If an insect attacks a plant, the plant likely won't just stand there and take it. It'll fight back in one or more ways. One long documented defense tactic is the production of chemical defenses that poison, repel or slow the growth of the plant's attackers.
The problem with this approach to self-defense is that often, the herbivorous insects evolve resistance to these defenses. Recent studies by the Max Planck Institute for Chemical Ecology documented a second way in which plants sometimes use chemicals to protect themselves.
The study focused on herbivore-induced volatiles from wild tobacco in the Great Basin Desert of the southwestern U.S.A. It found that chemicals released by that plant when it's being eaten increase the hunting efficiency of predators and parasitoids that eat the larvae and eggs of the plant's dominant herbivore, the five-spotted hawkmoth. (pictured above at left).
Parasitoids and predators can face a difficult challenge trying to locate their prey in a vast expanse of green plants. By releasing volatile signals that can be detected by other insects, plants assist the predators, essentially saying 'here's one for you to attack'. This phenomenon has also been reported from studies with corn, which attracts parasitic wasps when attacked by army worms, and by lima beans after damage by spider mites.
This type of knowledge could possibly be used to develop synthetic imitations of these chemicals that would give farmers another weapon in their fight against crop destroying insects, a weapon that would probably be less of a threat to the environment than many of the chemicals now in use.