Perception of an insect semiochemical primes host-plant defenses

Olfactory cues play a central role in many ecological interactions, including those among plants and insects. Well-documented examples of such interactions include pheromonal communication among insects and the use of plant odors as foraging cues by insect pollinators, herbivores and predators. Recent work demonstrates that plants themselves can also perceive and respond to olfactory cues. Some plant species ready their defenses against herbivores in response to volatile cues emitted by their insect-damaged neighbors. In this study, we demonstrate for the first time that plants can also perceive and respond to olfactory cues emitted by the insects themselves. Our research suggests that tall goldenrod (Solidago altissima) plants exhibit enhanced defense responses following exposure to the volatile emissions of a specialist herbivore, the gall fly Eurosta solidaginis. In a field experiment, female E. solidaginis flies avoided ovipositing in plants that had been exposed to male flies. In laboratory assays, goldenrod plants previously exposed to the fly or crude extracts of its emission suffered significantly less insect-feeding damage than control plants or plants exposed to the pheromone of an unassociated herbivore. Moreover, plants exposed to volatile compounds from the fly exhibited stronger induction of the key defense signaling hormone jasmonic acid following damage. These results suggest that goldenrod plants eavesdrop on signals of their insect antagonists and exploit them as indicators of impending herbivory, thus documenting an entirely new class of olfactory-mediated interactions with broad significance for the evolutionary ecology of plant-insect interactions.