Transgenic maize expressing toxins derived from the bacterium Bacillus thuringiensis (Bt) may be commercially planted in northern China where Bt cotton has been planted for more than two decades. While Bt maize brings additional benefits for insect control, it complicates the resistance management of cotton bollworm (CBW), Helicoverpa armigera (Lepidoptera, Noctuidae), a common target of Bt cotton and Bt maize.
We developed population genetic models to assess the risk of resistance in CBW when Bt cotton and Bt maize are planted concurrently. Model simulations showed that when natural refuges are absent, the time to resistance (TTR) is less than 10 generations in the case of one-toxin Bt cotton and one-toxin Bt maize, but is more than 30 generations in the case of two-toxin Bt cotton and two-toxin Bt maize. The differences in the TTR between the two cases become greater as the proportion of natural refuge increases. Among the parameters we investigated, the fitness cost has a relatively smaller effect on the TTR, while the dominance of resistance and the proportion of natural refuge have a much greater effect.
We concluded that planting the first generation Bt cotton with Bt maize could significantly increase the risk of CBW resistance to Bt toxins as compared to planting a pyramid two-toxin Bt cotton. The strategies for reducing the risk of CBW resistance include replacing the one-toxin Bt cotton with a pyramid two-toxin Bt cotton, adopting a pyramid two-toxin Bt maize, and maintaining a sufficient proportion of natural refuges.