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Suzukii: A New Fly in the Ointment

Suzukii: A New Fly in the Ointment header image
A SWAT team pursues a tiny new invader.
suzukii fly over strawberry
Photo by Michael Durham.

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 chamber.

“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.

infested blueberry
The first sign of infection is the appearance of tiny, threadlike snorkels that the suzukii larvae use to breathe. Oregon State University photo.

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.”

Amy Dreves
OSU entomologist Amy Dreves gets down to work as she monitors home-made fly traps in a Willamette Valley strawberry field. The spotted wing drosophila made its first big appearance in North America last fall, so Dreves and her fellow researchers have everything to learn about this new, potentially damaging pest. Photo by Lynn Ketchum.

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 the 1930s.

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?

SWD identification cards
Identification is crucial when tracking a new pest. Researchers from USDA and OSU provided growers with mounted specimens and hand lenses and asked for their help in reporting what they see in their orchards, vineyards, and fields. Photo by Lynn Ketchum.
spotted wing drosophila identification
The female spotted wing drosophila (left) is armed with a saw-like ovipositor she uses to bore through the skins of fresh fruit, including relatively tough-skinned fruit such as apples and pears. Photo by Eric LaGasa.

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.

Vaughn Walton
OSU entomologist Vaughn Walton (above) shares overwintering data with growers at one of the many “fly-by” workshops the researchers hosted during the winter. “We don’t know if the fly will show up again,” he said. “But we want to be ready if it does.” Photo by Lynn Ketchum.
workshop
Fruit growers learn how to identify SWD and how to make and set traps to monitor SWD activity at a joint OSU/USDA workshop held at the North Willamette Research and Extension Center. Photo by Lynn Ketchum.

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.

Joe DeFrancesco
Joe DeFrancesco (above), an OSU entomologist, checks a blueberry for signs of the spotted wing drosophila. Photo by Lynn Ketchum.
Suzukii drosophila by Michael Durham
Drosophila suzukii. Photo by Michael Durham.

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 eggs.

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.