Relative fitness of CrylA-resistant and -susceptible Helicoverpa armigera (Lepidoptera: Noctuidae) on conventional and transgenic cotton

Glasshouse and laboratory experiments were conducted to evaluate the relative fitness of CryIA-susceptible and laboratory-selected resistant strains of Helicoverpa armigera (Hubner). Life history parameters of H. armigera larvae feeding on young cotton plants showed a significant developmental delay of up to 7 d for the resistant strain compared with the susceptible strain on non-Bacillus thuringiensis (Bt) cotton. This fitness cost was not evident on artificial diet. There was no developmental delay in the F-1 hybrid progeny from the reciprocal backcross of the resistant and susceptible strains, indicating that the fitness cost is recessive. In two cohorts tested, survival to pupation of resistant larvae on Bt cotton expressing Cry1Ac was 54 and 51% lower than on non-Bt cotton, whereas all susceptible and F-1 larvae tested on Cry1Ac cotton were killed. Mortality of susceptible larvae occurred in the first or second instar, whereas the F-1 larvae were able to develop to later instars before dying, demonstrating that resistance is incompletely recessive. The intrinsic rate of increase was reduced by >50% in the resistant strain on Cry1Ac cotton compared with the susceptible strain on non-Bt cotton. There was a significant reduction in the survival of postdiapausal adults from the resistant strain and the F-1 strains, indicating that there is a nonrecessive overwintering cost associated with Cry1A resistance in H. armigera.