The impact of induced plant volatiles on plant-arthropod interactions

Plants release volatile organic compounds from their vegetative tissues into their environment during most of their life cycle. The functions of these volatiles are diverse and not always known but some of these volatiles repel foraging herbivores while others, in turn, attract them and are feeding stimuli. Upon herbivory the amount of volatiles increases dramatically while, simultaneously, also the composition of the blend changes thereby enhancing the attractiveness of the plant to foraging natural enemies and in some cases increasing repellency to herbivores. Hence, herbivore-induced volatiles promote a natural form of biological pest control referred to as “indirect plant defense” and it has often been suggested that this phenomenon could be exploited to enhance crop protection. Here we will introduce the concept of indirect plant defense via volatiles and via other means and outline the current state of knowledge to the extent in which it contributes to protecting a plant to maximize its fitness under natural conditions in an evolutionary and ecological context. Moreover we will summarize the different approaches that have been undertaken to manipulate indirect defenses, either via application of synthetic volatiles or via transgenic manipulation of plant-volatile production, to control the movements of foraging arthropods to improve biological control. Finally, we will discuss to which extent IPM can be improved or even be disrupted via manipulation of plant volatiles.