**8. Aphid-plant-natural enemy tritrophic interaction: the "cry or call for help"**

In response to aphid feeding, plants release a number of volatile compounds which are perceived by aphid natural enemies. Since plants employ these natural enemies to defend themselves, the release of volatile compounds is analogous to "cry or call for help" by plants. This type of defense is referred to as indirect defense. A number of insects are associated with natural suppression of aphid population which includes predators such as ladybird beetles (e.g., *Coccinella* spp., Brumus sp., *Adalia bipunctata* L., *Menochilus* sp., etc.), green lacewing (*Chrysoperla carnea* Stephens), syrphids (*Episyrphus balteatus* De Geer), mirid bugs, and parasitoids (*Aphidius* spp., *Diaeretiella rapae* M'Intosh, *Praon* spp., etc.). However, these natural control agents are not efficient in suppressing aphid population, and there is a lack of synchrony in the peak activity of aphids and their natural enemies [63]. Aphid populations generally develop early in the season (mostly in spring) with delayed action of natural control agents. But once their action has started, there is sudden decline in aphid population as observed in oilseed Brassica [66] and organic crops [70].

The feeding by aphids triggers the release of volatile compounds from infested plants making them more attractive to parasitoids. For example, *Acyrthosiphon pisum-*infested broad bean plants are six times more attractive to *Aphidius ervi* Haliday than uninfested plants [71]. Similarly, *Brassica rapa* L. var. *rapifera* plants infested either by *L. erysimi* or *M. persicae* become more attractive to *D. rapae*. This increase in attractiveness has potential implications in aphid control, and researchers are working to find possible ways to elicit this attractiveness in uninfested plants. For example, exogenous application of (Z)-jasmone, a compound derived from jasmonic acid, results in increased attractiveness of uninfested broad bean plants to *A. ervi* similar to those infested by *A. pisum* [72].
