Simulating the impact of cross resistance between Bt toxins in transformedclover and apples in New Zealand

Simulations were conducted to guide development of resistance management st rategies aimed at prolonging the usable life of B. thuringiensis (Bt) endot oxins in multiple cropping situations, where different crops expressing Bt endotoxins are host plants for a common pest. We used the New Zealand apple and clover model ecosystem to explore the relative impact on the rate of r esistance development of varying levels of cross-resistance between differe nt toxins expressed in these 2 potentially Bt-transformed crops. These 2 cr ops are hosts for a complex of leafrollers in New Zealand, including the li ghtbrown apple moth, used here as the model pest. Cross-resistance was vari ed between 0.0 and 0.5 (zero to partial cross-resistance) to allow for the case in which selection by one plant has a potential effect on resistance t o the toxin in another plant. The largest factor affecting the evolution of resistance was the total habitat area occupied by transgenic orchards. The proportion of the clover habitat that was transformed was also an importan t. factor, even in the absence of cross-resistance. The effect of increasin g the proportion of the second transformed crop (clover) acted on resistanc e evolution mainly by reducing the external refuge of susceptibility for th e transgenic orchards. Hence, the ecological implications of reducing the a vailable source of susceptible insects from clover, which can help to dow r esistance development in the orchard ecosystem, had a more significant impa ct than the presence of cross-resistance. Partial cross-resistance between different toxins in the separate crops was overall of relatively minor impo rtance. These simulations have implications for deployment decisions for in dividual transformed crops in multiple cropping systems, where there is the potential for the crops to serve as refuges for each other. These decision s may need to focus less on cross-resistance between toxins, than on econom ic trade-offs between the relative roles of individual crops as refugia mai ntaining susceptibility in the system as a whole.