Modeling the evolution of resistance in cotton bollworm to concurrently planted Bt cotton and Bt maize in China

Abstract

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. Here we developed two-locus population genetic models to assess the risk of resistance in CBW in which four cases of Bt cotton and Bt maize and two types of refuges are considered. Model simulations showed that the time to resistance (TTR) is longest in the case of two-toxin Bt cotton & two-toxin Bt maize, followed by the cases of two-toxin Bt cotton & one-toxin Bt maize, one-toxin Bt cotton & two-toxin Bt maize, and one-toxin Bt cotton & one-toxin Bt maize. With 25% of cotton seed mixed refuge and 20% of maize seed mixed refuge, the TTRs in the four cases by order are 54, 13, 9, and 7 generations, respectively. With additional natural refuges, the differences in the TTRs among the four cases are greater. Sensitivity analysis showed that among the parameters examined, the initial frequency of resistance alleles and fitness cost are the ones to which the TTRs are most and least sensitive, respectively. We concluded that when natural refuges are scarce, planting both two-toxin Bt cotton and two-toxin Bt maize instead of one-toxin ones are necessary to combat CBW resistance to concurrently planted Bt cotton and Bt maize in northern China.