Eco-evolutionary factors drive herbivore-induced plant volatiles, which intercept plant defense

Herbivore-induced plant volatiles (HIPVs) mediate a wide variety of interactions in ecosystems. However, many volatile compounds are found across plant taxa, and some are exploited to attract natural enemies for biological control. Subtle changes in HIPV blends can produce profound changes in the behavior of predators, parasitoids and herbivores, and may also alter the defensive processes of neighboring plants. I investigated different ecological and evolutionary factors to understand how plant volatile emissions are affected by feeding guild, herbivore diet breadth and domestication. I meta-analyzed 109 studies and found that specialists induce more total volatiles than generalists. Domesticated species have stronger green-leaf volatile induction that wild plants. Chewers induce more volatiles than sap-feeders in most biochemical classes except benzenoids/phenylpropanoids. Particularly, sap-feeders induce more methyl salicylate (MeSA) than chewers. MeSA is of particular interest because it is used commercially as a predator lure; it is the volatile analog of the phytohormone salicylic acid, and provokes strong responses in natural enemies. However, I hypothesized that exposure to MeSA may have unintended consequences on plant defensive pathways. In a field experiment, I investigated interactive effects of MeSA exposure and herbivory on proteins associated with plant defense, herbivore performance, and pathogen resistance. I found that MeSA increased plant resistance to a chewing herbivore, Manduca sexta, and improved resistance to secondary pathogen infection by 25%. This common volatile signal, which is often emitted in response to sap-feeding and other salicylic acid-associated attacks, may not compromise resistance to chewing herbivores due to tradeoffs in guild-specific expression of defensive compounds, and may also buffer against opportunistic pathogens