Weevil Gets Upper Hand With Unruly
On a hillside in the Clearwater River valley near Lewiston, Idaho, ARS
entomologist Steven Clement (right) and University of Idaho entomologist Joseph
McCaffrey inspect yellow starthistle for evidence of damage by Eustenopus
villosus. Biological control is starting to take hold in this
Bright-yellow flowers carpet canyon slopes above the Clearwater River in
Idaho. But its a small, denuded patch that excites scientists. That's because
the flowers belong to the noxious weed called yellow starthistle, and the bare
patch is one of the first signs of success in the biological control of the
A small weevil, Eustenopus villosus, gets credit for cutting the
plant down to size.
In their Eurasian homeland, insects and disease organisms keep yellow
starthistle at low levels. But when the spiny weed entered the United States in
the mid-1800sprobably in a hay shipmentit found no natural enemies
to hinder its spread. Now the weed inhabits tens of millions of acres in
California, Idaho, and other western states.
"When the thistle takes over areas like river canyons, it displaces
other vegetation and makes the land virtually worthless for livestock and
wildlife," says University of Idaho entomology professor Joseph McCaffrey.
The weed is toxicsometimes fatalto horses. Other animals will eat
only very young starthistle plants.
Biological control of introduced weeds has been an ongoing priority
nationwide for ARS. Preliminary work on
yellow starthistle began in the 1960s, but the research program shifted into
high gear in 1983.
"We investigated several weevils and flies that appeared to keep yellow
starthistle in check in its native habitat," says ARS entomologist Stephen
L. Clement, who works at the Western Regional Plant Introduction Station in
Pullman, Washington. "Then we tested the insects at our European
facilities to make sure they wouldn't feed on desirable plants in the United
Since then, with approval from USDA's Animal and Plant Health Inspection
Service, six of the insects have been imported, and five have become
established. So far, the Eustenopus weevilimported in
1990has been the most effective.
After ARS scientists release the insects, collaborators like McCaffrey
monitor and redistribute them to new sites. "Fewer than 200 weevils were
initially released at Clearwater," McCaffrey says. "Now we use the
site as a nursery to collect weevils for use in other weed-infested areas.
We've moved at least 10,000 insects," he says.
Sites that show dramatic weed reduction are gratifying, but biological
control success is usually hard to measure.
"This weevil is valuable because it attacks the weed twice," says
ARS entomologist Joseph K. Balciunas. Adults eat developing buds. Females lay
eggs in older buds that remain, and the larvae that hatch eat most of the seeds
before they mature.
However, in California, environmental conditions allow the weed to produce
more budsand seeds after the weevils have completed their life
cycle for the year.
"Biological controls play an important role in reducing the number of
seeds the weed produces every year," he says. "But in some areas
where plant numbers are reduced, individual plants grow larger to produce more
To better understand the weed, Balciunas plans to compare its seed
production in the United States with that in Turkey, where yellow starthistle
is native. "The weed doesn't persist in its homeland like it does here. If
the soil there is undisturbed, the weed disappears within 5 years," he
says. "If we can discover why that happens, it will give us clues for
controlling the weed." By Kathryn Barry Stelljes,
Agricultural Research Service Information Staff.
Joseph K. Balciunas is in the
USDA-ARS Exotic and
Invasive Weeds Research Unit, 800 Buchanan St., Albany CA 94710; phone
(510) 559-5975, fax (510) 559-5963.
"Weevil Gets Upper Hand With Unruly Weed" was published in
the February 1999 issue of
Agricultural Research magazine.