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Pest identified in Oregon wine grapes

October 26, 2009  By Oregon State University



oregongrapesOctober 26, 2009 — A newly
recognized pest in Oregon continues to concern fruit growers and researchers
with the recent discovery of a Spotted Wing Drosophila fly in a sample of
Willamette Valley wine grapes.

October 26, 2009 — A newly
recognized pest in Oregon continues to concern fruit growers and researchers
with the recent discovery of a Spotted Wing Drosophila fly in a sample of
Willamette Valley wine grapes.

Since the tiny fly,
Drosophila suzukii, was first confirmed in Oregon less than two months ago,
there have been an increasing number of reports of its occurrence in a variety
of fresh fruits, including blueberries, peaches, raspberries, strawberries,
blackberries, plums – and now grapes, according to Amy Dreves, a research
entomologist at Oregon State University.

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“This is an insect that,
up to last year, had never been seen in the continental United States,” Dreves
said. “Now, suddenly, it is showing up in lots of places.”

oregongrapes 
 Vineyards in Oregon. 

Losses to fruit crops have
been significant in some places this year, according to OSU entomologist Vaughn
Walton, who is working with Dreves and others on strategies to combat this
invasive fruit fly. California lost about one-third of its cherry crop from
Davis to Modesto. Willamette Valley peach growers were hit hard, especially in
the late season, with losses up to 80 per cent in some orchards. Crop losses up
to 20 per cent were seen in Oregon raspberries.

In early October, two
blueberry fields were sampled, and one showed no damage at all while another
had approximately 20 per cent infestation (sampled and dissected approximately
400 berries at each site). “We have frozen samples from an infected blueberry
field,” Walton explained. “The berries were picked at different times, and we
can dissect them to help determine times of the first infestations this season
in Benton County.”

New reports of its
occurrence have been confirmed almost every week since OSU researchers first
identified the fly in a sample of Oregon blueberries in August.

Dreves and Walton are now
reporting that adult Spotted Wing Drosophila had emerged from wine grapes that
had been collected in the northern Willamette Valley two weeks earlier. Also
confirmed by the Oregon Department of Agriculture were flies emerging from
infested red table grapes collected from the Willamette Valley.

At this stage, growers
have not seen noticeable damage to harvested grapes, Dreves said, and the
harvest of grapes is nearly complete in the Willamette Valley without signs of
Spotted Winged Drosophila impact.

Dreves and Walton are part
of a team of researchers from OSU, U.S. Department of Agriculture, and Oregon
Department of Agriculture
working to uncover the extent of infestation and to
test methods for controlling its spread.

Their work is one part
science, one part Extension, and one part detective work.

Native to Japan and parts
of Southeast Asia, D. suzukii had been introduced into Hawaii in the 1980s and
was first confirmed in Florida and California last year. Since August 2009, the
fly has been reported throughout California, from Vancouver, Wash., to
Abbotsford in British Columbia, and in 12 counties in Oregon.

“That’s because we’re
looking for it now and this year's environmental conditions were right,” said
Dreves. Because the Drosophila fly larvae are small, shapeless and pale, Dreves
and her colleague, OSU entomologist Vaughn Walton, culture suspicious larvae
from sampled fruit to confirm the identity of the insect in adulthood.

Other research partners,
in recent reports, suspect the fly has been found on pears in Oregon, Dreves
said. What might be good news for Oregon is that D. suzukii, at least in Japan,
only lays eggs in apples that are already damaged; apples seem not to be a
primary host.

The Spotted Wing
Drosophila is a close relative of the so-called fruit or vinegar fly associated
with overripe bananas. That fly, Drosophila melanogaster, feeds on spoiled and
rotting fruit and is the star attraction in high school biology classes when
students learn about genetics and mutations. The spotted wing Drosophila fly,
in contrast, infests fresh fruit, which presents a significant economic threat
to fruit growers.

Discovery of the Spotted
Wing Drosophila in wild Himalayan blackberries has the researchers worried.
Despite the efforts growers will put toward cleaning their orchards of all
leftover fruit, these feral areas could offer a refuge for over-wintering
populations of flies, according to Dreves. “We just don’t know.”

There’s a lot that
researchers don’t know about this new invader, but they are learning fast.
Dreves is scouring the scientific literature, going back to Japanese monographs
from the 1930s to learn everything that is known about D. suzukii.

“In Japan, these flies are
reported to reproduce up to 13 times in one season,” she said, which suggests
that the population could explode toward the end of the season, as seems to
have happened this year.

According to reports,
these flies thrive in cooler areas and are most active at temperatures of 68
degrees Fahrenheit. Activity, longevity and egglaying are said to decrease at
temperatures above 86 degrees Fahrenheit, although infestations have been found
in warm parts of California and Florida.

Much of western Oregon’s
growing season would seem to favour conditions favoured by these flies, which
means that most of Oregon’s berry crops could be at-risk during the growing
season, according to Dreves. And because Oregon has a variety of crops that
ripen at different times during the season, the spotted wing Drosophila fly
could move from one crop to another as the season progresses, and populations
could build up to high numbers in many crops.

On the other hand, the fly
might be gone by next season, Walton said, pointing out the uncertainty
associated with a new invasive species.

Planning for the worst,
the OSU team is working with colleagues in the USDA Agricultural Research
Service
and ODA to develop management plans for this new pest in Oregon. They
are sampling fruits at farmers’ markets and receiving samples from growers and
OSU Extension agents in the field to map the extent of the infestation. And
they are testing baits to monitor population levels this fall. In small areas,
it may be a possibility to lure flies away from vulnerable fruit by setting up
traps.

For now, Dreves said, two
principles are at the heart of controlling the fly regardless of crop. First,
reduce the fly's breeding sites by immediately removing and disposing of the
source – infested fruit. And monitor for the presence of adult flies before
they lay eggs.

Signs of possible
infestation include:

  • Spotted Drosophila flies
    with a pale black spot at the leading edge of the wing (only the male flies of
    this species have this marking).
  • Small puncture wound on
    hanging fruit, where female drilled in to lay her eggs.
  • Soft fruit on plant,
    starting at puncture scar. Secondary decay can establish at this point.
  • Small pale maggots in
    intact fruit on the plant.


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