Protein Deficient Diets: Cascade Effects on a Lepidopteran Pest and Its Parasitoid Wasp

The nutritional quality of herbivorous insects' food can not only directly affect the herbivorous insects themselves, but can also indirectly affect their parasitoids. To investigate these cascading, multi-trophic effects, we reared cabbage looper, Trichoplusia ni (Hubner) (Lepidoptera: Noctuidae), on artificial diets (8.1, 11.5, 16.75, 25.5, 34.25, and 43 g protein/liter diet) to assess how diet protein content affected the development of this common pest and its suitability as a host for the gregarious parasitoid, Cotesia vanessae (Reinhard) (Hymenoptera: Braconidae). Nonparasitized caterpillars experienced increased mortality when reared on 8.1 g protein/liter diet, and slower development and reduced pupal mass when reared on <= 16.75 g protein/liter diet. Host diet did not affect the percentage of hosts with parasitoid emergence nor the mass of individual parasitoids. However, parasitoid broods emerging from caterpillars reared on <= 25.5 g protein/liter diet were smaller and those reared on <= 16.75 g protein/liter diet exhibited prolonged development. The consequences of host diet on these latter F-1 parasitoids did not affect their reproductive fitness. Caterpillars compensated for nutrient stress, induced by either low quality diet or parasitism, by increasing the amount of diet that they consumed. These collective results demonstrate the plasticity of host-parasitoid systems. Compensatory feeding allows the host caterpillar to moderate the consequences of low quality diets, which may subsequently affect the F-1 parasitoids developing within the host, but not necessarily affect the F-2 parasitoid generation.