Suzukii: A New Fly in the Ointment
It began with a handful of blueberries. You couldn’t actually hold the berries
in your hand, or at least, you wouldn’t want to. Although they had been picked
fresh just a few days earlier, the berries now looked like they’d been frozen
and thawed; they were squashed and oozing liquid.
A Willamette Valley blueberry grower had brought the berries in to the office
of Vaughn Walton, an OSU research and extension entomologist, to get his opinion
on what could be wasting the fruit. Walton put a hand lens up to his eye and
zeroed in on one deflated berry. He saw a few thin, white threads wriggling
through the pulp, the larvae of some kind of fly.
Walton kept the infected berries and grew out the larvae in an incubator
chamber, to see what they were. In less than a week, tiny flies emerged within
“The adults looked very much like our common vinegar fly, so at first we assumed
that’s what they were,” Walton said in a soft, laconic South African accent.
But something wasn’t right.
That was August, 2009. Walton got on the phone with colleagues in other states.
A few California growers had reported similar disintegration of ripe raspberries
and strawberries the year before. By 2009, the problem was found in California
cherries and Florida strawberries. Researchers confirmed the culprit to be
the spotted wing drosophila, Drosophila suzukii, a vinegar fly native to southeast
Asia and never before reported in North America, until now.
News spread rapidly. The Oregon Department of Agriculture posted a pest alert
and more growers reported problems. By the end of the summer, the spotted wing
drosophila had been confirmed for the first time from California to British
Columbia and in parts of Europe, and in more than 16 different kinds of fruit.
And damage was mounting. By September 2009, about one third of the California
cherry crop was lost, as was a quarter of Oregon’s late season blueberries,
raspberries, and as much as 80 percent of some late season peaches. How could
a tiny fly never before seen in North America suddenly cause so much trouble
in so many places?
“You find things when you know to look for them,” said Amy Dreves, an OSU
entomologist who is working with Walton and others on an integrated pest
management strategy. “Since the fly is new to the continental United States,
we had to look everywhere and learn everything about it as fast as we could.”
Dreves is a small spitfire of a woman who can do many things at once, very
fast. As soon as the fly was identified, she dove into the scientific literature
to learn all that she could about the new invader. A Japanese researcher working
in Corvallis helped her translate references from Japan going back as far as
Mostly, the Japanese reports gave the researchers a starting point for understanding
the fly’s biology. They learned that it is most active in cool weather (between
50 and 80 degrees), which would make much of western Oregon’s long growing
season a comfortable home for these flies. And because Oregon has a variety
of fruit that ripen at different times throughout the season, the spotted wing
drosophila population could move from one crop to another and build up to high
numbers by the end of the season.
Dreves learned that the spotted wing drosophila is a problem for fruit producers
in Asia, where losses of up to 80 percent of the cherry crop have been reported
some years. So, how do Asian farmers control the fly?
There are millions of farmers in China, according to Wei Yang, an OSU blueberry
specialist at OSU’s North Willamette Research and Extension Center in Aurora.
Family farms are much smaller there and the farm labor force is much larger,
so monitoring and control are at a very different scale. Peach growers wrap
each immature peach in an individual paper bag to protect the ripening fruit
from the egg-laying fly.
Throughout the fall of 2009, the OSU researchers worked closely with colleagues
from other states, especially with fellow entomologists Jana Lee and Denny
Bruck, researchers from the USDA Agricultural Research Service. The team
launched field and laboratory tests to learn how the fly might survive the
winter, how quickly it reproduces, at what ripeness the fruit is most vulnerable,
and what kinds of controls would be most effective. One month after first identifying
this fly in Walton’s lab, the research SWAT team published an OSU Extension
bulletin with photos of the spotted-wing bandit and evidence of its crimes.
The evidence is barely noticeable at first. Look close with a magnifying glass,
and you might see a tiny pinhole where the female fly has laid her eggs. In
a couple of days, two thin white feathery threads poke out from the hole, like
little snorkels that the larvae use to breathe. Soon the fruit begins to soften
and collapse, eaten from the inside out. Berries turn mushy; peaches show
brown spots from secondary infections. Even without a hand lens, you can see
the tiny larvae looping through the pulp.
Throughout the winter, the researchers plowed through tests, desperate to
learn what they could about the fly. They confirmed that the spotted wing drosophila
will feed on a wide range of grapes, berries, cherries, peaches, and plums
in Oregon, California, and Washington.
An especially cold December made some people hopeful that the spotted wing
drosophila would be wiped out. But the first warm weather of February brought
the first few adult flies out of hiding.
Throughout the early spring, the OSU and USDA researchers met with hundreds
of Northwest fruit growers, mobilizing a monitoring force to help track the
fly’s presence throughout the region. They handed out small plastic displays
of mounted flies and descriptions for easy identification. They concocted inexpensive
traps from plastic cups baited with apple cider vinegar and loaded with sticky
flypaper. They showed growers how to test for the presence of the drosophila
larvae by dunking a sample of crushed fruit into a clear container of sugar-water
(the larvae, if present, quickly float to the top).
“We call this our ‘fly-by’ demonstration,” Dreves said as she gave each of
the growers a hand lens and encouraged them to zero in on a display of flies
the size of sesame seeds.
“Only the males have a spot on the wings,” she said. “But the females have
something far more impressive.”
The female spotted wing drosophila is armed with a serrated saw at the end
of her body that she uses to stab through the skin of a fruit and lay her eggs
in the flesh. With each puncture, she lays one to three eggs, eventually
depositing up to 350 eggs in her four-week lifetime. USDA’s Denny Bruck explains
that if you begin with one male and one female under ideal conditions, in two
weeks you’ll have 100 females; in another two weeks you’ll have 10,000; in
another month, 100 million; and onward toward something like the national debt.
The stakes are high and the potential for economic damage makes this a race
against time. After discovering the fly in his raspberries last fall, a fruit
grower in Jefferson shut down his harvest, losing his entire late season crop
of berries and peaches. “I didn’t want to lose my customers,” he said. The
fruit industry is a multi-million dollar enterprise in Oregon. The farm gate
value of Oregon berries is more than $100 million. Oregon wine grapes are valued
at about $68 million, Oregon cherries at $49 million, according to the Oregon
Department of Agriculture.
The researchers have tested the effectiveness of dozens of chemicals, from
full-spectrum insecticides to organic bait sprays that can be used to attract
and kill flies before they lay eggs. But they know this is not a problem that
can be wiped out with a barrage of chemicals. Jeff Miller, an OSU insect ecologist
on the research team, warned that controls must not harm pollinating insects
or other beneficial organisms that are necessary for healthy orchards and
fruit fields. The research team is concerned about increased human exposure
to pesticides and they want to avoid secondary pest outbreaks that might result
from new or more powerful pesticides.
In addition, chemical resistance is a problem when combating any insect with
up to 10 generations a year, as has been reported for this fly in Japan. Remember
that the spotted wing drosophila, Drosophila suzukii, is a cousin of the more
familiar, rotten-banana-loving vinegar fly, Drosophila melanogaster, that is
used to teach classroom genetics expressly because it mutates so rapidly.
As the flies emerge in the warmth of summer, the researchers have yet to witness
an entire season in the field. They don’t yet know what triggers the flies
to lay eggs or what predatory bugs could stop the flies before they lay their
But they are learning, as much and as fast as they can. The Oregon legislature
provided $225,000 for monitoring; and the USDA granted $5.7 million to the
three-state team to extend their research and extension. The “fly-by” outreach
continues. “The thing that keeps me awake at night,” Dreves said, “are all
those questions that we don’t have answers for.”
For up-to-date information, see the
spotted wing drosophila website.