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Entomologist Allard Cossé
prepares a sawfly antenna
for GC-EAD analysis.
(K9447-1)
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The wheat stem sawfly, Cephus
cinctus, is a pest that poses dilemmas for Northern Great Plains farmers,
and entomologist Allard A. Cossé and his colleagues hope to help make
their decisions a bit easier one day.
Thanks to plant breeders who have developed thick-stemmed, sawfly-resistant
wheat varieties, catastrophic yield losses can be avoided—but at the cost
of forgoing a potential bumper crop. Only in some years do hordes of the pest
bore through wheat stems and drastically cut yields. So first the farmer must
decide whether to plant a higher yielding, nonresistant variety or one that's
resistant and that directs more of its energy into making thick stems.
If the farmer chooses a nonresistant variety and the spring emergence of adult
sawflies is large, there is only a 1- to 2-week period in which to spray
insecticides. Once the eggs are laid and the larvae are feeding safely within
wheat stems, insecticides have little effect. Knowing exactly when and where
adults are emerging and whether the population is dangerously large could help
the farmer make timely and effective pest-control treatments. Someday, traps
baited with attractants for adult sawflies could provide the needed
information, says Cossé.
A chemical ecologist, Cossé is interested in sawfly attractants as
potential trap baits or for any other novel sawfly-control approaches. But the
first step is to learn what the natural sawfly
attractants—pheromones—are. To take some of the mystery out of what
causes sawflies of one gender to be attracted to the other, the scientist gets
down to one of the basics—the sawfly's antennae, where its sense of smell
is located.
In the laboratory at the National Center for Agricultural Utilization Research,
in Peoria, Illinois, a sawfly antenna that's suspended between tiny electrodes
sends out an electrical signal when exposed to any of the airborne components
of sawfly pheromone. An instrument called a gas chromatograph separates the
complicated blend of chemicals found in the air around wheat plants swarming
with sawflies. The compounds emerge from the instrument one by one and are sent
simultaneously to two detectors—one that senses all compounds and the one
that includes the antenna. The antenna detector senses just a few compounds,
such as pheromone components, that are essential to sawfly biology. The results
are recorded on two graphs, one for each kind of detector.
The technique is called coupled gas chromatographic-electroantennographic
detection (GC-EAD). It allows the scientist to pinpoint exactly which chemicals
are critically important to the sawflies from among the myriad, mostly inert,
compounds. And once scientists know which chemicals to concentrate on, they can
focus on determining their chemical structure and how they function.
"We've found that the sawfly pheromone system is unusual," says
Cossé. Unlike most insects, both genders of sawfly produce the same
odors, but in different proportions. Adding to the complexity is that scents
from the wheat plant also influence sawfly behavior. That's useful information
as the scientists formulate various mixes of volatile chemicals to research the
insect's behavior.
Wind-tunnel experiments simulate conditions sawflies might respond to in the
field. The wind tunnel offers a quick way to evaluate the importance of various
compounds as the scientists develop synthetic blends as trap baits. The
complete blend isn't always needed to get a response. David Weaver,
Cossé's collaborator at Montana State University, found that one of the
body odor chemicals, which project leader Robert J. Bartelt produced
synthetically, was potent enough by itself to attract sawflies to traps in the
field.
Among the odors produced mainly by male sawflies are 9-acetyloxynonanal,
phenylacetic acid, and tetradecanal. When two or more males are together, they
emit musky-smelling phenylacetic acid in amounts large enough to be sensed by
the human nose. In nature, clouds of aggregated males advertise themselves to
nearby females, which in turn signal their readiness to mate by emitting their
own unique blend of compounds. The emissions from female wheat stem sawflies
are relatively rich in hexadecanal.
While some sawfly species produce volatile chemicals similar to those of the
notoriously destructive pine sawfly, the wheat stem sawfly produces vastly
different chemicals.
The Peoria scientists are using GC-EAD to research other insects, such as sap
beetles, which are pests of figs, dates, and corn; other sap beetles that
spread a fungus which causes oak wilt of trees; exotic leaf beetles, which may
someday control weedy tamarisk trees along western U.S. streams; exotic flea
beetles, which are being used as biological control agents against leafy
spurge; and flea beetles, which are pests of canola and other cruciferous
crops.—By Ben
Hardin, Agricultural Research Service Information Staff.
This research is part of Crop and Commodity Pest Biology, Control, and
Quarantine (#304), an ARS National Program described on the World Wide Web at
http://www.nps.ars.usda.gov.
Allard A. Cossé and
Robert J. Bartelt are with
the USDA-ARS National Center for
Agricultural Utilization Research, 1815 N. University St., Peoria, IL
61604; phone (309) 681-6217, fax (309) 681-6693. |
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