Plants Send SOS When Caterpillars
Corn earworm on an immature cotton boll.
By emitting chemical distress signals, some corn, cotton, and tobacco plants
can summon tiny parasitic wasps to rescue them from hungry caterpillars like
the tobacco budworm and corn earworm. Breeding crop varieties specifically for
their wasp-calling traits is of keen interest to scientists seeking ways to
reduce insecticide use.
But until recently, researchers believed that plants use the same SOS
distress signal--no matter what the attacker. [See "Beneficials Are Money
in the Bank," Agricultural Research, February 1993, pp. 20-22.]
Now, a team of scientists has proven otherwise by showing that plants actually
emit signals specific to the caterpillar pest attacking them.
By releasing blends of 10 to 12 different chemical compounds called
volatiles, plants can quickly and accurately communicate their attacker's
identity to friendly parasitic wasps--a finding that could improve their use in
integrated pest management programs.
This chemical tête-à-tête "is a much more
sophisticated system than we ever suspected," says University of Georgia
graduate student Consuelo DeMoraes. She conducted the study under the direction
of Agricultural Research Service
entomologist W. Joe Lewis and his colleague, ARS chemist James H. Tumlinson.
They reported the finding in the June 1998 issue of Nature.
Before their work, "it had not been shown scientifically that plants
could send specific information," says Lewis. He is at ARS' Insect Biology
and Population Management Research Laboratory in Tifton, Georgia.
Specificity is key to survival of both the plant and the wasps, he adds. For
example, Cardiochiles nigriceps wasps can only reproduce using tobacco
budworms as hosts for their brood. In fact, the finicky wasp will ignore plants
signaling attack by the budworm's close relative, the corn earworm--a pest with
its own natural enemies.
By releasing a large, aromatic plume into the air, plants not only help
their wasp allies quickly find them, they also tell the wasp who their attacker
is. This helps the wasp avoid wasting precious energy.
"Without a specific signal from the plant, the parasitoid might
otherwise spend all its time tracking false cues," says Lewis. "It's
like finding a needle in a haystack. It has to find this one particular host in
order to reproduce."
In field trials at Tifton, scientists observed that C. nigriceps
wasps more often flew to plants signaling attack by tobacco budworms than to
those pestered by corn earworms. In experiments with tobacco, tobacco
budworm-infested plants accounted for 164 out of 198 total wasp visits.
In separate experiments, the scientists also monitored plants whose leaves
had been chewed by the caterpillars and subsequently removed. This eliminated
the possibility of the wasps' homing-in on chemical cues in the caterpillars'
saliva or feces, rather than on the plants' signals. As before, C.
nigriceps zeroed-in on the tobacco budworm-damaged plants. This happened 32
of 48 times.
The scientists also ran a chemical analysis of the plants' signals using a
special glass sleeve and gas chromatography techniques. This showed consistent
differences in concentrations of volatile compounds emanating from the plants,
depending on which caterpillar pest had attacked them.
Lewis is also monitoring the effects of too much nitrogen on the signaling
capacity of cotton plants.
"You can select varieties for their inherent ability to defend
themselves," he says. "But you also have to know how to manage them
so that trait is expressed."--By Jan Suszkiw, Agricultural Research
Service Information Staff.
W. Joe Lewis is at the
USDA-ARS Insect Biology and
Population Management Research Laboratory, P.O. Box 748, Tifton, GA 31793;
phone (912) 387-2348, fax (912) 382-9467.
"Plants Send SOS When Caterpillars Bite" was published in
the October 1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.