Effects of plant chemistry and egg microbes on the immune response of a specialist caterpillar

Background/Question/Methods

Insect herbivores represent a model system for investigating the complex interactions between plant nutrition, the immune response, and beneficial microbiota. In this study, we asked whether plant chemistry and the presence of egg microbes affect the immune response of a specialist herbivore and its ability to resist viral attack. Common Buckeye larvae (Junonia coenia) were inoculated with a densovirus (JcDNV) and reared on two host plant species, Plantago major and Plantago lanceolata. These plant species differ in their chemical makeup of monoterpene-derived compounds, iridoid glycosides, with P. major containing low concentrations and P. lanceolatacontaining high concentrations. In an additional treatment, a subset of eggs were surface sterilized to investigate whether microbes on the egg’s surface contributes to viral resistance. Multi-dimensional measures of survivorship, development time, pupal weights, hemocyte counts, and a colormetric assay of phenoloxidase (PO) enzyme activity were used to identify what role plant chemistry and microbes play on the outcome of infection by a pathogen.

Results/Conclusions

We found that individuals exposed to the virus had significantly lower PO activity compared to the unexposed individuals. However, there was no effect of plant diet on PO activity. Survival was significantly higher in individuals feeding on P. lanceolata, the host plant with high concentrations of iridoid glycosides. The effect of chemistry, however, was not directly on the immune response itself, but most likely on the virus, possibly interfering with the ability of the virus to successfully inoculate the Buckeye caterpillar. Individuals that were surface sterilized as eggs suffered significantly higher mortality than individuals that were not sterilized, indicating beneficial microbes on the egg’s surface that may interfere with viral inoculation success. Pupal weights were significantly higher on P. lanceolata, but did not differ by virus exposure or egg sterilization. Individuals feeding on P.major had significantly increased development time as did those larvae that were not exposed to the virus, and individuals that were surface sterilized as eggs. In summary, these results suggest that plant chemistry and egg microbes play an important role in defense against viral enemies and herbivore performance.