Abstract
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 strength of the insect immune response.
3. We reared larvae on two different host plant species: a native host Astragalus canadensis and a novel host Medicago sativa. Egg-associated microbes were removed through a series of antimicrobial egg washes prior to hatching. Immune response was measured through three assays: standing phenoloxidase (PO), total PO, and
melanization.
4. We detected strong effects of microbial removal. Egg washing resulted in larvae with an increased immune response as measured by total PO- contrary to reports from other taxa. The effect of washing was especially strong for larvae consuming the native host plant.