When I read this thread I was reminded of hearing about "screaming plants" years ago so I looked it up. Here is a link: http://www.celinho.com/vestibulares/vestibulares.php?page=unirio&ano=1994
Here is a quote from about 2/3rd of the way down the page:
Screaming Plants
"When attacked by insects, plants emit a chemical "scream"that summons predators to kill the invading pests, scientists have learned.
U.S. Department of Agriculture researchers in Gainesville, Fla., identified the elaborate defense system in corn plants whose leaves are chewed by caterpillars. Attracted by chemical cues from the injured plants, wasps lay their eggs in the larvae, which die in the ensuing days as the wasp offspring nourish themselves off their hosts.
The chemical "scream"is quite specific: It is released only after caterpillar saliva mixes with the damaged portion of a leaf. A plant that is cut by any other means does not give off the same chemical sign, nor does an uninjured plant. This explains how a wasp can find a caterpillar in a huge field of corn.
Soybeans, cotton plants, and probably many other plants use a similar defense against pests, says USDA scientist James H. Tumlinson. By enhancing this natural response in plants, researchers might reduce or eliminate the need for chemical pesticides, which can cause ecological damage. For example, scientists might breed plants for the trait or transplant specific genes to amplify the chemical's release."
Edited for spelling.