D3502: Hybrid vigor influences the estimated fitness costs and dominance levels of Bt resistance in Helicoverpa zea

Fitness costs and dominance levels are two important factors in insecticide-resistant management. However, hybrid vigor caused by crossing two inbred populations with different genetic backgrounds might promote the fitness of heterozygous resistant populations and thus cause an overestimation of the dominance levels and underestimation of the fitness costs. In this study, we used a known dual-protein Cry1A/Cry2A Bt resistant (RR) and susceptible (SS) populations of the corn earworm, Helicoverpa zea, as a model system to test this hypothesis. First, a series of crosses and backcrosses between RR and SS were performed to generate six insect populations with different genetic backgrounds. Then, larval mortalities of the six populations were examined on two independent test approaches: diet-overlay and corn leaf tissue bioassays. Both tests showed that the fitness of the heterozygous populations and the dominance levels of the resistance estimated based on the crossed-and-backcrossed populations were significantly lower compared to the values for the populations that were reared separately without backcrosses. The results validate that hybrid vigor significantly influences the measurement of fitness costs and dominance levels of resistance. Knowledge gained from this study will be useful in experimental designs in analyzing fitness costs and inheritance of insecticide/Bt resistance.