Interactions between transgenic plants, the diamondback moth and natural enemies

Several Brassica crops have been transformed with -endotoxin genes from Bacillus thuringiensis (Bt) to provide resistance to the diamondback moth (Plutella xylostella) and other lepidopteran pests. This paper describes studies of the direct and indirect effects of Bt plants on a parasitoid of P. xylostella using Cry1Acexpressing transgenic oilseed rape (canola) as an example. The braconid wasp Cotesia plutellae is a solitary endoparasitoid of P. xylostella larvae. We present evidence that, although C. plutellae larvae that were forced to develop in Bt-treated susceptible hosts inevitably died with their hosts, behavioural factors are likely to limit the impact of this effect on field populations. Cotesia plutellae mortality in susceptible hosts was not due to a direct toxic effect of Cry1Ac, but due to premature host mortality since C. plutellae larvae developed normally in Bt-resistant hosts on Bt plants. Adult C. plutellae females were highly attracted to Bt plants damaged by Bt-resistant hosts. Any tactic aimed at suppressing pest populations risks affecting the dynamics of some natural enemies, simply due to a reduction in host or prey densities. The extent of these effects will depend on the ecological specialisation of the different natural enemy species present in the crop ecosystem. They are likely to be most pronounced for species, such as hymenopteran parasitoids, that tend towards greater host or prey specificity. However, the apparent lack of direct effects of Bt plants on survival and host-seeking ability of C. plutellae implies an environmental advantage compared with the broadspectrum insecticides used for diamondback moth management in many areas.