Posted September 10, 2004: Wild multiflora
roses are considered a nuisance by hunters who have
to trudge through the thorny plants. Orchardists who
labor to remove them from along fence lines share the
same sentiment. But a change of attitude is taking place
around apple, pear, and cherry orchards in parts of
Washington and Oregon. There, some growers are actually
planting wild rose gardens next to their orchards. There's
a method to such madness, though.
The gardens are actually part of an areawide study
that Agricultural Research Service (ARS) and Washington
State University (WSU) scientists are conducting to
bolster spring populations of tiny, parasitic wasps
that attack several leafroller moths in tree fruits.
In the caterpillar stage, the moths eat the leaves and
fruit of apple, pear, or cherry trees, sometimes causing
yield losses greater than 50 percent.
In heavily infested areas of central Washington, tree
fruit growers may need to spray their orchards two to
four times a season to check the pests' appetite for
But results emerging from studies by entomologists Tom
Unruh and Bob Pfannenstiel, of ARS, and Jay Brunner,
of WSU, suggest that making these orchards more hospitable
to the parasitic wasps could ease or even eliminate
the need for spraying leafrollers. "I'm convinced
we can do away with most sprays," says Unruh, with
ARS's Tree Fruit and Vegetable Insects Research Unit
in Wapato, Washington.
War of the Rose Gardens
So what's the rose bush garden got to do with anything?
The gardens, in the researchers' scheme, serve as a
kind of winter sanctuary from which the wasps can emerge
en masse during the spring. The wild roses' job is to
shelter strawberry leafrollers, a secondary host on
which the wasps rely for room and board when the temperatures
drop.This story began when Brunner, who directs WSU's
Tree Fruit Research and Extension Center in Wenatchee,
Washington, discovered the wasp, Colpoclypeus florus,
parasitizing leafrollers in central Washington. This
tiny wasp is a native of western Europe and found its
way into the United States sometime after 1968, when
Agriculture Canada scientists released it in the Ontario
region to biologically control redbanded leafrollers.
While harmless to nonhost insects, the wasp stalks
its leafroller prey with the agility of a big-cat predator.
The hunt starts after a female C. florus wasp has mated
and picks up the chemical scent of the leafroller in
its lair, a leaf rolled onto itself with silk spun by
the caterpillar. The wasp must take care, however, because
the caterpillar is 20 times larger than she is. And,
it can be quite belligerent about intruders.
When the time is right, "She jumps onto the back
of the leafroller and stings it just behind its head.
She then jumps off and hides in a safe spot," explains
Unruh. If she isn't nimble enough, the leafroller may
rear back and snap her in half with its jaws.
"It's a real battle; it's like early hunters trying
to spear a mammoth," says Unruh of the 1- to 2-millimeter-long
wasp's attack. "Almost all the time, the leafroller
But the wasp's sting isn't what delivers the killing
blow. Rather, her sting is the means of "injecting
toxins that change the behavior of the leafroller so
it starts making extra-thick webbing," Unruh explains.
Once the silken web is to her liking, he continues,
"The wasp starts laying her eggs in the webbing
out of range from damage by the host. About 20 wasp
larvae hatch from the eggs, crawl down the webbing to
the leafroller, and start sucking externally on its
After 1 to 2 weeks of such feeding, the young wasps
pupate, then emerge as fully formed adults in another
week. These mate and then fly off to start the cycle
over again. In summer, parasitism of apple leafrollers
in the orchard can exceed 50 percent. But winter poses
a problem for the wasp in western orchards.
Until recently, the researchers couldn't figure out
why the wasp's springtime parasitism rate was very low
(just a few percent)—a bottleneck to its use as
biocontrol agent, since apple leafrollers generally
produce only two generations during the season.
"This is an old problem," says Unruh. "In
Europe, C. florus is the main parasite of leafrollers,
but there's nothing in the scientific literature regarding
where, and on which host, the wasp survives the winter.
Our leafrollers in apple are not suitable winter hosts
because they overwinter as very small larvae, too small
for the wasp."
The first clues to the mystery surfaced in the fall
of 1997, when Robert Pfannenstiel, then a WSU postdoctoral
researcher with Brunner, did some scouting along the
banks of Squilchuck Creek, which winds past hillside
orchards of apple, cherry, and pear outside Wenatchee.
There, among wild roses, dogwoods, and other native
plants that favor riparian habitats, Pfannenstiel collected
some leafroller larvae of a size apt to suit the wasp's
That day, "I collected two or three specimens
and actually found a female C. florus attacking the
leafroller," recalls Pfannenstiel, who has been
a full-time scientist with ARS's Beneficial Insects
Research Unit in Weslaco, Texas, since June 2000. Given
the lack of scientific information on the wasp's winter
hosts, "it was a kind of big deal," he says
of the observation.
From this and later collections, Pfannenstiel identified
two winter hosts for the wasp: Syndemis species that
feed on a native dogwood and Ancylis comptana (strawberry
leafroller). The strawberry leafroller is an exotic
species that favors domestic and wild strawberry and
several native varieties of wild rose, particularly
Rosa woodsii. Neither is a pest of apples, but they
share a common trait: large, mature larvae the wasp's
brood can feed on in fall, allowing them to enter diapause,
or hibernation, and survive until spring.
From these clues, Pfannenstiel and Unruh, in 1999,
started a 2,471-acre survey of orchards near the Yakima
River to test the hypothesis that parasitism of apple-feeding
leafrollers by C. florus would be highest close to riparian
habitats, where both wild roses and the wasp's winter
hosts could be found. More importantly, three-quarters
of this large orchard area is next to sage-grassland
habitat, not riparian. There, scientists observed virtually
no springtime parasitism by wasps of the apple-feeding
In July and August 2000, Unruh's team planted 10- by
50-foot gardens of wild rose and strawberry at four
survey sites farthest from the Yakima River banks. These
were sites where no spring parasitism by C. florus had
After establishing the gardens, the team seeded them
with strawberry leafrollers. Specimens collected the
following December and in February 2001 showed C. florus
had parasitized the strawberry leafrollers in three
of the four gardens. More importantly, the following
May, they observed parasitism of apple leafrollers in
the orchards near the three garden sites.
At the Annual Western Orchard Pest and Disease Management
Conference held January 2003 in Portland, Oregon, Unruh
described the study and contrasted the findings with
earlier observations: In orchard areas close to the
garden where no strawberry leafrollers had been parasitized,
there was also no C. florus parasitism of apple leafrollers.
What's more, no apple leafroller parasitism was observed
in garden-free orchard sites used as controls.
This spring, the researchers have expanded the study
to include 10 new rose/strawberry garden sites near
Yakima, and two gardens each in Milton-Freewater and
The Dalles, Oregon, and in the northern Okanagon area
Unruh says growers excited by the prospect of less
insecticide for leafroller control have planted their
own gardens in Royal Slope, Sunnyside, and Columbia
basin sites of central Washington. The researchers are
helping these volunteer efforts by making sure that
the strawberry leafroller becomes established at these
sites. "It's been a groundswell," notes Unruh
of the response. But "just planting roses isn't
enough," he adds. More research needs to be done,
including on the ecology of the garden itself.
For example, strawberry leafrollers do best as winter
hosts for C. florus when a few wild strawberry plants
are mixed in with the rose bushes. Problem is, wild
strawberry is low growing and much less hardy than the
rose, which adds another interesting wrinkle. "In
mixed patches of roses and strawberries, we found that
5 to 10 percent of leafrollers in strawberries had been
parasitized versus almost 100 percent of those in the
roses," says Unruh. "The parasite seems to
look for its hosts in bigger bushes and trees rather
than in ground cover."
Other research includes tests of more than a dozen
different kinds of wild roses and strawberries to identify
which ones will work best in the gardens. The test plants
are found naturally, purchased from nurseries, or obtained
from the USDA plant germplasm center in Corvallis, Oregon.
The team is also working on spray strategies that will
protect the wasps, tachinid flies, and other beneficial
insects if use of chemical controls for other pests,
such as codling moths, is unavoidable. As the approach
takes shape, says Unruh, "I think it will have
a substantial impact on how we manage leafrollers in
western tree fruit orchards."—By Jan Suszkiw,
Agricultural Research Service Information Staff.
This research is part of Crop Protection, Product Value,
and Safety, an ARS National Program (#304) described
on the World Wide Web at www.nps.ars.usda.gov.
Thomas R. Unruh is in the USDA-ARS Tree Fruit and Vegetable
Insects Research Unit, 5230 Konnowac Pass Rd., Wapato,
WA 98951; phone (509) 454-6563, fax (509) 454-5646.
"Rose Gardens Make Fruit Orchards More Inviting
to Friendly Wasps" was published in the January
2004 issue of Agricultural Research magazine.