Herbivorous insects tolerate chemical variation in their host plant diet by modulating several traits. Insect immune response is one trait that plays a crucial role in maintaining fitness but can be heavily influenced by variation in host plant quality. An important question is how the use of different host plants affects the ability of herbivorous insects to resist viral pathogens. Furthermore, the transcriptional changes associated with this interaction of diet and viral pathogens remain understudied. The Melissa blue butterfly (Lycaeides melissa) has colonized the exotic legume Medicago sativa as a larval host within the past 200 years. Here we study the interplay between the effects of host plant variation and viral infection on larval immune responses and global gene expression. We measured immune strength in response to infection by the Junonia coenia densovirus (JcDV) and performed transcriptional sequencing of L. melissa larvae exposed to different viral and host plant treatments. Our results demonstrate that viral infection caused total phenoloxidase (total PO) to increase and viral infection and host plant interactively affected total PO such that for infected larvae, total PO was significantly higher for larvae consuming the native host plant. Additionally, larvae differentially expressed hundredgenes in response to host plant treatment, but with minimal changes in gene expression in response to viral infection. These results demonstrate that in herbivorous insects, diet can alter both physiological and transcriptional responses relevant to viral infection, emphasizing the importance of considering immune and detoxification mechanisms into models of evolution of host range in insects.

Insect immune response plays a crucial role in how external threats influence overall fitness through life history traits. An understudied question is how the use of different host plants might affect the ability of herbivorous insects to resist viral pathogens. The Melissa blue butterfly (Lycaeides melissa) has colonized the exotic legume Medicago sativa as a larval host within the past 200 years. Here we investigate how novel host plant use affects the immune response of L. melissa when infected with the lepidopteran virus, Junonia coenia densovirus (JcDV). We measured immune strength in response to JcDV in two ways: 1) direct measurement of phenoloxidase activity and melanization, and 2) transcriptional sequencing of individuals exposed to different viral and host plant treatments. Viral infection caused total phenoloxidase (total PO) to increase. We detected an interaction between viral infection and host plant for total PO: for control larvae, host plant use had no effect on total PO, whereas for infected larvae, total PO was significantly higher for larvae consuming the native host. Within the exotic host plant treatment, few genes were differentially regulated due to viral infection. Approximately two times more genes were differentially regulated in response to infection for larvae eating the native or exotic host, with differential expression of few putative immune genes. These results demonstrate that consumption of a novel host plant can alter both physiological and transcriptional responses to infection, emphasizing the importance of understanding diet when studying the molecular basis of immune function.