Effects of seasonal changes in cotton plants on the evolution of resistance to pyramided cotton producing the Bt toxins Cry1Ac and Cry1F in Helicoverpa zea

BACKGROUND: In pests with inherently low susceptibility to Bacillus thuringiensis (Bt) toxins, seasonal declines in the concentration of Bt toxins in transgenic crops could accelerate evolution of resistance by increasing the dominance of resistance. Here, we evaluated Helicoverpa zea survival on young and old cotton plants that produced the Bt toxins Cry1Ac and Cry1F or did not produce Bt toxins. RESULTS: Using a strain selected for resistance to Cry1Ac in the laboratory, its parent strain that was not selected in the laboratory, and their F₁ progeny, we showed that resistance to Cry1Ac + Cry1F cotton was partially dominant on young and old plants. On Cry1Ac + Cry1F cotton, redundant killing was incomplete on young plants but nearly complete on old plants. No significant fitness costs on non-Bt cotton occurred on young plants, but large recessive costs affected survival on old plants. Simulation models incorporating the empirical data showed that the seasonal changes in fitness could delay resistance to Cry1Ac + Cry1F cotton by inducing low equilibrium frequencies of resistance alleles when refuges are sufficiently large. CONCLUSION: Our results suggest that including effects of seasonal changes in fitness of pests on Bt crops and refuge plants can enhance resistance risk assessment and resistance management.