Lifetime consequences of food protein‐carbohydrate content for an insect herbivore

Food protein–carbohydrate content has significant consequences for animal survival, growth and reproduction. Rarely, though, is its effect examined over an animal's entire lifetime. In this study, we reared newly hatched caterpillars [Heliothis virescens Fabricus (Lepidoptera: Noctuidae)] on diets containing different protein/carbohydrate (p/c) ratios. We recorded larval survival, time to pupation, pupal mass, eclosion success, time to eclosion and pupal body lipid content. Additionally, for each treatment, we mated eclosed males and females and measured egg production and egg viability. Larval performance (survival to pupation and time to pupation) was similar across all except the two most extreme treatments. In contrast, pupal performance (mass, eclosion success and time to eclosion) was best on diets that were balanced or slightly protein‐biased. However, eclosion success differed between sexes. For males, it was best on diets with balanced p/c ratios, while female eclosion was strong across all but the most carbohydrate‐biased diet. Pupal body lipid content in both males and females increased as the food p/c ratio decreased. Egg production was best on diets with balanced or slightly protein‐biased p/c ratios. We also estimated the effect of food p/c ratio at the population level, using the data generated in this study. Population size was largest on diets with a balanced p/c ratio and declined steadily and strongly as the food p/c ratio became increasingly more imbalanced. Our findings show, for the first time, the effect of food p/c content over an insect herbivore's entire life. Our data indicate that there is a narrow range of p/c ratios that maximize lifetime performance, and for H. virescens, this range coincides with its self‐selected p/c ratio.