Impact of Colorado potato beetle-resistant potatoes on non-target arthropods: A meta-analysis of factors potentially involved in the failure of a Bt transgenic plant

The relatively high specificity of transgenic plants based on Cry toxins of Bacillus thuringiensis (Bt) implies the possibility of upward agroecosystemic cascades toward new equilibria among arthropods associating with cultivated plants. We examine the hypothesis that exclusion of the Colorado potato beetle from potato expressing the Cry3a toxin increases the abundance of non-target herbivores, which indirectly favours the abundance of herbivore-dependent predators and omnivores foraging on agricultural plants. We examined the impact of Bt potato on non-target arthropod taxa, based on impact studies conducted during development of the Newleaf(R) Bt potato in North America. Of 32 field tests comparing Bt potato to non-transgenic controls, 14 (42%) revealed a significant, positive effect on the abundance of sucking insects (aphids, leathoppers, mirids, thrips). Among 72 tests on generalist predators that were simultaneously monitored, 14 (similar to 20%) also revealed significant positive effects. Such positive effects on predators can best be explained by their abundance being increased as a result of greater productivity due to overabundance of sucking insect prey, which are selectively favoured by the high specificity of the Cry3a Bt toxin. Our results support the idea that development of the Bt potato may have been hampered in part by its positive effects on sucking insect pests, and underline the importance of conserving the natural enemies of secondary pests that are indirectly favoured.