Pacific Northwest growers are vigilant following reports of spotted wing drosophila among Concord grapes in eastern Washington and the early appearance of the flies in traps in British Columbia’s Okanagan Valley in Canada.

While the susceptibility of wine grapes is not known, table grape and stone fruit ­growers fear the tiny fly could be a devastating new pest for the region.

The fly, native to Asia, was originally identified in strawberry fields around Watsonville, California, in August 2008. It was discovered in Pinot Noir grapes in the upper Willamette Valley of Oregon last year as well as in a noncommercial wine grape vineyard south of Abbotsford in British Columbia’s Fraser Valley. It has since been identified in the Okanagan Valley, where trapping suggests some flies may have successfully ­overwintered.

Drosophila suzukii, as the pest is known scientifically, is distinct from D. melanogaster on account of its predilection for healthy, mature fruit rather than damaged or rotting fruit. Thin-skinned fruits such as berries (including wild blackberries), cherries, and grapes are particularly vulnerable. Some peach growers in the Willamette Valley reported losses of up to 80 percent last year.

Both tree fruit growers and grape growers are concerned because a fruit fly infestation makes fruit unsightly to fresh-market consumers and raises the risk of spoilage. While the fly isn’t a threat to human health, no one wants damaged fruit.

“This is unacceptable,” Mark Bolda, a farm advisor specializing in strawberries and caneberries at the University of California, told growers attending the Pacific Agriculture Show in Abbotsford earlier this year.

Bagging

Unfortunately, he said, there are no simple solutions for controlling the pest.GF-120 NF Naturalyte fruit fly bait provides effective short-term control, while ­long-term results can be had with stronger substances such as malathion and Delegate (spinetoram). But if the fruit is ready to pick, chemical controls may not be possible.

Organic management is equally difficult.

Bolda recommends bagging and removing cull fruit, though he knows it’s labor ­intensive.

“I realize this is not a popular option,” he said. “[But] the number of flies, while not eradicated, went down significantly.”

Dr. Susanna Acheampong, an entomologist with the B.C. Ministry of Agriculture and Lands in Kelowna, suggests burying cull and dropped fruits at depths of 12 inches. Alternatively, an information sheet she’s prepared for growers notes that wrapping or bagging fruit in airtight plastic may kill fly larvae and pupae in three to seven days.

The sheet notes that Australian researchers recommend setting fruit bagged in plastic in the sun for at least three days, while trials at Oregon State University found that “placing a 1- to 2-­millimeter [1 to 2 thousandths of an inch] clear polyethylene plastic (with UV inhibitor) over an infested fruit pile and tightly sealing it will kill larvae and pupae after one week.”

Acheampong also advises postharvest spraying to kill remaining larvae and pupae and prevent the spread of the fly into other crops. Spraying may also help control populations of other pests, such as cherry fruit fly, in the following season.

Scant information is available regarding the susceptibility of wine grapes. Save for a private survey by Dr. Tom Lowery at 20 vineyard sites in the South Okanagan, trapping efforts have focused on other crops because the pest’s impact on wine grapes isn’t yet known.

Speaking to growers at the annual B.C. Wine Grape Council conference in Penticton in July, Dr. Sheila Fitzpatrick of the Pacific Agri-food Research Centre in Agassiz, British Columbia, said the primary information regarding the pest’s taste for grapes is from Japanese research from the 1930s.

That information suggests that varieties such as Black Muscat (Muscat Hamburg) and Muscat of Alexandria were susceptible to penetration by the fly, while the skins of Concord, Chasselas (Chasselas de Fontainebleau), and Niagara could resist the fly’s ovipositor, although they were susceptible to infestation if wounded.

Control strategy

Fitzpatrick advocated a control strategy addressing cultural, chemical, and biological elements.

Cultural practices such as prompt harvesting or removal of damaged or fallen fruit, and the prevention of damage to fruit from the beginning will help prevent infestations. Removing potential alternative hosts, including blackberries and ornamental plants such as wild rose, from the vicinity of the crop may also help.

Recent research from Japan in blueberries suggests covering fruit with a 0.98-millimeter wire mesh will ­prevent infestations, Fitzpatrick noted.

Chemical controls are useful, but Fitzpatrick echoed Bolda’s concerns regarding the effects of chemical controls on pollinators and beneficial insects such as bees and predatory mites.

Fortunately, research in British Columbia has identified a beetle with a taste for spotted wing drosophila ­larvae, Atheta coriaria. A soil-dwelling rove beetle, it is used commercially to control fungus gnats but may also ­provide control of D. suzukii larvae.

Given strong awareness of the threat the pest poses, and a vigilant monitoring program, Fitzpatrick believes B.C. growers will fare well in fighting the pest this year.