1. Maternally transmitted microbes are ubiquitous. In insects, maternal microbes can play a role in mediating the insect immune response. Less is known about how ecological factors, such as resource use, interact with maternal microbes to affect immunity. 2. In the context of a recent colonization of a novel host plant by the Melissa blue butterfly (Lycaeides melissa), we investigated the interaction between host plant use and vertically transmitted, extracellular egg-associated microbes in determining the...