Comparative phenology of Lepidoptera on genetically modified BT- and non-BT maize
Van Wyk, Annemie
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The maize stem borers, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) and Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) are economically important pests of maize in South Africa. Genetically modified Bt maize (MON810) expressing Cry1Ab protein is used to control these pests on approximately 425 000 hectares in South Africa. Before this study no information was available on the diversity of Lepidoptera on maize in South Africa or the potential impact of Bt maize on non-target Lepidoptera species under field conditions. There was also no information on the susceptibility to Bt maize of another stem borer species, Sesamia calamistis (Hampson) (Lepidoptera: Noctuidae), which is not a target species of Bt maize. The aims of this study were to determine which Lepidoptera species occur and feed on maize and could be directly exposed to Bt toxin as well as to assess the levels of infestation of target stem borer species and non-target Lepidoptera species on Bt- and non-Bt maize fields. Field collections of Lepidoptera that were directly exposed to Bt toxin through feeding on Bt maize plants were done between January 2004 and May 2006. Surveys were conducted in the North-West, Free State, Gauteng and Limpopo provinces. In order to quantify infestation levels and incidence of larvae on plants, sampling was done by inspecting between 300 – 900 plants per field. Studies were also done to compare the incidence of damaged plants and larvae on plants between Bt- and adjacent non-Bt maize fields. The susceptibility of S. calamistis to several Bt maize hybrids was evaluated under laboratory and greenhouse conditions. Fifteen species of Lepidoptera were recorded on maize plants. The following six species were recorded to feed on Bt maize and were reared on Bt maize until the adult stage: Acantholeucania loreyi (Noctuidae), Agrotis segetum (Noctuidae), B. fusca (Noctuidae), Helicoverpa armigera (Noctuidae), Eublemma gayneri (Noctuidae) and Nola phaeocraspis (Nolidae). Although Bt maize was damaged by several species of leaf, stem and ear feeding Lepidoptera in this study, the incidence of damage was always significantly lower on Bt maize fields than susceptible fields. This study provided base line data on Lepidoptera that feed on Bt maize in South Africa. Non-target Lepidoptera species that are directly exposed to Bt toxin was identified. An ecological model wasused to develop a preliminary risk assessment for Bt maize through which priority species for research and monitoring was identified as well as species that are at risk of resistance development.
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