Spotted-wing drosophila numbers up after harvest

By DAN WHEAT

Capital Press

WENATCHEE, Wash. -- Spotted-wing drosophila, a dreaded pest that just this summer invaded eastern Washington, was thought to be active mainly in midsummer.

But the tiny fly is puzzling scientists by apparently increasing in number in early October, well after the fruit it prefers has been harvested.

It's taken to peaches, nectarines, apricots, cherries, berries and some grapes, but, as expected, is believed to have left thicker-skinned pears and apples alone if their skin isn't broken.

From June 28 to Aug. 15, the average number of the fly found per week in apple cider vinegar traps was less than five, according to data posted on a new Washington State University spotted wing drosophila website.

On Aug. 22, the number peaked at 18, dropped to 7 by Sept. 12 but rose to 14 on Sept. 26.

"It has me mystified," said Elizabeth Beers, entomologist at the WSU Tree Fruit Research and Extension Center in Wenatchee.

Then, during the first full week of October, more than 1,000 flies were found in a single trap, the biggest catch of the season, she said.

"It was an isolated cherry block in the Columbia Basin that had been harvested months ago and had nothing (no fruit) in it," she said.

"I thought it would be tied to the availability of cherries, late June through early August but numbers were low then," Beers said. It will be interesting to see if the same lateness occurs next year or if it was a first-year phenomenon, she said.

The lateness probably means a chance of a high population over wintering if it does so in eastern Washington, she said.

About 18,000 spotted wing drosophila were found in traps in 340 locations from June 28 through Oct. 1, Beers said. There's no easy way of knowing the actual size of the infestation, she said.

The largest numbers were found near the north central Washington towns of Orondo, Wenatchee, Tonasket and Quincy (in order of greatest to least) with lesser amounts in Brewster, Chelan, Royal City and Prosser. Very few were found in Yakima, the Tri-Cities and Walla Walla.

That may be because there was more trapping in north central Washington, Beers said.

Trapping will continue for a couple of weeks after the last spotted wing drosophila are found, she said.

So far grapes have turned out to be less of a host than expected, but that could change as the crop matures, she said.

It's unknown if the pest will winter over in eastern Washington or reinvade every year from western Washington and Oregon.

An Asian pest that attacks ripened fruit, it was discovered in California strawberries in 2008 and damaged about 20 percent of California's cherry crop and one-fourth of late-season berries and peaches in Oregon in 2009. It was more controlled by pesticides in California and Oregon this year, scientists have said.

Beers said pesticides of four different chemical groups are effective and that once scientists know more about the insect's biology they will be able to finesse the use of pesticides within integrated pest management.

Online

http://extension.wsu.edu/swd/Pages/default.aspx