Interactions between two biological control agents released on Pereskia aculeata Miller (Cactaceae), in South Africa
- Authors: Mnqeta, Zezethu
- Date: 2017
- Subjects: Pereskia aculeata -- Biological control , Cactus -- Biological control -- South Africa , Alien plants -- Biological control -- South Africa , Flea beetles -- South Africa , Coreidae -- South Africa , Insects as biological pest control agents -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/7949 , vital:21327
- Description: Pereskia aculeata Miller (Cactaceae) is an alien invasive plant introduced into South Africa from Brazil, which has negative impacts on native biodiversity in South Africa. Phenrica guerini Bechyne (Chrysomelidae) and Catorhintha schaffneri Brailovsky & Garcia (Coreidae) are two biological control agents released against P. aculeata in South Africa. Phenrica guerini was first released against P. aculeata, in 1991, followed by C. schaffneri in 2014. The overall aim of this study was to improve the biological control programme against P. aculeata in order to decrease its density to a level where it does not threaten the native biodiversity of South Africa. The first part of this study evaluated the efficacy of P. guerini on P. aculeata at Port Alfred (Eastern Cape) to better understand the role of P. guerini in the biological control of P. aculeata. An insecticide exclusion experiment was conducted over 100 days. Plots with P. guerini had a mean of 187 (SE ± 62) fewer leaves/m2 than plots without P. guerini. The agent reduced percentage cover in plots with P. guerini, with a mean of 19.42% (SE ± 3.15) lower cover than plots without P. guerini. Although P. guerini had an impact on P. aculeata at Port Alfred previous studies have indicated that a reduction to below 50% cover is required for native biodiversity to recover and the agent only reduced cover to 62% at Port Alfred. Phenrica guerini has therefore not reduced percentage cover sufficiently to completely control the weed. The data collected from Port Alfred was compared to the performance of the agent nationwide. Although P. guerini was found at far more sites than previously recorded, there were very few sites with comparable levels of damage to Port Alfred. This evidence suggests that P. guerini is not sufficiently damaging to reduce P. aculeata to acceptable levels and other biological control agents should be considered.Interactions between two biological control agents can have complex and unexpected impacts for a biological control programme. The second part of this study was to investigate interactions between C. schaffneri and P. guerini under laboratory conditions to test whether the two agents, individually or jointly, enhanced or reduced their impact on P. aculeata. Potted P. aculeata plants were exposed to one of four treatments: control (no agents), P. guerini only, C. schaffneri only and both species in combination. Four stocking densities, ranging from 2 to 12 insects per plant were used. Catorhintha schaffneri alone at high densities was more damaging than all other treatments with a significantly greater reduction in the mean number of leaves, 11.7 (SE ± 1.29), and shoot lengths, 2.17cm (SE ± O. 75). Even at lower density treatments, the combination of the two agents was not significantly more damaging than C. schaffneri alone and C. schaffneri was always more damaging than P. guerini alone. Mortality of P. guerini was significantly higher than C. schaffneri at the highest stocking density when in combination. Phenrica guerini contributes towards the biological control of P. aculeata at some sites in South Africa but not enough to completely control the weed. The antagonistic interaction between P. guerini and C. schaffneri suggests that these agents should not be released together because this would impact negatively on the overall biocontrol programme against P. aculeata. Catorhintha schaffneri should be released at sites were P. guerini is not present and evaluations of the success of this agent in the field should be conducted. Extrapolation of laboratory-based studies into the field is often challenging so mass-rearing of P. guerini should continue until there is convincing proof that C. schaffneri alone is more effective than P. guerini in the field.
- Full Text:
- Authors: Mnqeta, Zezethu
- Date: 2017
- Subjects: Pereskia aculeata -- Biological control , Cactus -- Biological control -- South Africa , Alien plants -- Biological control -- South Africa , Flea beetles -- South Africa , Coreidae -- South Africa , Insects as biological pest control agents -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/7949 , vital:21327
- Description: Pereskia aculeata Miller (Cactaceae) is an alien invasive plant introduced into South Africa from Brazil, which has negative impacts on native biodiversity in South Africa. Phenrica guerini Bechyne (Chrysomelidae) and Catorhintha schaffneri Brailovsky & Garcia (Coreidae) are two biological control agents released against P. aculeata in South Africa. Phenrica guerini was first released against P. aculeata, in 1991, followed by C. schaffneri in 2014. The overall aim of this study was to improve the biological control programme against P. aculeata in order to decrease its density to a level where it does not threaten the native biodiversity of South Africa. The first part of this study evaluated the efficacy of P. guerini on P. aculeata at Port Alfred (Eastern Cape) to better understand the role of P. guerini in the biological control of P. aculeata. An insecticide exclusion experiment was conducted over 100 days. Plots with P. guerini had a mean of 187 (SE ± 62) fewer leaves/m2 than plots without P. guerini. The agent reduced percentage cover in plots with P. guerini, with a mean of 19.42% (SE ± 3.15) lower cover than plots without P. guerini. Although P. guerini had an impact on P. aculeata at Port Alfred previous studies have indicated that a reduction to below 50% cover is required for native biodiversity to recover and the agent only reduced cover to 62% at Port Alfred. Phenrica guerini has therefore not reduced percentage cover sufficiently to completely control the weed. The data collected from Port Alfred was compared to the performance of the agent nationwide. Although P. guerini was found at far more sites than previously recorded, there were very few sites with comparable levels of damage to Port Alfred. This evidence suggests that P. guerini is not sufficiently damaging to reduce P. aculeata to acceptable levels and other biological control agents should be considered.Interactions between two biological control agents can have complex and unexpected impacts for a biological control programme. The second part of this study was to investigate interactions between C. schaffneri and P. guerini under laboratory conditions to test whether the two agents, individually or jointly, enhanced or reduced their impact on P. aculeata. Potted P. aculeata plants were exposed to one of four treatments: control (no agents), P. guerini only, C. schaffneri only and both species in combination. Four stocking densities, ranging from 2 to 12 insects per plant were used. Catorhintha schaffneri alone at high densities was more damaging than all other treatments with a significantly greater reduction in the mean number of leaves, 11.7 (SE ± 1.29), and shoot lengths, 2.17cm (SE ± O. 75). Even at lower density treatments, the combination of the two agents was not significantly more damaging than C. schaffneri alone and C. schaffneri was always more damaging than P. guerini alone. Mortality of P. guerini was significantly higher than C. schaffneri at the highest stocking density when in combination. Phenrica guerini contributes towards the biological control of P. aculeata at some sites in South Africa but not enough to completely control the weed. The antagonistic interaction between P. guerini and C. schaffneri suggests that these agents should not be released together because this would impact negatively on the overall biocontrol programme against P. aculeata. Catorhintha schaffneri should be released at sites were P. guerini is not present and evaluations of the success of this agent in the field should be conducted. Extrapolation of laboratory-based studies into the field is often challenging so mass-rearing of P. guerini should continue until there is convincing proof that C. schaffneri alone is more effective than P. guerini in the field.
- Full Text:
Investigating herbivory and plant origin on tall-statured grasses in South Africa
- Authors: Canavan, Kim N
- Date: 2017
- Subjects: Insects as biological pest control agents -- South Africa , Arundo donax , Giant reed -- South Africa , Giant reed -- Biological control -- South Africa , Phragmites australis , Phragmites mauritianus , Phragmites , Tetramesa romana , Biological invasions -- South Africa , Wasps -- Host plants , Wasps -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/6147 , vital:21051
- Description: South African riparian zones have been heavily degraded through anthropogenic activities such as dam construction and extraction of water for irrigation, which has resulted in a loss of ecosystem services and functioning. A consequence of such disturbances to riparian areas is in their susceptibility to invasive alien species (IAS). One such IAS is the giant reed, Arundo donax L. (Poaceae), introduced to South Africa in the 1700s largely for erosion control. Arundo donax has since greatly expanded in the country and is now one of the most abundant IAS. Arundo donax has been found to displace native vegetation and in South Africa this will most likely lead to the displacement of the native tall-statured grasses, Phragmites australis (Cav.) Trin. ex Steud. and Phragmites mauritianus Kunth. This study aimed to enhance our understanding of the tall-statured grasses A. donax, P. australis and P. mauritianus to better manage them in riparian areas. For A. donax, biological control is seen as the most viable option to control stands in the long-term. However, before such a programme is put in place, it is important to first collect baseline data that can be used to guide the direction of the biological control project in South Africa. For the Phragmites spp., despite being a dominant vegetative type in riparian areas, very little is known about their status in South Africa. Furthermore, there have been increasing reports of both Phragmites species having an expansion of their range and abundance. In North America, there has been a similar trend of reed expansion and through molecular work it was determined that a cryptic invasion has occurred with the introduction of an invasive non-native haplotype from Europe. It is therefore unknown if Phragmites spp. populations are expanding due to anthropogenic activities or due to a cryptic invasion. To address these shortfalls in knowledge the study investigated the tall-statured grasses in two parts; firstly, molecular techniques are used to explore the plant origin and genetic diversity of A. donax, P. australis and P. mauritianus and secondly using the Enemy Release Hypothesis as a framework, herbivore assemblages for each reed was determined across their distribution in South Africa. Molecular-techniques determined that both P. australis and P. mauritianus had only one haplotype - known as haplotype K and haplotype V respectively, across their distribution. For P. australis, haplotype K shares a close connection with populations from a Mediterranean lineage and this was further confirmed with a shared grass-waxy band. The direction and timing of genetic exchange between the two regions could not be ascertained and thus still remains unknown. Microsatellite analysis determined that both Phragmites spp. had a high genetic diversity compared to worldwide lineages. With no evidence of any cryptic invasions of haplotypes from other regions, both Phragmites spp. populations are likely to be native to South Africa. For A. donax all populations across South Africa were determined to be haplotype M1; a cosmopolitan haplotype that has an ancient native range in Afghanistan and Pakistan (Indus Valley). Populations were found to have no genetic diversity and thus can be considered one clone. A pre-introductory survey determined a list of herbivores associated with each tall- statured grass. For A. donax, a total of seven herbivores were found. Of these, one herbivore, a galling wasp, Tetramesa romana Walker (Hymenoptera: Eurytomidae) was found to be highly abundant and widely distributed in South Africa. Tetramesa romana is already a biological control agent in North America and thus is likely exerting some pressure on A. donax populations in South Africa. For both Phragmites spp. a total of ten herbivores were found, although having higher species richness compared to A. donax, when compared to other regions, these native species have a relatively low species richness. Providing baseline data on plant origin, genetic diversity and herbivory on A. donax, P. australis and P. mauritianus has provided important information on managing these species in riparian ecosystems in South Africa. For the Phragmites spp. with no evidence of any cryptic invasions, it is recommended that reed stands continue to be managed as native species. Phragmites spp. are important dominant vegetative species and thus should be protected; however, if reed stands become expansive, control methods can be put in place to focus on managing spread and abundance. For A. donax, this study was able to provide pivotal information in guiding the biological control programme. By determining the ancient lineage of South African populations, research can be focused in this area to find potential biological control agents. Lastly, the pre-introductory survey determined that a biological control agent, T. romana was already established with an unknown introduction and also highlighted potential plant parts that should be targeted. In particular, no rhizome feeding herbivores were found in South Africa and therefore this highlights an important niche that should be explored in biological control agents.
- Full Text:
- Authors: Canavan, Kim N
- Date: 2017
- Subjects: Insects as biological pest control agents -- South Africa , Arundo donax , Giant reed -- South Africa , Giant reed -- Biological control -- South Africa , Phragmites australis , Phragmites mauritianus , Phragmites , Tetramesa romana , Biological invasions -- South Africa , Wasps -- Host plants , Wasps -- South Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/6147 , vital:21051
- Description: South African riparian zones have been heavily degraded through anthropogenic activities such as dam construction and extraction of water for irrigation, which has resulted in a loss of ecosystem services and functioning. A consequence of such disturbances to riparian areas is in their susceptibility to invasive alien species (IAS). One such IAS is the giant reed, Arundo donax L. (Poaceae), introduced to South Africa in the 1700s largely for erosion control. Arundo donax has since greatly expanded in the country and is now one of the most abundant IAS. Arundo donax has been found to displace native vegetation and in South Africa this will most likely lead to the displacement of the native tall-statured grasses, Phragmites australis (Cav.) Trin. ex Steud. and Phragmites mauritianus Kunth. This study aimed to enhance our understanding of the tall-statured grasses A. donax, P. australis and P. mauritianus to better manage them in riparian areas. For A. donax, biological control is seen as the most viable option to control stands in the long-term. However, before such a programme is put in place, it is important to first collect baseline data that can be used to guide the direction of the biological control project in South Africa. For the Phragmites spp., despite being a dominant vegetative type in riparian areas, very little is known about their status in South Africa. Furthermore, there have been increasing reports of both Phragmites species having an expansion of their range and abundance. In North America, there has been a similar trend of reed expansion and through molecular work it was determined that a cryptic invasion has occurred with the introduction of an invasive non-native haplotype from Europe. It is therefore unknown if Phragmites spp. populations are expanding due to anthropogenic activities or due to a cryptic invasion. To address these shortfalls in knowledge the study investigated the tall-statured grasses in two parts; firstly, molecular techniques are used to explore the plant origin and genetic diversity of A. donax, P. australis and P. mauritianus and secondly using the Enemy Release Hypothesis as a framework, herbivore assemblages for each reed was determined across their distribution in South Africa. Molecular-techniques determined that both P. australis and P. mauritianus had only one haplotype - known as haplotype K and haplotype V respectively, across their distribution. For P. australis, haplotype K shares a close connection with populations from a Mediterranean lineage and this was further confirmed with a shared grass-waxy band. The direction and timing of genetic exchange between the two regions could not be ascertained and thus still remains unknown. Microsatellite analysis determined that both Phragmites spp. had a high genetic diversity compared to worldwide lineages. With no evidence of any cryptic invasions of haplotypes from other regions, both Phragmites spp. populations are likely to be native to South Africa. For A. donax all populations across South Africa were determined to be haplotype M1; a cosmopolitan haplotype that has an ancient native range in Afghanistan and Pakistan (Indus Valley). Populations were found to have no genetic diversity and thus can be considered one clone. A pre-introductory survey determined a list of herbivores associated with each tall- statured grass. For A. donax, a total of seven herbivores were found. Of these, one herbivore, a galling wasp, Tetramesa romana Walker (Hymenoptera: Eurytomidae) was found to be highly abundant and widely distributed in South Africa. Tetramesa romana is already a biological control agent in North America and thus is likely exerting some pressure on A. donax populations in South Africa. For both Phragmites spp. a total of ten herbivores were found, although having higher species richness compared to A. donax, when compared to other regions, these native species have a relatively low species richness. Providing baseline data on plant origin, genetic diversity and herbivory on A. donax, P. australis and P. mauritianus has provided important information on managing these species in riparian ecosystems in South Africa. For the Phragmites spp. with no evidence of any cryptic invasions, it is recommended that reed stands continue to be managed as native species. Phragmites spp. are important dominant vegetative species and thus should be protected; however, if reed stands become expansive, control methods can be put in place to focus on managing spread and abundance. For A. donax, this study was able to provide pivotal information in guiding the biological control programme. By determining the ancient lineage of South African populations, research can be focused in this area to find potential biological control agents. Lastly, the pre-introductory survey determined that a biological control agent, T. romana was already established with an unknown introduction and also highlighted potential plant parts that should be targeted. In particular, no rhizome feeding herbivores were found in South Africa and therefore this highlights an important niche that should be explored in biological control agents.
- Full Text:
Post-release evaluation of the biological control programme against Cereus jamacaru De Candolle (Cactaceae), in South Africa
- Authors: Sutton, Guy Frederick
- Date: 2017
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/7982 , vital:21330
- Description: Cereus jamacaru De Candolle (Cactaceae) is an environment-transforming weed of both agricultural and wildlife habitats in South Africa. Weed infestations reduce carrying capacity of the land, and pose a risk to livestock, wildlife and human safety due to the plant’s spiny thorns. The weed is considered to be under complete biological control in South Africa, due to its introduced control agent, Hypogeococcus festerianus Lizer y Trelles (Hemiptera: Pseudococcidae), although observational reports suggest that the level of success achieved is variable. In this thesis, a formal post-release evaluation of this biological control programme was conducted, specifically to determine the efficacy of H. festerianus as a biological control agent, and to identify factors which may limit or constrain the level of success achieved by the control agent. These data were collected with the intention of improving the control of C. jamacaru in South Africa. A field-based study of C. jamacaru population demographics investigated the efficacy of H. festerianus as a biological control agent of the weed, by integrating weed growth, fecundity and survival metrics with C. jamacaru population dynamics and demographic patterns from 8 sites where H. festerianus was present and 14 sites where the control agent was absent. The findings indicated that H. festerianus significantly reduced weed fecundity, which resulted in fewer seedling recruits, and that levels of plant mortality were greater at sites where H. festerianus was present. The reduction in weed fecundity and survival translated into negative population-level consequences for H. festerianus. Weed-population age frequency distributions in the absence of H. festerianus demonstrated a “reverse J-shaped” distribution, indicative of high recruitment rates and population stability, while C. jamacaru populations infected with H. festerianus were described by bell-shaped distributions, and were typified by limited recruitment, or a complete lack thereof. By constraining recruitment and inhibiting selfregeneration, H. festerianus appears to regulate populations of C. jamacaru. Predation and parasitism of H. festerianus was believed to be a limiting factor for the biological control programme against C. jamacaru in South Africa, although no formal evaluation of this claim had been undertaken. Accordingly, the assemblage of natural enemies acquired by H. festerianus in South Africa was identified by field-collections of infected H. festerianus gall-material. Further, timed point-count surveys of natural enemies associated with H. festerianus were performed and integrated with the data on the impact of H. festerianus on weed population dynamics to assess the impact of two prominent predaceous taxa on H. festerianus efficacy as a biological control agent. Although H. festerianus had acquired a diverse suite of novel natural enemies in South Africa, this has not prevented the biocontrol agent from having an impact on C. jamacaru populations, although other subtler effects cannot be ruled out. This study showed that biological control efforts employing H. festerianus for the management of C. jamacaru have been successful. Furthermore, these data demonstrated the utility of retrospective analyses in developing and improving the science of biological control, specifically how to improve candidate agent prioritisation, determining how many agents are required for successful biological control, and how to evaluate the success of biological control efforts. Improvements in our theoretical understanding of biological control will undoubtedly reduce costs of biological control programmes, improve success rates, and increase the predictability of biological control.
- Full Text:
- Authors: Sutton, Guy Frederick
- Date: 2017
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/7982 , vital:21330
- Description: Cereus jamacaru De Candolle (Cactaceae) is an environment-transforming weed of both agricultural and wildlife habitats in South Africa. Weed infestations reduce carrying capacity of the land, and pose a risk to livestock, wildlife and human safety due to the plant’s spiny thorns. The weed is considered to be under complete biological control in South Africa, due to its introduced control agent, Hypogeococcus festerianus Lizer y Trelles (Hemiptera: Pseudococcidae), although observational reports suggest that the level of success achieved is variable. In this thesis, a formal post-release evaluation of this biological control programme was conducted, specifically to determine the efficacy of H. festerianus as a biological control agent, and to identify factors which may limit or constrain the level of success achieved by the control agent. These data were collected with the intention of improving the control of C. jamacaru in South Africa. A field-based study of C. jamacaru population demographics investigated the efficacy of H. festerianus as a biological control agent of the weed, by integrating weed growth, fecundity and survival metrics with C. jamacaru population dynamics and demographic patterns from 8 sites where H. festerianus was present and 14 sites where the control agent was absent. The findings indicated that H. festerianus significantly reduced weed fecundity, which resulted in fewer seedling recruits, and that levels of plant mortality were greater at sites where H. festerianus was present. The reduction in weed fecundity and survival translated into negative population-level consequences for H. festerianus. Weed-population age frequency distributions in the absence of H. festerianus demonstrated a “reverse J-shaped” distribution, indicative of high recruitment rates and population stability, while C. jamacaru populations infected with H. festerianus were described by bell-shaped distributions, and were typified by limited recruitment, or a complete lack thereof. By constraining recruitment and inhibiting selfregeneration, H. festerianus appears to regulate populations of C. jamacaru. Predation and parasitism of H. festerianus was believed to be a limiting factor for the biological control programme against C. jamacaru in South Africa, although no formal evaluation of this claim had been undertaken. Accordingly, the assemblage of natural enemies acquired by H. festerianus in South Africa was identified by field-collections of infected H. festerianus gall-material. Further, timed point-count surveys of natural enemies associated with H. festerianus were performed and integrated with the data on the impact of H. festerianus on weed population dynamics to assess the impact of two prominent predaceous taxa on H. festerianus efficacy as a biological control agent. Although H. festerianus had acquired a diverse suite of novel natural enemies in South Africa, this has not prevented the biocontrol agent from having an impact on C. jamacaru populations, although other subtler effects cannot be ruled out. This study showed that biological control efforts employing H. festerianus for the management of C. jamacaru have been successful. Furthermore, these data demonstrated the utility of retrospective analyses in developing and improving the science of biological control, specifically how to improve candidate agent prioritisation, determining how many agents are required for successful biological control, and how to evaluate the success of biological control efforts. Improvements in our theoretical understanding of biological control will undoubtedly reduce costs of biological control programmes, improve success rates, and increase the predictability of biological control.
- Full Text:
- «
- ‹
- 1
- ›
- »