Watch Out, Soft-Bodied Pests!
A wild tobacco plant, Nicotiana glutinosa, is a source of sugar esters
used as an experimental natural insecticide.
New weapons from the Agricultural
Research Service could spell better control over silverleaf whiteflies and
other soft-bodied insect pests, such as aphids.
When silverleaf whiteflies, Bemisia argentifolii, infest a
plant, they look like tiny specks of ash covering leaves and stems. The pests
suck sap from more than 600 kinds of plants including many fruit, vegetable,
fiber, and ornamental crops. Their saliva can also transmit destructive plant
viruses and disorders. Every year, whiteflies cause multimillion-dollar crop
losses in the United States.
But ARS scientists are adding to the limited arsenal against this pest. They
recently came up with a new way to apply insecticides that whiteflies can't
readily escape. They also ranked wild species of tobacco plants for their
commercial potential as sources of a natural insecticide known as sugar esters.
And they devised lookalike synthetic versions of the natural compounds that
were the forerunner of a commercial version soon to be a registered
Conventional insecticide sprayers have had only limited success against this
"That's because a high percentage of the adult and immature stages of
B. argentifolii feeds and breeds on the lower surface of plant
leaves," says ARS entomologist Alvin M. Simmons. "Most sprayers do
not deliver insecticides effectively to the underside of plant leaves."
For this and other reasons--including insecticide cost and effectiveness and
ecological, health, and aesthetic concerns--Simmons and his colleague,
entomologist D. Michael Jackson, have been exploring alternative technologies
to reduce the amounts of insecticide used and to better target them on fruits
All Fogged In
Under a cooperative research and development agreement with Strauch and
Sons, Inc., of Bethesda, Maryland, the scientists tested and evaluated an
ultrasonic fogging device.
Originally, the fogger was designed as a humidifier and fungicide applicator
for produce in storage. The scientists wanted to see if it could effectively
deliver low dosages of contact insecticides to whitefly-infested plants growing
in a greenhouse.
Greenhouse whitefly control is crucial--not only because seedlings can be
severely damaged, but also because this is where field outbreaks can get their
start. Safe from winter and the weather, whiteflies can spend all year breeding
and feeding on greenhouse plants. Then, when infested seedlings are
transplanted outdoors, so are the whiteflies.
The scientists conducted fogger tests on whitefly-infested collard plants in
"The fogger dispenses about 4 gallons of water per hour, producing
droplets of about 5 microns in diameter," Simmons says. "The droplets
are so small, they act much like a gas."
The scientists showed that the fogger could successfully apply two
whitefly-killing sprays in a mix with water that reached and coated the lower
One of the sprays used imidacloprid--one of the newest commercially
available whitefly insecticides. The other spray was an experimental natural,
or "biorational," insecticide consisting of sugar esters extracted
from a species of wild tobacco plant known as Nicotiana glutinosa.
"Sugar esters break down the insects' outer coating, causing the pests
to shrivel as they lose water," says Simmons. "The esters are
relatively nontoxic to hard-bodied beneficial insect predators like lady
beetles and to other beneficials."
With the fogger, similar amounts of the sprays coated both the top and
bottom of infested leaves.
"In just 6 minutes, the fogger delivered less than half as much
imidacloprid as the label recommended for control," Simmons says. But the
result was a 100-percent whitefly kill.
The scientists say the new fogger could work in commercial greenhouses as an
automated method of controlling whiteflies.
"It would be ideal for controlling whiteflies and other target pests on
high-value crops like poinsettias that have low damage thresholds and on
greenhouse-grown vegetable crops for which low levels of contact insecticides
are preferred," Simmons says.
Simmons and Jackson believe the fogger could also be adapted for field use,
but the fog would need to be housed to protect it from the wind until it could
reach the plants. The fogger is available commercially from Shira Aeroponics,
Ltd., of Rehovot, Israel.
While the fogger applies sugar esters in an effective manner, "current
processing and production practices have been too expensive to make growing
plants for their esters feasible commercially," says Jackson. "For
the amounts of sugar esters that have been produced per acre, relative to the
cost of extraction, it has been too expensive."
One problem has been not knowing which, of all the natural esters available
from Nicotiana, would have the best commercial potential. To find out,
Jackson and Simmons worked with other ARS scientists to grow and screen 21 wild
Nicotiana species in field tests.
Cooperators included chemist Orestes T. Chortyk, who is retired from the
former ARS Phytochemical Research Unit in Athens, Georgia; ARS agronomist
Michael G. Stephenson at Tifton, Georgia; entomologist Chris D. Harlow and
geneticist Vernon A. Sisson from the North Carolina State University in Oxford;
and entomologist Albert W. Johnson at Clemson University in Florence, South
"We selected eight candidate species, including N. glutinosa,
for further study to determine which were the best sources of sugar
esters," Jackson says.
In 1995, the scientists grew plants of all eight species in field plots at
Florence and Charleston, South Carolina, and at Tifton. Three times during the
season at each location, they cut the aboveground portion of the plants,
weighed them, and extracted chemicals from the leaves.
"We believe the best commercial candidate overall is N.
trigonophylla. Its esters are the least complex, very concentrated, and
comparatively easy to extract," says Jackson.
Toward a Suitable Synthetic
At another ARS laboratory, in Kearneysville, West Virginia, ARS entomologist
Gary Puterka focuses on a different soft-bodied insect pest--the pear psylla,
Cacopsylla pyricola. This yellowish-green insect may be the
primary reason the East Coast pear industry has disappeared.
Sugar esters have been among the most successful biorational compounds
Puterka has used against these pests in tests at ARS' Appalachian Fruit
Research Station. When he sprayed esters mixed with water on pear leaves, the
compounds killed both nymphs and adults.
Searching for an easier and cheaper way to mass-produce the esters, Puterka
worked with Ava Chemical Ventures of Portsmouth, New Hampshire. Recently, the
firm produced a lookalike synthetic version of the active ingredient in natural
Working with Ava Chemical, he identified a form of synthetic sugar ester
that is readily water soluble, yet remains active in controlling insects. Says
Puterka, "The major problem with earlier versions of synthetic sugar
esters was their inability to dissolve in water."
Puterka is conducting further studies under a cooperative research and
development agreement with the New Hampshire company. He believes the synthetic
esters will soon be fully registered and could be commercially available in the
United States by 1999.--By Hank
Becker, Agricultural Research Service Information Staff.
Alvin M. Simmons and D. Michael Jackson are at the USDA-ARS
Vegetable Laboratory, 2875 Savannah Hwy., Charleston, SC 29414; phone (843)
556-0840, fax (843) 763-7013.
Gary J. Puterka is at the
USDA-ARS Appalachian Fruit
Research Station, 45 Wiltshire Rd., Kearneysville, WV 25430; phone (304)
728-3451, ext. 361, fax (304) 728-2340.
"Watch Out, Soft-Bodied Pests!" was published in the July
1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.