Back off. Cabbageworms excrete a residue that apparently sends ants running.

Caterpillar Pests Ooze Insecticide

If you have a garden, one of the most successful insect marauders in the world probably lurks among the cauliflower and broccoli. Now researchers think they know why the cabbage butterfly is so ubiquitous. The velvety-green caterpillars of the tiny butterfly secrete an unusual insect repellant from glandular hairs, a powerful defense mechanism against predators.

The white and pale yellow Pieris rapae is as plain-Jane as butterflies go. It's both despised by agriculturists and grudgingly respected for its success in colonizing much of the planet. Originally a native of Eurasia and Northern Africa, the butterfly now flits about roadsides and vegetable farms throughout North America, Mexico, Australia, and Hawaii. Its cabbageworm caterpillar, lime green with thin yellow racing stripes, is an eating machine that sports fine glandular hairs. Each is topped with a fluted cup that holds an oily fluid. Although glandular hairs are a common chemical defense mechanism in plants, they're rare in insects.

Reasoning that the secretions play a defensive role that could explain the bug's competitive edge, a team led by biologist Thomas Eisner of Cornell University in Ithaca, New York, analyzed the fluid. The team raised thousands upon thousands of caterpillars and painstakingly collected the tiny fluid droplets over several years. In the 7 May issue of the Proceedings of the National Academy of Sciences, the team reports that the fluid is a complex, unstable mixture of fatty acids, which they've named the mayolenes. The newfound compounds are chemically similar to linolenic acid, a natural insect repellent. Mayolenes appear to work: Ants exposed to P. rapae caterpillars beat a hasty retreat, kept their distance, and repeatedly cleaned themselves--apparently to remove Mayolene residue.

The compounds were named in honor of leading entomologist May Berenbaum of the University of Illinois, Urbana-Champaign, who calls the research appealing. Chemical signals dominate the insect world, says Berenbaum, and this research shows that "new things can be discovered about even the most dirt common of insect pests."

Related site
USGS Northern Prairie Wildlife Research Center--Pieris rapae