Diet quality improves survival and mediates larval response to densovirus infection in the Melissa blue butterfly

Entomopathogenic viruses affect insect populations by reducing fitness and influencing other measures of performance such as development and longevity. Given the huge diversity of viruses, studies are needed to understand the mechanisms underlying how viruses affect their insect hosts. The Junonia coenia densovirus (JcDV) infects a wide range of Lepidoptera and can cause death in the larval and pupal stages. In this study, we used the Melissa blue butterfly (Lycaeides melissa: Lycaenidae) to describe infection progression at the individual level, the effects of infection on development, survival, and longevity, and whether plant diet influenced these factors. Caterpillars were infected with JcDV and reared on either a low-quality host plant (Medicago sativa) or one of two high-quality host plants (Astragalus canadensis or Lotus nevadensis). We found that average viral load was similar across six days post-inoculation with a slight peak on day four. Viral load was highest among individuals reared on the low-quality host plant. Infection resulted in faster development time compared to uninfected individuals, and diet-specific effects were evident in survival to adulthood, with the native host plant (A. canadensis) offering optimal conditions for survival. None of the infected individuals that reached adulthood successfully eliminated JcDV, but viral loads were much lower in adults than larvae, indicating individuals reduced or maintained low viral loads between life stages. These results suggest diet-dependent response to JcDV infection and have implications for understanding the mechanisms that facilitate or impede the colonization of novel host plants.