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.