All organisms invest in nutrient storage to fuel energetic requirements needed for survival and reproduction. The consistency with which organisms invest in nutrient storage, in particular lipids, makes it remarkable that numerous parasitic insects lack lipid synthesis. The parasitic larval lifestyle adopted by these parasitoids has facilitated a means to carry over valuable resources from their host and allows them to refrain from costly investment in storage reserves. The lack of lipogenesis has thus evolved concurrently with that of the parasitic lifestyle in insects. However, several parasitoids deviate from this pattern, in which lipogenesis is recurring, suggesting that ecological conditions rather than evolutionary adaptations may underlie the lack of this trait. I propose to explore how ecological conditions affect the lack of lipogenesis in parasitoids, and the costs and benefits associated with this strategy. I will determine how nutritional conditions affect gene transcription and nutrient dynamics in two species of parasitic wasp differing in their propensity for lipogenesis. Biological systems will be experimentally pushed to switch their lipogenic strategy using flight to increase energy-demand and the ecological setting necessary to maintain lack of lipogenesis as a successful strategy will be determined using a modeling approach. This integrative study will be the first to identify the causal link between the molecular and physiological mechanisms and the ecological conditions that underlie the lack of an essential metabolic trait in parasitic insects. It will provide important insights into how shifts in metabolic regulation occur and function, and help to unravel how and when organisms can survive while lacking a metabolic trait that is crucial for survival and reproduction in all other organisms.