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. 

5.     This result may explain why consumption of the egg casing is not a universal behaviour in insects, due to negative effects on larval immunity.