When they find themselves being munched on by caterpillars, many kinds of plants don't just stand there and take it. They call in air strikes by parasitic wasps, which kill the voracious worms.
Biologists have discovered that when certain plants are chewed by caterpillars, they respond by releasing a special blend of aromatic compounds to the air. The compounds attract some species of wasps that inject their eggs into the bodies of caterpillars. The worm immediately slows its feeding by up to 90 percent, and when the wasp egg hatches, the larva feeds on the worm's internal organs.
"The female wasp just takes a second to lay its egg and then it flies off again, presumably looking for another caterpillar," said Ted C. J. Turlings of the U.S. Department of Agriculture's Agricultural Research Service laboratory in Gainesville, Fla.
Turlings and his colleagues, who have been studying this surprising alliance of plants and wasps in recent years, report in today's issue of the Proceedings of the National Academy of Sciences that plant behavior can be even more sophisticated. In experiments on corn plants grown in greenhouses, Turlings and James H. Tumlinson found that the aromatic compounds don't just emerge passively from the place where the caterpillar is chewing the leaf. Instead, some kind of signal goes out to the entire plant and the wasp-recruiting odors are emitted from undamaged leaves as well.
Turlings said the response appears to be triggered by something in the caterpillar's saliva. The scientists found, for example, that they could elicit the response simply by rubbing the saliva into a scratch on a corn leaf. They then snipped off the leaves and tested them in special chambers to see whether wasps flew to them. It turned out that wasps not only flew to damaged leaves but to undamaged leaves from the same plant -- even if the rest of the plant was not in the chamber. Leaves from normal corn plants did not draw wasps.
The government scientists have found the plant-wasp alliance not only in corn but in lima beans, eggplants, cowpeas and, to a lesser extent, cotton. The long-range goal of the research is to find plants that are especially good at recruiting insect allies and then breed those traits into various agricultural crops. The result could be new strains that need less insecticide.
Turlings said he suspects the emitted aromatics evolved originally not to draw wasps but as chemical weapons to be used directly on the attacking insect, to repel it, if not kill it. Some of the substances probably originated as insect toxins, but many insects may long ago have evolved resistance. Turlings said the compounds also may function as antibiotics, preventing the plant's wound from being infected by bacteria or fungi.
"But what we think happened is that the parasitic wasps learned that these volatiles can guide them to the caterpillars," Turlings said.
Parasitic wasps are non-stinging forms whose various species are rarely bigger than a rice grain and more typically measure less than one-tenth as large. Each is specialized to lay its eggs inside the bodies of one or a few species of soft-bodied insects.
Turlings said he suspects that as female wasps evolved the ability to detect plant odors and follow them to caterpillars and other hosts for their young, plants evolved the ability to emit the substances from all their leaves. Presumably this would improve their chances of luring wasps. No one knows how a plant transmits the signal throughout its body.
A similar phenomenon has been found in which plants infested with spider mites emit substances that attract predatory mites that kill pest species.
Still other research, reported a few years ago, has shown that some tree species can detect the aromatic compounds themselves and respond by deploying chemical internal defenses. What happens is that when one tree is attacked by insects, it releases substances that act as an alarm signal, triggering neighboring trees to start making bitter-tasting compounds in their leaves. This way the other trees are better able to repel the pests when they arrive.