Host plant associated enhancement of immunity and survival in virus infected caterpillars

A path analysis of the relationship between viral infection, pupal mass, development time, and the immune response as measured by PO activity (the best fit model to our data; X2 = 0.7429, df = 2, P = 0.6798). The only causal hypothesis strongly supported by the model is that viral infection caused significantly shorter development times for caterpillars (t = 3.42, df = 2, P = 0.0006). All other pathways were non-significant (P > .05). [Display omitted] •We tested host plant, virus, and egg microbe effects on immunity and survival.•Infected caterpillars had higher survival and PO when reared on a high IG plant.•Infected caterpillars had lower hemocyte couts and shorter development time.•When faced with pathogen challenges host plant association can facilitate survival. Understanding the interaction between host plant chemistry, the immune response, and insect pathogens can shed light on host plant use by insect herbivores. In this study, we focused on how interactions between the insect immune response and plant secondary metabolites affect the response to a viral pathogen. Based upon prior research, we asked whether the buckeye caterpillar, Junonia coenia (Nymphalidae), which specializes on plants containing iridoid glycosides (IGs), is less able to resist the pathogenic effects of a densovirus infection when feeding on plants with high concentrations of IGs. In a fully factorial design, individuals were randomly assigned to three treatments, each of which had two levels: (1) exposed to the densovirus versus control, (2) placed on a plant species with high concentrations of IGs (Plantago lanceolata, Plantaginaceae) versus low concentrations of IGs (P. major), and (3) control versus surface sterilized to exclude surface microbes that may contribute to viral resistance. We measured phenoloxidase (PO) activity, hemocyte counts, and gut bacterial diversity (16S ribosomal RNA) during the fourth larval instar, as well as development time, pupal weight, and survival to adult. Individuals infected with the virus were immune-suppressed (as measured by PO response and hemocyte count) and developed significantly faster than virus-free individuals. Contrary to our predictions,mortality was significantly less for virus challengedindividuals reared on the high IG plant compared to the low IG plant.This suggests that plant secondary metabolites can influence survival from viral infection and may be associated with activation of PO. Removing egg microbes did not affect th