The corn earworm, Bt transgenic corn and Bt-resistance evolution in a mixed cropping system

Varieties of Bt corn and Bt cotton that express the Lepidoptera-specific Cry1Ab and Cry1Ac toxins from Bacillus thuringiensis have recently been commercialized and are active against larvae of the corn earworm, Helicoverpa zea. Field studies presented here show that Cry1Ab Bt corn varieties from transformation events Bt-11 and Mon-810 kill 75% to 95% of H. zea larvae feeding on them, by steady attrition from neonate to pupa. The survivors weigh 33% less than those that fed on non-Bt corn ears, and their development is delayed by around six days. Female longevity and fecundity are not significantly affected by having fed on Bt as larvae. The effects of a Cry1Ab Bt corn variety from transformation event 176 are weaker. Bt corn in the whorl stage does not appear to support H. zea larvae.; These effects of Bt corn raise concerns about resistance evolving in H. zea to the Cry1A family of Bt toxins if Bt crops are widely deployed. A stochastic computer simulation model presented here shows how the cannibalistic behavior of H. zea larvae on Bt corn ears can hasten resistance evolution. The faster-developing resistant larvae are the likely victors in cannibalistic encounters with slower-developing susceptible larvae.; In regions such as eastern North Carolina where Bt corn and Bt cotton may be grown together ad H. zea feeds extensively on them, the interactions among crop phenology, agronomy, the environment, insect biology and genetics will determine the rate at which resistance evolves over wide areas. A spatially-explicit stochastic computer simulation model incorporating population dynamics and genetics is presented and used to investigate the importance of several factors in the success of strategies to manage resistance through planting refugia. The initial frequency and functional dominance of resistance alleles and the larval mortality induced by Bt crops have large effects on the rate of resistance evolution. Several abiotic and operational factors beyond the scope of insect pest management are also important. The spatial distribution of refugia affects the local resistance allele frequency: areas of high Bt deployment can create local resistance problems that are not mitigated by low Bt deployment in surrounding areas.