Busseola fusca (Lepidoptera: Noctuidae) moth and larval behaviour in Bt- and non-Bt maize: an IRM perspective
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The African maize stemborer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is one of the most damaging pest species of maize in Africa. Genetically modified (GM) Bt maize that expresses insecticidal Cry proteins could soon be a primary control method for this pest on the continent, since, over the past few years, several African countries have been conducting regulatory field trials that are required for approval of GM crops for commercial release. However, since the sustainability of the Bt maize technology is threatened by the evolution of resistance by pest populations, development of insect resistance management (IRM) strategies such as the high-dose/refuge (HDR) strategy are required. Current HDR strategies require the expression of Bt toxins in a dose high enough to kill heterozygous-resistant individuals, as well as a source of non-Bt host plants (refuge area) near the Bt field, which acts as a source of homozygous susceptible target pest individuals. The refuge area can be structured (e.g. blocks or strips of non-Bt plants within the Bt field) or unstructured, where a blend of non-Bt and Bt plants (seed mixture) is planted within a single field. The functioning of the HDR strategy is based on the validity of several key assumptions about the biology and behaviour of the target pest species. The rapid evolution of resistance to Bt toxins in B. fusca populations in South Africa demonstrated the necessity for deployment of an effective IRM strategy. However, the design of an IRM strategy is complicated by the heterogenous nature of the agricultural systems in Africa, which makes the implementation of a standardized, universal IRM strategy impossible. Although smallholder farmers find it challenging to implement separate refuges due to their limited scale of production, the use of seed mixtures is not an appropriate strategy to delay resistance evolution in pests with highly mobile larval stages. Therefore, an effective IRM strategy must take into consideration both the practical limitations of the agricultural system that it intends to serve, as well as the behaviour of target pest species (especially oviposition and larval migration behaviour). However, when the target pest forms part of a mixed population of pest species, for example B. fusca, Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) and Spodoptera frugiperda (JE Smith) (Lepidoptera: Noctuidae), the adjustment of the IRM strategy to local conditions becomes complicated. Even closely related pest species generally differ in various biological and behavioural aspects. The aim of this study was therefore to investigate oviposition preference and larval migration behaviour of B. fusca in Bt and non-Bt maize, to review aspects of the biology and ecology of B. fusca, which occur in mixed populations, and develop a synthesis on the possible impact of mixed pest populations on insect resistance management for Bt maize in Africa. Oviposition preferences of moths and feeding preferences of larvae, when offered a choice between Bt and non-Bt maize, were investigated for both resistant and susceptible B. fusca populations in laboratory bioassays. Additionally, larval migration behaviour and the effect of Bt toxin, plant density, and plant age on this behaviour were evaluated in laboratory, semi-field (glasshouse), and field experiments. Results indicated a lack of oviposition preference in B. fusca between undamaged Bt and non-Bt maize, which suggests that separate refuges can be implemented in an IRM strategy against this species (assuming a single-species pest population of B. fusca and the presence of ample refuge plants). In contrast, larvae displayed feeding avoidance behaviour on Bt maize, suggesting that the larvae migrate more readily off Bt maize plants. However, the field trials conducted during this study indicated that, although B. fusca larvae do migrate between plants, the migration distance is limited and survival is very low (even in pure stands of non-Bt maize). This contradicts earlier accounts which erroneously reported significant larval movement between plants. This thesis also concluded that the use of separate refuges as an IRM tactic against mixed populations in smallholder Bt maize fields in Africa would be unwise, since the domination of the refuge by a single species will undermine the efficacy of the IRM strategy for another pest species. This synthesis concludes that no single, generic, standardized IRM strategy can be prescribed across the different agroecological zones of Africa. The dissimilarity between the African agricultural regions necessitates a tailored approach to IRM and development of strategies for each region/agro-ecological zone to ensure that it meets the needs of the farmers that are tasked with its implementation. Future research should focus on generating data required for use in models to improve IRM strategies for specific regions and agricultural systems.