Multi-trophic effects of protein-restricted diet on the fitness of Trichoplusia ni (Lepidoptera: Noctuidae) and of one of its parasitoid, Cotesia vanessae (Hymenoptera: Braconidae)

Understanding trophic effects of unbalanced diets on the parasitoids of crop-pest insects can improve our understanding of parasitism in the field and the results of laboratory studies. In this study, casein levels in an artificial diet were manipulated to test the effects of a diet primary metabolite on a Lepidoptera larva, the cabbage looper, Trichoplusia ni, and on its parasitoid, Cotesia vanessae. Cabbage looper larvae reared on diets with less than 25.5 g/L of protein exhibited delayed growth and lighter pupae. Similarly, parasitoids emerging from hosts reared on diets containing less than 25.5 g/L of protein exhibited delayed growth and smaller broods. The percentage of hosts successfully parasitized and the mass of individual parasitoids were not affected by diet quality. An increase in cabbage looper death was observed only for individuals being reared on the most protein-poor diet (8.1 g/L of protein), which suggests that cabbage loopers can tolerate high variation in protein availability. As a compensatory mechanism, cabbage loopers increasing their food consumption on diets of lower protein content. Food consumption of cabbage loopers was also higher in parasitized individuals (only on high-protein diets), likely to compensate for nutrients lost to parasitoids. No parental effect of hosts’ diet quality on parasitoids was found: offspring of parasitoids that developed on caterpillars reared on high-protein diet and offspring of parasitoids that developed on caterpillars reared on the most protein-poor diet showed no difference of development time, brood size, individual parasitoid mass, and brood mass.