Volatile herbivore-induced terpenoids in plant-mite interactions: Variation caused by biotic and abiotic factors

Plants may defend themselves against herbivores by enhancing the effectiveness of natural enemies of herbivores. This is termed “indirect defense,” which may be induced by herbivore damage. An important aspect of induced indirect defense is the attraction of the herbivore's natural enemies to infested plants by the plant emitting so-called “herbivore-induced synomone” (HIS) in response to herbivore damage. In this paper, we review the role of terpenoids in the induced indirect defense of plants against herbivorous mites. HIS are emitted from both damaged and undamaged areas of infested plants, and the composition of HIS varies among different plant species. The emission of HIS may also vary within a plant species, depending upon: (1) plant cultivar, (2) leaf growth stage, (3) the herbivore species that is attacking, and (4) abiotic conditions (light intensity, time of year, and water stress). Predatory mites cope with this variation of HIS by innate recognition as well as temporary specialization to a certain HIS via learning.