This study examined the influence of dietary protein and carbohydrate balance in a chemically defined artificial diet for Manduca sexta larvae on development of the gregarious parasite Cotesia congregata. Levels of blood protein nitrogen and trehalose, nutrients supporting growth and development of C. congregata, varied with diet and were influenced by parasitism. The levels of dietary protein and carbohydrate had a significant effect on the numbers and biomass of parasites developing in individual host larvae. Parasites included individuals that developed and eventually emerged as 2nd instar larvae, moulted to 3rd instars and pupated. Failing to emerge, however, were many apparently mature 2nd instar parasites. The proportion of non-emerging individuals varied with diet, and in some cases, parasites failing to emerge were greater in number and total biomass than those that did emerge to complete development. Three dimensional models developed to demonstrate the relationships between blood protein nitrogen and trehalose levels and parasite burden and biomass established that the levels of both metabolites are important for supporting growth and development of emerged and non-emerged parasites. Blood metabolite levels supporting the greatest parasite burden and biomass of emerged and non-emerged parasites occupy a region of two dimensional space corresponding to approximately 60 to 200 mg/insect of protein nitrogen and 60 to 100 mg/insect of trehalose. Despite the differences in the response of emerged and non-emerged parasites to host nutrition, the present results indicate that host nutrition is not the critical factor determining parasite emergence.