Baseline susceptibility of the corn earworm (Lepidoptera : Noctuidae) to the Cry1Ab toxin from Bacillus thuringiensis

Susceptibility to Cry1Ab toxin from Bacillus thuringiensis,lsis (Bt) was de termined for 12 field populations of neonate corn earworm, Helicoverpa zea (Boddie), from the United States. Earworm larvae were exposed to artificial diet treated with increasing Pt concentrations, and mortality and growth i nhibition were evaluated after 7 d. The range of variation in Bt susceptibi lity indicated Ly Growth inhibition was very similar to that indicated by m ortality. Although interpopulation variation in susceptibility to both prot eins was observed, the magnitude of the differences was small (less than or equal to fivefold). These results suggest that the observed susceptibility differences reflect natural variation in Bt susceptibility among corn earw orm populations rather than variation caused by prior exposure to selection pressures. Therefore, corn earworms apparently. are susceptible to Pt toxi ns across most of their geographic range.