Status of resistance of Helicoverpa armigera (Lepidoptera: Noctuidae) and Diparopsis castanea (Lepidoptera: Noctuidae) to Bt cotton in South Africa
Pretorius, Johannes Diederik
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Genetically modified (GM) cotton expressing Cry1Ac proteins was released in South Africa in 1997 for control of the bollworm complex on this crop. No reports of the failure of Bollgard® cotton to control these pests have yet been made. Throughout the world there are concerns about the development of resistance of target pests to Bt cotton due to the use of only one Bt gene. The aim of this study was to determine if Helicoverpa armigera (Lepidoptera: Noctuidae) and Diparopsis castanea (Lepidoptera: Noctuidae) developed resistance to Bt cotton in South Africa. To determine if H. armigera developed resistance, laboratory experiments were conducted to determine the levels of larval survival and development time when feeding on Bt and non–Bt cotton. Bollworm populations were collected on maize and cotton at different sites in South Africa and reared on Bt and non–Bt cotton under laboratory conditions. Results showed that some populations survived on Bt cotton and that a significant proportion of the individuals successfully completed their life cycles on Bt cotton. Surveys were also conducted amongst cotton farmers to determine the levels of compliance to the refuge strategy that has to be implemented by farmers as an insect resistance management (IRM) strategy to delay resistance development. The levels of compliance to refugia requirements were low and farmers generally only started planting refugia several years after they planted Bt cotton for the first time. The development of resistance of H. armigera to Bt cotton in South Africa can possibly be ascribed to non–compliance to the prescribed refuge requirements. No conclusions can be made on resistance of D. castanea to Bt cotton but the relatively long time to mortality of larvae could indicate development of tolerance to Cry1Ac proteins. The new generation Bollgard II® cotton, expressing both Cry1Ac and Cry2Ab2 proteins, has been released in South Africa during the 2010/11 growing season and field observations showed effective control of the bollworm complex at several sites in the country. Monitoring of refuge compliance levels as well as resistance development in the bollworm complex to Bollgard II® cotton is necessary to ensure the future success of GM cotton.
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