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The ratio of dietary protein to carbohydrate (P:C) affects the feeding performance of insect herbivores and was recently shown to affect susceptibility to Bt toxins and fitness costs of resistance in Trichoplusia ni Hübner. Here, we used laboratory experiments with larvae reared on artificial diet to determine whether dietary P:C ratio affects responses to Bt toxin Cry1Ac and fitness costs of resistance in Helicoverpa zea Boddie (Lepidoptera: Noctuidae). We tested a H. zea strain that was selected for resistance to Cry1Ac in the laboratory (GA‐R) and its field‐derived parent strain (GA), which was not selected in the laboratory. Relative to GA, GA‐R had 14‐fold resistance to Cry1Ac. The P:C ratio of diet did not affect susceptibility to Cry1Ac of GA‐R. However, against GA, Cry1Ac was least toxic on the diet with the lowest P:C ratio (35:65) and most toxic on the diet with the highest P:C ratio (90:10). Diets of all P:C ratios except 80:20 induced a fitness cost of lower pupal weight in GA‐R relative to GA for males, females, or both. Considered together with previous results, the new findings reported here indicate that the effects of P:C ratio on responses to Cry1Ac and fitness costs vary between species and between strains within species.

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