Multitrophic effects of herbivore-induced plant volatiles in an evolutionary context

Herbivorous and carnivorous arthropods use plant volatiles when foraging fo r food. In response to herbivory, plants emit a blend that may be quantitat ively and qualitatively different from the blend emitted when intact. This induced volatile blend alters the interactions of the plant with its enviro nment. We review recent developments regarding the induction mechanism as w ell as the ecological consequences in a multitrophic and evolutionary conte xt. It has been well established that carnivores (predators and parasitoids ) are attracted by the volatiles induced by their herbivorous victims. This concerns an active plant response. In the case of attraction of predators, this is likely to result in a fitness benefit to the plant, because throug h consumption a predator removes the herbivores from the plant. However, th e benefit to the plant is less clear when parasitoids are attracted, becaus e parasitisation does usually not result in an instantaneous or in a comple te termination of consumption by the herbivore. Recently, empirical evidenc e has been obtained that shows that the plant's response can increase plant fitness, in terms of seed production, due to a reduced consumption rate of parasitized herbivores. However, apart from a benefit from attracting carn ivores, the induced volatiles can have a serious cost because there is an i ncreasing number of studies that show that herbivores can be attracted. How ever, this does not necessarily result in settlement of the herbivores on t he emitting plant. The presence of cues from herbivores and/or carnivores t hat indicate that the plant is a competitor- and/or enemy-dense space, may lead to an avoidance response. Thus, the benefit of emission of induced vol atiles is likely to depend on the prevailing faunal composition. Whether pl ants can adjust their response and influence the emission of the induced vo latiles, taking the prevalent environmental conditions into account, is an interesting question that needs to be addressed. The induced volatiles may also affect interactions of the emitting plant with its neighbours, e.g., t hrough altered competitive ability or by the neighbour exploiting the emitt ed information. Major questions to be addressed in this research field comprise mechanistic aspects, such as the identification of the minimally effective blend of vo latiles that explains the attraction of carnivores to herbivore-infested pl ants, and evolutionary aspects such as the fitness consequences of induced volatiles. The elucidation of mechanistic aspects is important for addressi ng ecological and evolutionary questions. For instance, an important tool t o address ecological and evolutionary aspects would be to have plant pairs that differ in only a single trait. Such plants are likely to become availa ble in the near future as a result of mechanistic studies on signal-transdu ction pathways and an increased interest in molecular genetics.