The United States Department of Agriculture has learned that plants "scream" chemically when attacked by caterpillars. Corn plants, for example, release a chemical signal when mixed with the saliva of the caterpillar. This signal attracts wasps which lay their eggs in the caterpillar, eventually killing the caterpillar and saving the plant. If the leaf is cut or injured in some other way, the chemical signal is not emitted. Only when the caterpillar saliva mixes with the damaged portion of the leaf is the signal given off.
Because of this elaborate system, a wasp can seek out caterpillars in a huge corn field and stop large scale damage. James Tumlinson, of the USDA says soybeans and cotton plants have a similar defense against pests. We are only beginning to understand how many natural protective mechanisms are built into the environment to help us and protect us. Some pesticides may kill the wasps and not the caterpillars, defeating the system designed to protect the plants. There is much to learn about the design of living things--a design so complex that we maintain it cannot be a product of chance.
--Source Popular Science, October, 1993, page 33.