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Doug Walsh, holding a mealybug pheromone trap, says that such traps have been used throughout the state to look for vine mealybug. Thus far, the destructive pest that's prevalent throughout California grape regions, has not been found in Washington.

Doug Walsh, holding a mealybug pheromone trap, says that such traps have been used throughout the state to look for vine mealybug. Thus far, the destructive pest that’s prevalent throughout California grape regions, has not been found in Washington.

Melissa Hansen

While exotic and invasive pests threaten grape industries in other West Coast states, Washington’s grape industry has thus far been relatively unscathed. Vine mealybug has not yet been detected in Washington vineyards, and spotted wing drosophila larvae do not appear to be able to develop on Washington’s juice or wine grapes, says Washington State University’s Dr. Doug Walsh. However, the big unknown to grapes is the brown marmorated stinkbug.

Last year was the second that university entomologists followed infestations of spotted wing drosophila in Washington State. It was first detected in the United States in 2008 in California. Since then, the small fruit fly has been found along the West Coast and throughout the United States. Literature reports an extensive host range for spotted wing drosophila, including tree fruit, berries, and grapes. A national research effort is under way to gain knowledge about the fruit fly, crop damage, monitoring protocols, and management options.

Walsh, WSU environmental and agrichemical education specialist and statewide integrated pest management coordinator, has spent the two years learning more about the tiny pest and its behavior.

Eight species of drosophila have been identified in central Washington in vineyards, backyard fruit trees, and blueberries, he said during a pest session of the Washington State Grape Society meeting last fall. Of the eight, the D. obscura group was the most prevalent in their trapping, with the peak number of flies captured in May to June, dropping off as grape harvest neared. The D. melanogaster and D. similis group are commonly seen during crush around wineries. Populations for this group peak during grape harvest in late September through October.

Walsh notes that for the D. suzukii, the spotted wing drosophila, WSU researchers are not trapping the first flies until mid-June, with low populations continuing through the summer. Peak trap catches were from September through November.

Traps for spotted wing drosophila in 2011 were placed in Concord and wine grapes as well as blackberries, backyard fruit trees, and blueberries. Trap catch numbers in blackberries were as high as eight females per week. In the Concord grape traps, one or less were caught per week, while in the wine grape traps, six females in one week was the high, with an average of two or less during the other weeks.

“There seemed to be an extra low abundance of these flies in the grape traps,” he said.

Walsh repeated a “no-choice” grape study in 2011 to measure the spotted wing drosophila’s affinity for grapes. Flies were placed inside sleeved cages around grape clusters (Riesling, Merlot, and Concord) and monitored to see if larva and eggs developed into adults.

“Results were nearly identical to 2010,” said Walsh, adding that while eggs were laid in the fruit and some larva were detected on the caged fruit at dates near harvest, no adults were detected.

“We got some eggs laid in grapes and observed larva, but they were never able to complete development and complete a generation in eastern Washington grapes. Between our cold weather and some factor of our grapes, we’re not having an issue, despite what appears in some of the literature about grapes being a host of D. suzukii.”

He now believes there will not be much issue with spotted wing drosophila in ­Washington’s juice and wine grapes. However, western Washington, Oregon’s Willamette ­Valley, and California are dealing with serious infestations that could impact grapes.

Vine mealybug

Vine mealybug spread rapidly through California vineyards, but Walsh reports that the bug has not been detected in Washington. Grape mealybug is the only mealybug species that has been found here. For several years, WSU has trapped for four species of mealybug using pheromones developed by the University of California’s Dr. Jocelyn Miller.

“Vine mealybug is a major concern because if it gets here, it will have more generations (three to four) over the course of the year than the one to two generations of grape mealybug,” he said, noting that mealybugs and soft scale insects can vector diseases like grapevine leafroll. “With more generations, it means more eggs, more honeydew, and more opportunity for the spread of virus through our vineyards. Instead of vines exposed twice to crawlers, the most efficient life stage for vectoring diseases, vines will be exposed three to four times a year.”

Another negative aspect of vine mealybug is that it can feed under soil on the vine roots, areas that are protected from the reach of chemical sprays.

Walsh stressed the importance of planting clean rootstock and cuttings. The vine mealybug’s appearance in California has been genetically traced by UC scientists to a vineyard in Israel. Muscat cuttings from the Israeli vineyard were bootlegged into the state and planted in California’s Kern County. Ground zero for vine mealybug in the Napa-Sonoma area of California was traced to nursery stock, he said.

That’s why it’s so important that Washington growers buying grape planting material from California purchase certified planting material and also request that the material be treated with a hot water dip to eliminate vine and other mealybug species.

All types of mealybug can survive crush and hide in the must and pomace. Wineries in California are advised to cover pomace with plastic to kill the mealybug before ­spreading the winery byproduct into vineyards.

Brown marmorated stinkbug

Walsh is particularly concerned with potential impact on grapes from the brown ­marmorated stinkbug, another pest newly introduced to the United States.

The brown marmorated stinkbug has been observed in commercial and research vineyards in Mid-Atlantic vineyards, and has become a statewide problem in Maryland vineyards, according to Dr. Joseph Fiola, viticulture extension specialist for University of Maryland. Brown marmorated stinkbug can cause direct injury to grapes by piercing and feeding on berries and impart a detectable taint to the wine if stinkbugs are inadvertently crushed with the berries.

The stinkbug has yet to be detected in eastern Washington, but it has been found in Vancouver, Washington, and in the Portland, Oregon, area. Sentinel traps have been placed in high-traffic areas in eastern Washington, though none have caught the stinkbug—yet, said Walsh. “But I’m not very confident with the aggregation lures
we’re using.”