Post-release evaluation of Megamelus scutellaris Berg. (hemiptera: delphacidae): a biological control agent of water hyacinth Eichhornia crassipes (Mart.) Solms-Laub (Pontederiaceae) in South Africa
- Authors: Miller, Benjamin Erich
- Date: 2019
- Subjects: Megamelus scutellaris Berg. , Delphacidae , Noxious weeds -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Water hyacinth -- Biological control -- South Africa , Biological pest control agents
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/92330 , vital:30710
- Description: Water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub. (Pontederiaceae) is a free-floating aquatic macrophyte from South America that was introduced to South Africa in the 1900s for its attractive ornamental flowers. The plant was classified as a serious invader in the country in the 1970s, eventually becoming the worst invasive aquatic plant in South Africa. Biological control is widely regarded as the most effective method of managing water hyacinth, as it is ecologically safe, cost-effective, and self-sustaining. To date, nine biological control agents have been released in South Africa against water hyacinth, including eight arthropods and a pathogen. Due to the cumulative effects of highly eutrophic waterbodies, which mitigate the damage caused by biological control, and the cold winters which inhibit the rate of biological control agent population build up, South Africa currently has more biological control agents released on water hyacinth than anywhere else in the world. The need for a cold-tolerant agent that can reproduce and develop quickly, while still being damaging to water hyacinth in eutrophic systems, led to the introduction of the most recently released water hyacinth biological control agent, the planthopper Megamelus scutellaris Berg (Hemiptera: Delphacidae), which was initially collected from Argentina. This thesis formed the first post-release evaluation of M. scutellaris since its release in South Africa in 2013. It included a greenhouse experiment to measure the agent’s feeding damage in relation to different nutrient levels and stocking rates, as well as a field component to evaluate both the post-winter recovery of M. scutellaris, and a nationwide survey to measure the establishment of the agent around the country in relation to climate, water quality, and plant health. In the greenhouse experiment, the feeding damage was quantified using measurements of plant growth parameters and chlorophyll fluorometry. It was found that, like other biological control agents of water hyacinth, M. scutellaris was most damaging when released in high numbers on plants grown at medium nutrient levels, and less effective on plants grown at elevated nutrient levels. A water hyacinth infestation on the Kubusi River was selected for the evaluation of the post-winter recovery of M. scutellaris. The Kubusi River is both the first site where M. scutellaris was released, and the coldest site where water hyacinth biological control agents have established successfully in South Africa. Monthly visits tracking seasonal plant health characteristics and agent population densities indicated that the populations of M. scutellaris were impacted most significantly by the season. Low temperatures led to the water hyacinth plants being of poor quality during the winter, which had a subsequent negative effect on the agent populations. The agents could only fully recover by late summer, which meant that the plants were without any significant biological control through the initial phases of the growing season, when they were most vulnerable, and a significant lag-phase occurred between the recovery of the plants and the recovery of the agent population after the winter bottleneck. A survey of all sites where M. scutellaris had been released in South Africa yielded 16 sites where the agents had successfully established, having survived at least one full winter. Among these sites were four sites where the agents were found without them having been released, indicating that they can disperse unaided to new sites. The temperature was a major factor responsible for the success or failure of establishment, with very few agents surviving in the hot areas of South Africa or in areas with a high frost incidence. The density of M. scutellaris was higher in nutrient-rich water, and on plants with more leaves, suggesting that the quality of the plants also contributed to establishment. The results of this thesis showed that M. scutellaris is able to establish successfully in South Africa, and that the agents are capable of causing significant damage to water hyacinth, making it a promising addition to the biological control programme. Novel methods of measuring subtle insect feeding damage in plants and quantifying agent populations are also discussed, along with suggestions for the future implementation of M. scutellaris in South Africa.
- Full Text:
- Authors: Miller, Benjamin Erich
- Date: 2019
- Subjects: Megamelus scutellaris Berg. , Delphacidae , Noxious weeds -- Biological control -- South Africa , Aquatic weeds -- Biological control -- South Africa , Water hyacinth -- Biological control -- South Africa , Biological pest control agents
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/92330 , vital:30710
- Description: Water hyacinth, Eichhornia crassipes (Mart.) Solms-Laub. (Pontederiaceae) is a free-floating aquatic macrophyte from South America that was introduced to South Africa in the 1900s for its attractive ornamental flowers. The plant was classified as a serious invader in the country in the 1970s, eventually becoming the worst invasive aquatic plant in South Africa. Biological control is widely regarded as the most effective method of managing water hyacinth, as it is ecologically safe, cost-effective, and self-sustaining. To date, nine biological control agents have been released in South Africa against water hyacinth, including eight arthropods and a pathogen. Due to the cumulative effects of highly eutrophic waterbodies, which mitigate the damage caused by biological control, and the cold winters which inhibit the rate of biological control agent population build up, South Africa currently has more biological control agents released on water hyacinth than anywhere else in the world. The need for a cold-tolerant agent that can reproduce and develop quickly, while still being damaging to water hyacinth in eutrophic systems, led to the introduction of the most recently released water hyacinth biological control agent, the planthopper Megamelus scutellaris Berg (Hemiptera: Delphacidae), which was initially collected from Argentina. This thesis formed the first post-release evaluation of M. scutellaris since its release in South Africa in 2013. It included a greenhouse experiment to measure the agent’s feeding damage in relation to different nutrient levels and stocking rates, as well as a field component to evaluate both the post-winter recovery of M. scutellaris, and a nationwide survey to measure the establishment of the agent around the country in relation to climate, water quality, and plant health. In the greenhouse experiment, the feeding damage was quantified using measurements of plant growth parameters and chlorophyll fluorometry. It was found that, like other biological control agents of water hyacinth, M. scutellaris was most damaging when released in high numbers on plants grown at medium nutrient levels, and less effective on plants grown at elevated nutrient levels. A water hyacinth infestation on the Kubusi River was selected for the evaluation of the post-winter recovery of M. scutellaris. The Kubusi River is both the first site where M. scutellaris was released, and the coldest site where water hyacinth biological control agents have established successfully in South Africa. Monthly visits tracking seasonal plant health characteristics and agent population densities indicated that the populations of M. scutellaris were impacted most significantly by the season. Low temperatures led to the water hyacinth plants being of poor quality during the winter, which had a subsequent negative effect on the agent populations. The agents could only fully recover by late summer, which meant that the plants were without any significant biological control through the initial phases of the growing season, when they were most vulnerable, and a significant lag-phase occurred between the recovery of the plants and the recovery of the agent population after the winter bottleneck. A survey of all sites where M. scutellaris had been released in South Africa yielded 16 sites where the agents had successfully established, having survived at least one full winter. Among these sites were four sites where the agents were found without them having been released, indicating that they can disperse unaided to new sites. The temperature was a major factor responsible for the success or failure of establishment, with very few agents surviving in the hot areas of South Africa or in areas with a high frost incidence. The density of M. scutellaris was higher in nutrient-rich water, and on plants with more leaves, suggesting that the quality of the plants also contributed to establishment. The results of this thesis showed that M. scutellaris is able to establish successfully in South Africa, and that the agents are capable of causing significant damage to water hyacinth, making it a promising addition to the biological control programme. Novel methods of measuring subtle insect feeding damage in plants and quantifying agent populations are also discussed, along with suggestions for the future implementation of M. scutellaris in South Africa.
- Full Text:
The thermal physiology of Stenopelmus rufinasus and Neohydronomus affinis (Coleoptera: Curculionidae), biological control agents for the invasive alien aquatic weeds Azolla filiculoides and Pistia stratiotes respectively
- Authors: Mvandaba, Sisanda F
- Date: 2018
- Subjects: Beetles -- South Africa , Curculionidae -- South Africa , Azolla filiculoides -- South Africa , Water lettuce -- South Africa , Aquatic weeds -- Biological control -- South Africa , Stenopelmus rufinasus , Neohydronomus affinis
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/62362 , vital:28158
- Description: Water lettuce, Pistia stratiotes L. (Araceae), and red water fern, Azolla filiculoides Lam. (Azollaceae), are floating aquatic macrophytes that have become problematic invaders in numerous South African waterbodies. Two weevils, Neohydronomus affinis Hustache 1926 (Coleoptera: Curculionidae) and Stenopelmus rufinasus Gyllenhal 1936 (Coleoptera: Curculionidae), are successful biological control agents of these two species, respectively, in South Africa. However, nothing is known about the thermal physiology of these two species Therefore, the aim of this study was to investigate the thermal physiologies of these two species to explain their establishment, distribution and impact in the field. Laboratory based thermal physiology trials showed that both weevils were widely tolerant of cold and warm temperatures. The CTmin of N. affinis was determined to be 5.5 ± 0.312°C and the CTmax was 44 ± 0.697°C, while the CTmin of S. rufinasus was 5.4 ± 0.333°C and the CTmax was 44.5 ± 0.168°C. In addition, the lower lethal temperatures were -9.8 ± 0.053°C and -7.2 ± 0.19°C, and the upper lethal temperatures were 42.8 ± 0.053°C and 41.9 ± 0.19°C respectively. These results suggest that both species should not be limited by cold winter temperatures, as previously thought. This is evident in the field, where S. rufinasus has established widely on A. filiculoides, despite local cold climates in some areas of the plant’s distribution. Even though N. affinis has a similar thermal range, and should therefore theoretically reflect a similar distribution to S. rufinasus throughout South Africa, its distribution is limited by the range of its host, which is restricted to the warmer regions of the country, as is its biocontrol agent. Using the reduced major axis regression method, the development for N. affinis was described using the formulay=12.976x+435.24, while the development of S. rufinasus was described by y=13.6x+222.45. These results showed that S. rufinasus develops much faster, in fact almost twice as quickly, than N. affinis. Using these formulae and temperature data obtained from the South African Weather Service, N. affinis was predicted to complete between 4 and 9 generations per year in South Africa, while S. rufinasus was predicted to complete between 5 and 14 generations per year around the country. This study showed that although the native range of these two species is warm temperate to tropical, they possess sufficient thermal plasticity to not only establish, but also damage their respective host plants in far cooler climates. Thus, in South Africa N. affinis and S. rufinasus are limited by the distribution of their target weeds and not climate.
- Full Text:
- Authors: Mvandaba, Sisanda F
- Date: 2018
- Subjects: Beetles -- South Africa , Curculionidae -- South Africa , Azolla filiculoides -- South Africa , Water lettuce -- South Africa , Aquatic weeds -- Biological control -- South Africa , Stenopelmus rufinasus , Neohydronomus affinis
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/62362 , vital:28158
- Description: Water lettuce, Pistia stratiotes L. (Araceae), and red water fern, Azolla filiculoides Lam. (Azollaceae), are floating aquatic macrophytes that have become problematic invaders in numerous South African waterbodies. Two weevils, Neohydronomus affinis Hustache 1926 (Coleoptera: Curculionidae) and Stenopelmus rufinasus Gyllenhal 1936 (Coleoptera: Curculionidae), are successful biological control agents of these two species, respectively, in South Africa. However, nothing is known about the thermal physiology of these two species Therefore, the aim of this study was to investigate the thermal physiologies of these two species to explain their establishment, distribution and impact in the field. Laboratory based thermal physiology trials showed that both weevils were widely tolerant of cold and warm temperatures. The CTmin of N. affinis was determined to be 5.5 ± 0.312°C and the CTmax was 44 ± 0.697°C, while the CTmin of S. rufinasus was 5.4 ± 0.333°C and the CTmax was 44.5 ± 0.168°C. In addition, the lower lethal temperatures were -9.8 ± 0.053°C and -7.2 ± 0.19°C, and the upper lethal temperatures were 42.8 ± 0.053°C and 41.9 ± 0.19°C respectively. These results suggest that both species should not be limited by cold winter temperatures, as previously thought. This is evident in the field, where S. rufinasus has established widely on A. filiculoides, despite local cold climates in some areas of the plant’s distribution. Even though N. affinis has a similar thermal range, and should therefore theoretically reflect a similar distribution to S. rufinasus throughout South Africa, its distribution is limited by the range of its host, which is restricted to the warmer regions of the country, as is its biocontrol agent. Using the reduced major axis regression method, the development for N. affinis was described using the formulay=12.976x+435.24, while the development of S. rufinasus was described by y=13.6x+222.45. These results showed that S. rufinasus develops much faster, in fact almost twice as quickly, than N. affinis. Using these formulae and temperature data obtained from the South African Weather Service, N. affinis was predicted to complete between 4 and 9 generations per year in South Africa, while S. rufinasus was predicted to complete between 5 and 14 generations per year around the country. This study showed that although the native range of these two species is warm temperate to tropical, they possess sufficient thermal plasticity to not only establish, but also damage their respective host plants in far cooler climates. Thus, in South Africa N. affinis and S. rufinasus are limited by the distribution of their target weeds and not climate.
- Full Text:
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