In a study done by the University of Georgia, scientists showed that plants send out very specific signals for parasitic wasps to come to their aid when under attack by herbivores. Parasitic wasps are the natural enemies of some of the plant herbivores (insects that eat plants). The scientists used cotton, tobacco and maize, all which emitted the signals for the wasps.
These wasps are only parasitic on certain herbivore species. Apparently, the signal sent out by the plant attracts the very species of wasp that parasitizes the herbivore.
In the study, plants were infected with two species of herbivores. Only one species was the host for the parasitic wasp. The wasps were able to distinguish which plants were infested by their host, and which plants were being attacked by another species of herbivore that they do not parasitize. These they left alone.
The scientists left some of the plants with the damaged parts and herbivores intact, and on other plants the damaged material and pests were removed. There was no significant difference in attraction to the wasps, showing that the wasps were not just attracted by seeing damaged plants or the herbivore, but by the chemicals released by the plant.