Signal-Sending Plants Identify Their Attackers

When caterpillars attack, some corn, cotton and tobacco plants release chemical SOS signals to recruit help from friendly parasitic wasps. But the wasps may not answer unless the caterpillar in question is a species they prefer.

To avoid "no-shows," scientists recently found, plants customize their signals, advertising their attackers' identity and ensuring that the right wasp comes calling.

The finding refutes an earlier belief that wasp-calling plants emit an all-purpose signal, regardless of the caterpillar species. So say entomologist Joe Lewis and chemist James Tumlinson, of the Agricultural Research Service, and University of Georgia graduate student Consuelo DeMoraes. ARS is the U.S. Department of Agriculture's chief scientific arm.

Their work aims to curb insecticide use through integrated pest management and breeding crops with stronger signaling traits. DeMoraes led the study under the guidance of Tumlinson, in Gainesville, Fla., and Lewis, at ARS' Insect Population Management Research Laboratory in Tifton, Ga.

In Georgia field trials, they saw Cardiochiles nigriceps wasps flying to plants signaling an attack by tobacco budworms—a host these wasps prefer—more often than to plants chewed by corn earworms, a related caterpillar species. Budworm-infested plants accounted for 164 of 198 total wasp visits.

The team also monitored plants after removing any leaves that caterpillars had chewed on. This ensured the wasps weren't homing in on chemical cues in their caterpillar host's saliva or in feces, rather than the plant's own signals Indeed, the Cardiochiles wasps visited budworm- damaged plants 32 of 48 times.

Gas chromatography revealed consistent differences in concentrations of about 12 chemical volatile compounds rising from the plants. The differences depended on which caterpillar species was attacking.

An article about the research, in this month's Agricultural Research magazine, is also on the World Wide Web at:

http://www.ars.usda.gov/is/AR/archive/oct98/sos1098.htm

Scientific contact: Joe Lewis, ARS Insect Biology and Population Management Research Laboratory, Tifton, Ga., phone (912) 387-2348, fax (912) 382-9467, wjl@tifton.cpes.peachnet.edu.