Dietary costs of genetic resistance to the microbial insecticide Bacillus thuringiensis

The microbial insecticide Bacillus thuringiensis (Bt) is the most successful biological control agent used in agriculture. However, its future efficacy is in jeopardy due to the evolution of insect resistance, most commonly through the modification of insect midgut binding sites for Bt toxins. As resistance to Bt is gut-based and associated with significant fitness costs, we asked whether Bt resistance alters the requirement for dietary protein and carbohydrate, and how changes to dietary requirements are associated with fitness and resistance to Bt. We report that Bt-resistant Trichoplusia ni self-compose a higher ratio of protein to carbohydrate than susceptible T. ni, and the self-composed ratio maximized their resistance to Bt. Instead, Bt-susceptible T. ni self-composed a diet that maximized fitness. Our findings demonstrate that maintenance of a resistance trait can alter the nutritional intake of an insect to optimize that trait.