The maize stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae), is of economic importance throughout sub-Saharan Africa. The seasonal activity pattern of this pest is characterized by two to three distinct generations during spring and summer followed by a diapause period of approximately six months during autumn and winter. Genetically modified Bt maize (MON810) that expresses an insecticidal Cry1Ab protein has been deployed in South Africa since 1998 to manage stem borers. The first report of field resistance of B. fusca to Bt maize was made during the 2006 cropping season. Resistant strains of this pest are expected have reduced fitness compared to susceptible individuals. Information regarding fitness of resistant individuals that survive on Bt maize could contribute to the understanding of resistance evolution as well as to development of improved resistance management strategies. Life history parameters of different B. fusca populations were compared in a laboratory study using diapause (spring) as well as second-generation (summer) populations. Sex ratio, pupal mass, fecundity and longevity of moths of field-collected Bt-resistant and susceptible B. fusca populations were compared. Slight adverse effects of Bt maize on fitness of the resistant summer-population were observed. The sex ratio was biased towards males in some resistant populations and towards females in susceptible populations. The resistant population had a lower mean pupal mass, shorter longevity of moths and reduced fecundity.
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Vol. 20 • No. 1