The tiny larvae of the willow leaf beetle, no bigger than this i, defend their feeding grounds from competitors such as hulking caterpillars -- thousands of times heavier -- by using a kind of chemical warfare, a University of Maryland entomologist has discovered.

Normally the beetle larvae spend their days quietly munching weeping willow leaves. When a major competitor nears, however, special glands in the larva's body exude an aromatic substance onto the leaf. One whiff induces such a powerful response in the caterpillars that they immediately vomit and crawl away.

"You can find these beetle larvae on weeping willow trees all over the Washington area and smell the chemical for yourself," said Michael J. Raupp, the scientist. "To me it smells kind of pleasant, sweet. But to the caterpillars it's an irritant, and they give the barf response."

Raupp learned how the beetle's defenses work by bringing some into the laboratory, letting them feed on willow leaves and introducing caterpillars. He found the insect behemoths quickly learned to avoid leaves occupied by the little larvae and to feed elsewhere. Raupp proved the chemical was the key by squeezing some out of larvae and smearing it on leaves himself. The caterpillars' vomiting response was just the same. Dead larvae, unable to exude the chemical, did not repel caterpillars.

Raupp, along with colleagues Frank R. Milan and Pedro Barbosa of the University of Maryland and Barbara A. Leonhardt of the Agricultural Research Service in Beltsville, reported the findings in last week's Science.