Maize stemborer predator activity under ‘push – pull’ system and Bt-maize: A potential component in managing Bt resistance

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Abstract
Studies were conducted between 2002 and 2004 in experimental plots at two sites in western Kenya and one site in South Africa, to assess the potential role of the ‘push – pull’ strategy in stemborer resistance management in Bt-maize through its influence on the pests' oviposition and mortality from predation. Experiments in Kenya were conducted with non-Bt-maize while a combination of Bt-maize (expressing Cry1Ab proteins) and its non-Bt-maize isoline was used in South Africa. Treatments comprised of a maize monocrop and an intercrop of maize and desmodium, with Napier grass as a trap crop around the field in each site (‘push – pull’). A combination of natural and artificial infestation procedures were employed. Oviposition preference and predation rates of naturally infested stemborer eggs were assessed. Screen house-reared plants (non-Bt-maize) were infested with eggs, early-instar larvae, late-instar larvae and pupae of Busseola fusca and Chilo partellus in natural enemy exclusion studies. Significantly more egg batches were oviposited in the maize monocrop than in the ‘push – pull’ plots by C. partellus, but not B. fusca. Predation rates of the naturally oviposited C. partellus eggs were significantly higher in the ‘push – pull’ than in the maize monocrop plots, but not different with B. fusca eggs. Disappearance rates of C. partellus eggs on control plants (those exposed to predators), attributable to predatory activity, was significantly higher in control than exclusion plants and similarly higher in the ‘push – pull’ than in the maize monocrop plots. There were no differences in disappearance rates of B. fusca eggs between treatments. Recovery of early-instar larvae was generally low for both species, similar between control and exclusion plants, but significantly lower in the ‘push – pull’ plots in one of the sites. Recovery of late-instar larvae and pupae in control plants was similar between the two cropping systems for both species. The study revealed reduced oviposition by C. partellus and enhanced predatory activity on C. partellus eggs in the ‘push – pull’ system, pointing to the system becoming a component in Bt resistance management for the pest.