Impact of a habitat management system and BT-maize on stemborrer natural enemies and biodiversity of arthropods and soil fauna in Kenya and South Africa
Lepidopteran stemborers are a major constraint to efficient production of cereals in Subsaharan Africa. As part of continuing efforts to manage these pests, the International Centre of Insect Physiology and Ecology and its partners have developed a habitat management strategy by use of gramineous and leguminous fodder plants that provide a diversionary strategy whereby stemborers are repelled from the maize and subsequently attracted to a discard or trap crop around the field ('push-pull' strategy). The European corn borer, Ostrinia nubilalis (Hubner), is also a major pest of corn in North America. Bt corn hybrids, in which a gene from a bacterium, Bacillus thuringiensis subsp. kurstaki has been sliced into the DNA of corn plants, have been developed as a management option for European corn borer due to its insecticidal properties. This technology is also used for Busseola fusca in South Africa. Bt maize therefore represents a promising technology for reducing losses from stemborer infestations in Africa. The current studies were conducted to assess the impact of both technologies on stemborer natural enemy abundance and activity and arthropod abundance and diversity in western Kenya and South Africa. Treatments consisted of a maize monocrop and an intercrop of maize and desmodium with Napier grass as a perimeter crop around the plots ('push-pull') with varied number of replications in each site and study. Oviposition preference rates were significantly higher in the maize monocrop than in the 'push-pull' systems at all sites indicating that the stemborer moths preferred the maize monocrop to the 'push-pull' system for oviposition. Results showed that the incidence of the larvae and pupae was significantly higher in the maize monocrop than in the 'push-pull' systems. Similarly, plant damage in terms of number of entry/exit holes, foliar damage, broken stems and dead plants, were significantly higher in the maize monocrop than in the 'push-pull' systems. Maize yields in terms of average dry kernel weight per plant and average weight of kernels per plot were significantly higher in the 'push-pull' than in the maize monocrop plots in Kenya but were the same between the two cropping systems in South Africa. Maize stemborer egg parasitism rates were significantly higher in the 'push-pull' than in the maize monocrop plots in Kenya and insignificant in South Africa. Larval and pupal parasitism rates followed the same trend at all sites while larval and pupal mortality did not differ between treatments. Generalist predators were significantly more abundant in the 'push-pull' than in the maize monocrop systems at all sites while species diversity did not show a consistent trend between the two systems. Stemborer egg predator activity was significantly more enhanced in the 'push-pull' than in the maize monocrop systems at all sites. There were, however, no significant differences with regard to larval and pupal predation rates between the two cropping systems at all sites. Spiders were more abundant in the 'push-pull' than in the maize monocrop systems at all sites. Their overall species diversity and dominance were however not affected by either the cropping system or Bt-maize, except for the family Lycosidae which were more species diverse in the 'push-pull' than in the maize monocrop systems. Both individual families and total springtail numbers did not differ between the 'push-pull' and the maize monocrop plots. The same was observed for these treatments under the Bt and non-Bt maize. Bt-maize did not affect spnngtail diversity and abundance. These studies imply that 'push-pull' systems enhance maize stemborer natural enemy abundance and activity, arthropod abundance and has no deleterious effects on spnngtail populations. Similarly Bt-maize has no effect on spider and springtail abundance and diversity.