rkrehn:

The lovely scent of cut grass is the reek of plant anguish: When attacked, plants release airborne chemical compounds. Now scientists say plants can use these compounds almost like language, notifying nearby creatures who can “rescue” them from insect attacks.

A group of German scientists studying a wild tobacco plant noticed that the compounds it released - called green leaf volatiles or GLVs - were very specific. When the plants were infested by caterpillars, the plants released a distress GLV that attracted predatory bugs who like to eat the caterpillars in question.

Plants emit green leaf volatiles (GLVs) in response to herbivore damage, thereby attracting predators of the herbivores as part of an indirect defense. The GLV component of this indirect defense was thought to be a general wound signal lacking herbivore-specific information. We found that Manduca sexta–infested Nicotiana attenuata attract the generalist hemipteran predator Geocoris spp. as the result of an herbivore-induced decrease in the (Z)/(E) ratio of released GLVs, and that these changes in the volatile bouquet triple the foraging efficiency of predators in nature. These (E)-isomers are produced from plant-derived (Z)-isomers but are converted by a heat-labile constituent of herbivore oral secretions. Hence, attacking herbivores initiate the release of an indirect defense a full day before the attacked plants manufacture their own defensive compounds.