were determined using an inverse linear model with plant weight as the yield variable. In the absence of herbivory, P. crassipes, was 4 times more competitive than S. minima, but as competitive when exposed to M. scutellaris feeding. Salvinia minima was 1.2 times as aggressive as P. crassipes in the absence of herbivory, but 2.6 times as competitive when M. scutellaris was established on P. crassipes. In the presence of herbivory on P. crassipes, interspecific competition coefficients from P. crassipes on S. minima were no longer statistically significant. These results indicate that the competitive ability of P. crassipes was reduced through herbivory when grown with S. minima, explaining the temporal dominance between the two species at Hartbeespoort Dam. Some studies have reported that P. crassipes releases allelochemicals as a competitive strategy to algae and phytoplankton. Previously identified allelochemicals include N-phenyl-1-naphthylamine, N-phenyl-2-naphthylamine and linoleic acid. This thesis investigated the presence or absence of key chemical compounds released by P. crassipes during allelopathy with microbes and phytoplankton to determine its potential to inhibit S. minima growth. Methanol extracts from P. crassipes roots, leaves, and field samples underwent LC-ESI-MS/MS analysis, creating a molecular network to match chemical profiles. Of the investigated compounds, N-phenyl-1-naphthylamine and N-phenyl-2-naphthylamine were absent in P. crassipes and field samples, while linoleic acid was consistently found. Its presence suggests its potential defensive role against S. minima. Future research should explore allelochemical production in the presence of S. minima and algae to confirm if S. minima elicits an allelochemical response by P. crassipes, or whether the methods used here were insufficient to detect allelochemical production. Understanding whether P. crassipes employs allelochemicals, especially in the presence of S. minima, could shed light on its competitive advantage beyond its invasive nature at the Dam. , Thesis (MSc) -- Faculty of Science, Botany, 2024
- Full Text:
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:
- 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:
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:
- 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
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