Unlike other pests, Drosophila suzukii has a wide range of potential hosts

By MITCH LIES

Capital Press

In the winter of 1987, the Russian wheat aphid dominated talk at Northwest wheat conferences.

Crop losses approaching 40 percent had been reported in Texas and Oklahoma, and Northwest growers braced for the worst.

"It was rolling in from the south," said Russ Karow, head of the Department of Crop and Soil Sciences at Oregon State University. "It was in Texas, and the Plains, and then Idaho. Then it was found in Ontario (Ore.)

"It was going to roll across the countryside wreaking havoc," Karow said.

Fears similar to those that accompanied the Russian wheat aphid today are flaring over the Drosophila suzukii vinegar fly.

Every decade or so a plant disease or pest like the Russian wheat aphid comes along, Karow said. In some cases, pests and diseases have driven crops out of areas.

White rot is slowly moving garlic seed production from Central Oregon: Acreage has dropped from around 3,000 acres in the early 1990s to less than 1,000 today.

Powdery mildew forced hops out of New York state in the 1930s to Oregon and Washington.

In most cases, however, fears subside, and the pests become part of the landscape.

The wheat aphid, for example, is still present in the Northwest, Karow said, but it is not considered a major pest.

"It's like grasshoppers," Karow said. "You have incidents where you get isolated breakouts. But it's not an epidemic."

In less than two years, D. suzukii has spread along the West Coast, from California's Central Valley to British Columbia. In its first full year in California, the pest reduced the state's cherry crop by a third, according to industry estimates.

In Oregon, despite the fact the pest didn't arrive until mid-summer, D. suzukii destroyed upwards of 20 percent of peaches in some Willamette Valley orchards and caused sizable crop losses in blueberries.

Growers and researchers hope the pest's eruption this summer was caused by unique climatic conditions.

But several factors indicate the pest is much more than a minor problem -- and much more destructive than the Russian wheat aphid.

D. suzukii originates in Southeast Asia. It was first identified in the U.S. by University of California researchers, who discovered it in the fall of 2008 in raspberries and strawberries in the Watsonville area of Santa Cruz County.

The pest was immediately considered significant. Unlike most vinegar flies, which lay eggs on damaged and decaying fruit, D. suzukii damages healthy, ripening fruit.

Scientists have learned the pest has as many as 13 generations per year and has multiple hosts, allowing it to flourish throughout a growing season.

"You have all the different fruits ripening through the season," said OSU entomologist Vaughn Walton. "In Oregon, you have some early ripening ones, and then it just continues the whole way through until late in the season. They'll just move from one to the other because they have so many hosts."

Unlike the Russian wheat aphid -- which is not a big concern in its native range -- the spotted wing vinegar fly is an ongoing problem for fruit producers in Asia, Walton said.

The fly can be controlled with most commonly used insecticides, Walton said. But growers say applying multiple pesticide treatments to control the pest is too expensive given today's slim profit margins.

"The challenge with the spotted wing drosophila will be to find a natural enemy that would bring its numbers down," Walton said.

Also concerning scientists is the fact that fresh fruit -- which can carry the fly -- is shipped through a sophisticated network along the West Coast and around the world. The movement provides the fly easy access to new areas, Walton said.

In a matter of weeks this past summer, the fly moved from California -- where researchers found it in June in cherries, raspberries and strawberries -- to Oregon.

By August, the pest's presence in Oregon was widespread, according to Oregon Department of Agriculture entomologist Helmuth Rogg.

Walton identified several similarities and differences between the Russian wheat aphid and D. suzukii. Among the differences is the fact that breeders relatively easily developed wheat varieties resistant to the aphid. Developing resistance in the perennial crops that host D. suzukii, Walton said, will be far more difficult.

Also, unlike the Russian wheat aphid, D. suzukii attacks the end product, not the plant, Walton said.

"With the suzukii, you're actually damaging the product you're going to harvest," Karow said. "That is a whole different ball game as far as the economic impact."

The USDA in recent weeks assigned two scientists full time to the pest. And scientists from University of California-Davis, OSU, Washington State University, Agriculture Canada and the Oregon Department of Agriculture have begun a coordinated effort to research the pest's biology and control factors.

For now, Rogg said, orchardists should remove unpicked fruit from trees and pick up fruit from orchard floors to reduce overwintering habitat.

"Eliminate every potential host," Rogg said. "Keep your orchard clean. That certainly will help cut it down."

Come next spring, Rogg said scientists will closely watch strawberries -- typically the first crop out of the gate -- for signs of the pest.

"We want to be ready," he said.

Staff writer Mitch Lies is based in Salem. E-mail: mlies@capitalpress.com.

 

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