Olfactory cues play important roles 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 parasitic plants grow toward host-plant odors and other plant species ready their defenses against herbivores in response to volatile cues emitted by their insect-damaged neighbors. Despite such intriguing discoveries, we currently know relatively little about the occurrence and significance of plant responses to olfactory cues in natural systems. In this study, we demonstrate for the first time that plants can also perceive and respond to olfactory cues emitted by insect herbivores. Our research indicates 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.
Results/Conclusions
Male E. solidaginis perch on S. altissima stems and release copious amounts of a volatile emission attractive to females (which we hypothesize to act as a sex pheromone). In a field experiment, E. solidaginis females avoided ovipositing into plants previously exposed to the emissions of male flies. Furthermore exposed plants received less overall damage from foliar herbivores than controls. In laboratory assays, S. altissima plants exposed to the male fly (or a crude extract of its emission) likewise suffered less insect feeding damage than control plants, and also exhibited stronger induction of the key defense signaling hormone jasmonic acid following damage. Together, these results indicate that goldenrod plants respond physiologically to the emissions of a specialist herbivore and may in fact exploit the fly emission as a cue of impending risk of herbivory. This finding thus documents an entirely new class of olfactory-mediated interactions with broad significance for the evolutionary ecology of plant-insect interactions. Additionally, these results may have applied relevance for management of many important agricultural and forest insect pests that release pheromones in proximity to host plants.