Tritrophic interactions between a Bt plant, the diamondback moth and a braconid endoparasitoid

Studies of the interactions between transgenic plants and natural enemies of pest insects are not only important to assess the risks associated with these new crops but also to assess the role of natural enemies in delaying pest adaptation to the transgenic plants. We have studied the effect of transgenic Brassica napus (canola, oilseed rape), expressing the Cry1Ac toxin of Bacillus thuringiensis (Bt), on the solitary larval parasitoid Cotesia plutellae . This parasitoid is an important natural enemy of the diamondback moth, Plutella xylostella , a pest of brassicas that has developed resistance to a wide range of insecticides and is one of the main model species for the study of Bt resistance. Although normal P. xylostella larvae are highly susceptible to Cry1Ac, laboratory-selected highly resistant strains of this pest are able to survive on Bt plants. We present evidence that although C. plutellae larvae that were forced to develop in Bt-treated susceptible P. xylostella larvae inevitably died with their hosts, behavioural factors may limit the scale of this effect under field conditions. 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 also highly attracted to Bt plants damaged by Bt-resistant hosts suggesting that parasitoids, such as C. plutellae , could assist with delaying the build-up of Bt-resistant pest populations.