The impact of nutrition on susceptibility to Cry endotoxins in Helicoverpa zea (Lepidoptera: Noctuidae)

Incidents of insecticide resistance in agricultural pest species represent a costly and ever-increasing problem that poses significant challenges for meeting the nutritional demands of our growing global population. The development and spread of genetic mutations conferring resistance is the primary explanation for observations of low pesticide efficacy and population outbreaks in agricultural systems. Unfortunately, in many cases it takes years to identify the mutations and alleles responsible for these resistant phenotypes, with most incidents assumed to be the result of mutations without any substantiation.  As a result, the potential for gene-by-environment interactions to play a dominant role in mediating pesticide resistance is often overlooked. One environmental factor that has been shown to be highly variable is plant macronutrient content. Macronutrients such as protein (P) and carbohydrates (C) have also been shown to strongly affect insect behavior, physiology, and performance, including detoxification potential. Studies have shown that dietary protein-to-carbohydrate ratios (P:C) can influence feeding behavior and detoxification ability in locusts, as well as immune function in caterpillars, indicating that the impact of nutritional variability on insecticide resistance is probative. In this study we explored the potential for plant macronutrient content to impact susceptibility to Cry1Ac endotoxin, found in transgenic cotton, in the generalist herbivore Helicoverpa zea. Using artificial diets, we mimicked the macronutrient content of different cotton tissues and reared larvae on diets without Cry1Ac, with a low concentration of Cry1Ac, and with a high concentration of Cry1Ac incorporated. We then measured the effect that Cry had on survival, pupal mass, and eclosion success on each of the diets.