Response of Chilo partellus (Lepidoptera : Crambidae) to Bt maize in South Africa
Maize is an important food resource for humans and their livestock. South Africa is one of the top maize producing countries and also one of the largest producers of genetically modified maize in the world. Maize pests provide important challenges to the sustainable production of maize in Africa and pests such as Busseola fusca (Lepidoptera: Noctuidae), and Chilo partellus (Lepidoptera: Crambidae), pose a serious threat to maize production since borer damage contributes to yield losses and poor grain quality. Bt maize was developed to target lepidopteran stem borers and C. partellus has been effectively controlled by Bt maize since its cultivation commenced during 1998. Over the years several Lepidoptera pests developed resistance to transgenic Bt crops. In South Africa, the African maize stem borer, B. fusca developed resistance to the MON810 Bt event, only eight years after the first release of Bt maize. However, the spotted stem borer C. partellus has not developed resistance to Bt maize in South Africa. The non-compliance to refuge requirements as part of an insect resistance management strategy contributed to resistance evolution of B. fusca and should therefore also have resulted in relatively quick resistance evolution in C. partellus. The aims of this study were to determine how C. partellus moths and larvae respond towards Bt maize in choice and no-choice tests and to determine the effect of larval size and survival on Bt maize. An attempt was also made to select for resistance in a population under laboratory conditions. Different sizes of larvae of four populations were used in survival studies and it was observed that older larvae were able to survive on Bt maize tissue. It was possible to select for tolerance to Bt over a two generation life cycle of this pest by allowing neonate larvae to feed on Bt maize tissue for short periods of time and then allowing the survivors to completed their life cycles on non-Bt maize. This study showed that C. partellus moths did not exhibit any oviposition preference towards the Bt or non-Bt treatments used in this study. This study concluded that if neonate larvae that hatch on a natal Bt maize plant move off the Bt maize plant onto a non-Bt plant within 24 hours, larvae will be able to survive. This study further concluded that C. partellus is still highly susceptible to Bt maize in South Africa and that behavioural characteristics such as larval movement between plants, may contribute to resistance evolution.