The ability of organisms to deal with adversity is essential for survival, reproductive success. Nutrition has strong impacts on all physiological processes, including stress responses. Several studies have shown diet macronutrients (protein and carbohydrates) have significant effects on the ability of insect herbivores to deal with toxins; however, despite the economic importance, few studies have focused on how macronutrients may impact a pest's susceptibility to insecticides. Therefore, the general goal of this dissertation was to use cotton as a model system to explore the impact of nutrition on susceptibility to an insecticide stressor.
To do this, I first characterized cotton as a resource. This was achieved by measuring the variability in dietary protein (p) and carbohydrates (c) across plant tissues, genotypes, developmental stages, and growing conditions (Chapter II). Secondly, I determined the macronutrient requirements of two polyphagous pest species, the sucking pest Lygus hesperus (Western tarnished plant bug) and the chewing pest Helicoverpa zea (cotton bollworm) (Chapters III and IV). Finally, I determined the effect of dietary macronutrients on the susceptibility of H. zea to the endotoxin Cry1Ac, produced in transgenic Bt cotton (Chapter V).
Our results showed that even in an agricultural monoculture, like cotton, there is high variability in p and c content, across tissues, throughout plant development, and between different growing conditions. Despite this variability, I found that both L. hesperus and H. zea feed selectively to ingest a specific food p:c ratio, or intake target (IT). Both species selected for a slightly p-biased IT of 1.2:1 (L. hesperus) and 1.6:1 (H. zea). Dietary macronutrients also had significant impacts on survival and performance for H. zea when Cry1Ac was present. Larval survival and performance were best on diets close to the IT; however, at higher concentrations of Cry1Ac, total macronutrient concentration was the most important dietary factor. The diet that produced the worst performance under control conditions, produced the best performance when Cry was present, identifying an interesting physiological trade-off between. These results show that nutrition can effect pesticide efficacy may be more important than currently acknowledged as an environmental mediator of resistance.
- Behmer, Spencer Professor