The Effect of Phenoloxidase Activity on Survival Is Host Plant Dependent in Virus-Infected Caterpillars

Abstract

An important goal of disease ecology is to understand trophic interactions influencing the host-pathogen relationship. This study focused on the effects of diet and immunity on the outcome of viral infection for the polyphagous butterfly, Vanessa cardui Linnaeus (Lepidoptera: Nymphalidae) (painted lady). Specifically, we aimed to understand the role that larval host plants play when fighting a viral pathogen. Larvae were orally inoculated with the entomopathogenic virus, Junonia coenia densovirus (JcDV) (Parvovirididae: Densovirinae, Lepidopteran Potoambidensovirus 1) and reared on two different host plants (Lupinus albifrons Bentham (Fabales: Fabaceae) or Plantago lanceolata Linnaeus (Lamiales: Plantaginaceae)). Following viral infection, the immune response (i.e., phenoloxidase [PO] activity), survival to adulthood, and viral load were measured for individuals on each host plant. We found that the interaction between the immune response and survival of the viral infection was host plant dependent. The likelihood of survival was lowest for infected larvae exhibiting suppressed PO activity and feeding on P. lanceolata, providing some evidence that PO activity may be an important defense against viral infection. However, for individuals reared on L. albifrons, the viral infection had a negligible effect on the immune response, and these individuals also had higher survival and lower viral load when infected with the pathogen compared to the controls. Therefore, we suggest that host plant modifies the effects of JcDV infection and influences caterpillars' response when infected with the virus. Overall, we conclude that the outcome of viral infection is highly dependent upon diet, and that certain host plants can provide protection from pathogens regardless of immunity.

Keywords: Junonia coenia densovirus; fitness; host plant; immune response; pathogen.

Fig. 1.

Panels (a) and (b) show posterior probability plots depicting the main effect of host plant on PO activity. The distribution of PO activity for the host plant, L. albifrons, is in the left panel and P. lanceolata in the right panel. The vertical line within the plot shows a zero effect size, or no difference between groups (see Supp Table 1 [online only] for unscaled mean PO values). Boxplots beneath posterior distributions show the interaction between host plant and virus on PO activity. Negative values indicate that the predictor variable decreased PO activity and vice versa for positive values. Thus, in the case of Plantago × Virus (right panel), there is a negative effect of virus on PO activity, while the Lupine × Virus interaction (left panel) shows PO values close to zero, or no difference between virus and control groups.

Fig. 2.

Top panel (a) shows differences in survival frequency for V. cardui caterpillars on each host plant under infected (virus) and uninfected (control) conditions. Notably, survival was higher for control caterpillars on Plantago while virus-infected caterpillars had higher survival on Lupine. The bottom panel (b) shows virus load of caterpillars reared on each host plant (expressed as log-transformed viral equivalent genomes). Individuals reared on Lupine had lower viral loads compared to individuals reared on Plantago, putatively explaining differences in survival frequency.