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The Causes And Consequences Of Variation In The Insect Immune Response
AuthorLangus, Tara C.
AdvisorSmilanich, Angela M.
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Plants produce an array of secondary metabolites that play an important ecological role in defense against herbivores in addition to having varied effects on herbivore natural enemies. Here, I investigate the complex interactions between diet, the immune response, and microbiota. In my first experiment, I asked whether plant chemistry and the presence of egg microbes affect the immune response of a specialist herbivore and its resistance to viral attack. Common buckeye larvae (Junonia coenia) were inoculated with a densovirus (JcDNV) and reared on two host plant species (Plantago lanceolata and Plantago major) that differ in their composition and concentration of iridoid glycosides. In addition, a subset of eggs were surface sterilized to investigate whether microbes on the egg’s surface contribute to viral resistance. Survivorship, development time, pupal weight, hemocyte counts, and a colormetric assay of phenoloxidase (PO) enzyme activity were measured to identify what role plant chemistry and microbes play on the outcome of infection by a pathogen, Junonia coenia densovirus. I found that individuals exposed to the virus did not have significantly lower PO activity compared to the unexposed individuals. Survival was significantly higher in individuals feeding on high iridoid diets. Individuals reared from surface sterilized eggs suffered higher mortality from the virus than individuals that were not sterilized, indicating beneficial egg microbes may interfere with viral inoculation success. Individuals feeding on P. lanceolata had significantly larger pupal mass and increased development time as did those larvae that were exposed to the virus. In summary, these results suggest that plant chemistry and egg microbes play an important role in defense against viral enemies and herbivore performance.Insect herbivores represent a model system for investigating complex tri-trophic interactions between their parasitoid counterparts, the insect immune response, and plant defense. The selective pressures from natural enemies and secondary plant metabolites play a key role in driving narrow herbivore diet breadth. In parallel, plants utilize a variety of chemical defense strategies to deter and reduce the fitness of natural enemies via direct and indirect mechanisms. In my second study, I investigated how intraspecific variation of amides in P. cenocladum affect the immune response of associated specialist neotropical caterpillars, Eois apyraria and Eois nympha (Lepidoptera: Geometridae). This plant is defended by colonies of Phiedole bicornis ants that live in the stems and petioles. When the ants are not present P. cenocladum produces high concentrations of three amides: piplartine, cenocladamide, and 4 -desmethylpiplartine. I hypothesized that intraspecific variation of plant chemistry would affect tri-trophic interactions. Specifically, I predicted there would be a positive affecting parasitoid success through the negative effects on the caterpillar immune response. Phenoloxidase activity was measured from 65 Eois caterpillars that were collected from 18 sites across the La Selva Biological station in Costa Rica. Abiotic and biotic factors such as light availability and ant-plant mutualisms that may influence chemotype-mediated effects on the caterpillar immune response were also explored. I concluded that intraspecific chemical variation is influenced by light availability and the presence/absence of ant mutualists. Overall, the immune response of Eois caterpillars was not affected by feeding on host plants that varied in their concentration of Piper amides.