When they are attacked by herbivores, many plants call in reinforcements. To this end, they emit odours. These odours attract wasps, for example, that are parasites and in search for host animals. The wasps lay their eggs into the caterpillars, thereby killing them: this means fewer butterflies and voracious caterpillars in the next generation.
An international research team has tested the effects of twelve types of herbivores on field mustard (Brassica rapa). The researchers found that the plants consistently adapt the odours they emit upon attack to the characteristics of the respective herbivore. This helps the plant to specifically attract natural enemies that feed on the herbivores eating them. Most surprisingly, they emit different odour bouquets in response to exotic as opposed to native herbivores.