Viral pathogens can have major fitness consequences for insects through decreased survivorship, lower weight, reduced reproductive output, and increased development time.  The butterfly Lycaeides melissa has colonized feral alfalfa (Medicago sativa) during the past 200 years; this novel host plant differs nutritionally, phytochemically, and microbially from the common native host plant, Astragalus canadensis. Here, we investigated whether novel host plant use or population effects impact the ability of L. melissa to resist a viral pathogen, Junonia coenia densovirus (JcDV). Maternal females were sourced from both native and exotic populations: offspring acquired from these females were reared on both the native and exotic host plants. Half of the larvae from each host treatment were left as controls and the other half were infected orally with JcDV at the beginning of their fourth instar. Survival, adult weight, and development time were recorded after pupation. We found that infected individuals consuming the novel host plant were less likely to survive infection to adulthood, and individuals consuming the novel host plant had longer development times and lower adult weight. Overall, these results suggest that an understudied cost of novel host plant use is manifest as an increased impact of viral infection, though differences among source populations and years highlight the context dependence of these viral-caterpillar-plant interactions.