Host-size selection in parasitoids is shown in many studies to increase fitness in ovipositing mothers due to female progeny emerging from larger (higher quality) hosts. We hypothesized that parasitoids of non-growing hosts (idiobionts) are selective in host size in that they prefer larger individuals, and preferentially use larger hosts for female offspring. Our study system is a unique tritrophic interaction involving a generalist seed-feeding beetle (Stator limbatus) that shows egg-size plasticity based on host tree species, and its egg parasitoid Uscana semifumipennis (Hymenoptera: Trichogrammatidae). On seeds of less favorable host species (suboptimal for development and survival), S. limbatus lays larger eggs than on seeds of favorable host species. We are testing our hypothesis using laboratory and field experiments composed of behavioral assays, performance measures (estimated host egg size, average hind tibia length as proxy for wasp body size), and field distribution of parasitism (absence/presence of parasitism, parasitoid sex ratio). Laboratory data shows that U. semifumipennis mothers are strongly size-selective, preferring larger host eggs. Data is currently being collected in the field to confirm this result. Furthermore, we predict that field data will show that because host egg-size variation corresponds to host plant species, beetles on certain species enjoy less parasitism. Due to variation in the distribution of host plant species, both level of parasitism and wasp body size may vary correspondingly. Our reasoning may also suggest that host-size selection in U. semifumipennis acts to reduce selection on S. limbatus for egg-size plasticity, and also promote host-parasitoid coevolution.