Macronutrient regulation in adult spotted-wing Drosophila and nutritional impacts on susceptibility to three commonly-used insecticides

Nutrition provides the resources necessary to carry out all the physiological functions organisms require. This has important implications for insect pest management, as polyphagous species can experience considerable nutritional variability, which may impact their response to management tactics. Many studies have shown that dietary protein and carbohydrates play a primary role in insect fitness and that insects regulate their intake of these macronutrients to reach an optimal balance or intake target. Importantly, this intake target can vary across developmental stages, gender, and environmental conditions as physiological requirements change. In this study, we determined the intake target for adult spotted-wing Drosophila, Drosophila suzukii, an invasive polyphagous fruit pest. We found that adult D. suzukii regulate their intake to meet a 1:4–1:5 protein-to-carbohydrate ratio, which is slightly more carbohydrate-biased than the larval intake target. We also examined the effect of nutritional state on susceptibility to three commonly-used insecticides by rearing flies on diets with different field-relevant protein-to-carbohydrate ratios and exposing adults to an LC₅₀ dose of zeta-cypermethrin, spinetoram, or pyrethrin. Overall, female flies had higher survivorship than males, and diet effects varied across insecticides. Diet had no impact in the spinetoram treatments, while the extreme carbohydrate- and protein-biased diets produced the lowest survivorship and reproductive performance in the zeta-cypermethrin treatments. Survivorship increased with dietary protein in the pyrethrin treatments but reproductive performance did not differ across diets. Our results suggest that tactics, which reduce fly access to dietary protein sources, such as sterilisation, could increase fly susceptibility to insecticides.