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Learning to live with drosophila

How well will it survive the winter?

The spotted wing drosophila, first found in the United States a couple of years ago in California and Florida, appears now to be established in the Pacific Northwest and has spread this summer throughout central Washington.

“This is a step backwards for IPM,” said Dr. Elizabeth Beers, entomologist with WSU in Wenatchee, noting that the new pest’s arrival will be particularly tough on organic growers.

The spotted wing drosophila was found in Oregon and British Columbia last year, but the first fly in Washington wasn’t trapped until June 28 this year in a cherry orchard in Mattawa. That prompted extensive trapping, which found the pest in many parts of central Washington. Beers said that by late August, 30 percent of the 240 traps out in Washington orchards this summer caught drosophila flies.

Doug Walsh, environmental and agrichemical education specialist with Washington State University in Prosser, said he had been trapping for spotted wing drosophila in cherries, grapes, and riparian areas in Walla Walla, Benton, and Franklin counties (in south central Washington) since late February but caught nothing until after the cherry season was over. “I was assuming it was going to show up in warm locations,” he said, “But we just kept coming up blank.”

Within ten days of the Mattawa find, flies were trapped along the Yakima River in hawthorn and blackberries but it was not until several weeks later that it began showing up in commercial fruit growing operations in the southern part of the state. He believes it’s now almost everywhere in the state.

Overwinter

Little is known yet about how the pest functions in the Pacific Northwest; most of the information about the insect comes from Asia, where it originated. It is thought to overwinter as a mated female, which is ready to lay eggs in the spring as soon as the temperatures get above about 50 degrees and there is a suitable host. Unlike its close relative the common vinegar fly, the spotted wing drosophila attacks ripening fruit on the tree as well as overripe fruit. Its extensive host list include cherries, peaches, nectarines, plums, berries, and grapes.
Female adults lay eggs under the skin of fruit, and larvae develop inside. The larvae pupate either inside the fruit or crawl out and pupate in the soil.

Although some entomologists believe the pest was probably in Washington State last season and overwintered, Walsh thinks not. The traps he used contain volatiles that should have attracted the flies if they were around, but he caught nothing this spring. “We were out diligently looking from February in a wide swath of crops. If it had overwintered, we should have picked them up.”

He thinks the pest was brought into the state this season, possibly on fruit from California, where it has been established for a couple of years.

But Tim Smith, Washington State University extension educator for north central Washington, said its wide distribution throughout Washington this summer makes him think it did overwinter. “It’s been found from well up in Canada down to the Tri-Cities, and in all kinds of crops,” he said. “It looks to me as if it didn’t overwinter in high numbers, but it overwintered in low numbers.”
Control

The attract-and-kill product GF-120 Naturalyte, which many cherry growers had been using to control cherry fruit fly in recent years, is not effective against drosophila. It might provide some suppression, Smith said, “But you don’t need suppression with an insect like this. You need control.”

GF-120, which was used by both organic and conventional cherry growers, allowed predators and parasites to build up in the orchard to the point where the black cherry aphid was not a problem.

Smith said Entrust (spinosad) is an alternative for organic production but is limited to three applications annually and is only effective for about a week. A major drawback is that it disrupts biological control of aphids and the black cherry aphid is likely to become a problem that needs to be addressed.

“It’s going to shake up the pest management in cherries and all the stone fruits,” he said.

Walsh said for conventional orchards, Success (spinosad), Delegate (spinetoram), organophosphates, and pyrethroids are all effective against the fly, but are more disruptive of IPM. “Now, to start getting back on the pesticide treadmill is a shame,” he
Walsh still hopes that cherry growers will be able to continue using the GF 120 bait, along with several cover sprays of insecticide if monitoring indicates a need. “That’s the hope,” he emphasized.

Sanitation

It’s been recommended that growers try to reduce the fly’s breeding sites by immediately removing and disposing of infested fruit, but Lynn Long, with Oregon State University Extension in The Dalles, said there are always some cherries left in an orchard after harvest. In the fall of 2009, large populations of drosophila were found in cherry orchards where the fruit had been left unharvested because there was no market for it.

Walsh also thinks good sanitation will help. He doesn’t recommend postharvest sprays in cherries or areawide spray programs because of the disruptive effect on IPM. He’d rather see growers remove the remaining fruit after harvest, though he acknowledges that with cherries it would be labor intensive.

Grapes

Walsh hopes that grape growers will be able to continue their reduced insecticide programs, despite the new pest. “I would hate to see growers spraying willy-nilly for this,” he said.

He has done laboratory work to find out at what stage of maturity the grapes become vulnerable to attack by the spotted wing drosophila in order to help wine growers to manage the pest. “I don’t want to be recommending prophylactic sprays since we have such a nice IPM program for grapes,” he explained.

The pest could have major impact on Concord grape growers, who have not had to apply insecticides for the past 10 years or so, he said. Juice grapes, which are irrigated more and have denser canopies than wine grapes, could provide a better habitat for the spotted wing drosophila than wine grapes.

Winter

Scientists will be watching closely to see how the drosophila survives Washington winters. Walsh will continue to trap even during cold weather to learn about its population dynamics.

“I’m still hopeful that our winter weather will be old enough to really clobber the overwintering population,” he said. “The thing we need to figure out soon here is what kind of survival will we expect to see over the winter. My purpose of the trapping is to see when we stop trapping them.”

It’s believed that the flies become inactive at the first frost, which is typically in October, but Walsh wonders if they might become active again if the weather warms up after the frost.

He’s also hoping to learn about the ability of the fly to recolonize. “We’re our own worst enemy, moving fruit everywhere,” he said. “Now that everyone’s aware of the pest, maybe we can do a better job of not moving fruit around. Let’s not bring in fruit from California that hasn’t been thoroughly looked at.”

Smith said it could take years to gain confidence in how the pest behaves in the Pacific Northwest. In the meantime, growers must assume that it’s a pest they’ll need to manage every year.
 

To learn more about the biology and management of the pest and access a phenology model, go to OSU’s spotted wing drosophila Web site. Information about the pest and how to trap it can be found on WSU’s Orchard Pest Management online under “Direct pests” and then “Fruit flies.” Also, check the WSU IPM Web site.

Look for an in-depth article on the spotted wing drosophila in the October Good Fruit Grower magazine.