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. In the context of a recent colonization of a novel host plant by the Melissa blue butterfly We reared larvae on two different host plant species: a native host 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. This result may explain why consumption of the egg casing is not a universal behaviour in insects, due to negative effects on larval immunity.
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