Good Fruit Grower

March 1

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16 MARCH 1, 2015 GOOD FRUIT GROWER www.goodfruit.com Five years later, what have we learned? by Melissa Hansen N ow that many of the basic questions about spotted wing drosophila (SWD) have been answered, scientists can focus on bring- ing integrated pest management back to impacted crops like cherries. When spotted wing drosophila showed up in the United States more than five years ago, little was known about the invasive fruit fly that was native to Asia. The pest, first detected in California in 2008, was found in western Washington and parts of Oregon in 2009, and by 2010, it was detected in east- ern Washington, Michigan, and six other states. The fruit industry had zero knowledge about the pest when it was first discovered, says Dr. Betsy Beers, Washington State University entomologist. "It's been a five-year voyage of discovery, and we've gone from knowing essentially nothing about spotted wing drosophila to learning quite a bit about it." That voyage has been a collaborative, regional research effort for stone and small fruits funded through the federal Specialty Crops Research Initiative. A team of nearly 30 university and private industry scientists from Washington, Oregon, and California participated. Researchers in other parts of the country, including Michigan, have also contributed to research efforts. Everywhere An extensive network of traps that were placed throughout Washington during the last five years showed that spotted wing drosophila was "everywhere we looked for it," Beers said. The peak number of traps in the state was almost 360 in 2013. Researchers focused their traps on cherries and blueberries in eastern Washington because those were the known high-risk crops, but traps were also placed in grapes, peaches and nectarines, apricots, caneberries, apples, and some feral plants (chokecherries, Himalayan blackberries). "Trapping was important because we needed to know when and where spotted wing drosophila would show up and when sprays should be applied," she said, summa- rizing five years of data during the annual meeting of the Washington State Horticultural Association. "Initially, our recommendation to cherry growers was to spray when the first fly was caught and fruit are susceptible." Spray recommendations based on a single fly detec- tion have been necessary because researchers don't have enough confidence in the trapping systems. Spotted wing drosophila is unlike ordinary fruit fly species because females don't need to wait for fruit to be damaged or over-ripe and can lay eggs in unripe fruit. "Cherries have to be dead green to not be susceptible," said Beers. "They don't like cherries as much at straw color, but they will lay eggs then (at least in the lab). Fruit are fully susceptible two weeks after straw color." Cherries are also vulnerable at straw color to damage from Western cherry fruit fly, she said, which means that timing for control of both species likely can be wrapped together. Greater establishment Five years of trapping data in eastern Washington showed that the seasonal phenology of the pest is remark- ably consistent. "Population numbers are low from January through spring, and then in August, numbers begin to kick up. They go crazy in fall through November, if temperatures remain mild," Beers said. Though abundance patterns have been consistent, researchers have noticed big year-to-year differences in the density of flies caught per trap. Strict comparisons between years cannot be made because trap design and locations have changed, but Beers said population trends show increasing numbers which reflect better establish- ment in Washington. "I believe the trend of better establishment is a real one," she said. "We're seeing seasonal variation. Population buildup in 2014 was later than 2013, and pest timing of the two years was different, but they were both big years in terms of numbers trapped." Beers believes winter temperatures affect spotted wing drosophila by inflicting higher levels of winter mortality, which delays population buildup. Cold winters with sin- gle digit temperatures, such as those in 2011 and 2014, shifted the timing when flies were first caught in most regions. "In 2011, things shifted dramatically, and most regions didn't catch any flies until after the first of August, which is after peak cherry harvest," she said. But during the 2012 season, which followed a mild winter, fly catches were spread throughout the season. In 2013, after a very warm winter, catches were also early, coming out in May. "The 2013 season had a few flies caught during the winter, and then nearly every trap in the state lit up in mid-May," she said. "It was a very high-pressure year for cherries because they were susceptible for essentially the entire season." But the seasonal variation in populations also presents Living with SPOTTED WING DROSOPHILA Pest Management Average SWD per trap 2010 2011 2012 2013 2014 20 40 60 80 100 120 140 Population trends Spotted wing drosophila is showing greater establishment in eastern Washington. SOURCE: BETSY BEERS, WASHINGTON STATE UNIVERSITY opportunities for IPM controls, Beers said. Trapping design and baits have also evolved in the last five years. Effective traps are needed so growers know if spotted wing drosophila is in their orchard, and they can use them to time sprays. Just about every style of container that holds liquids has been studied. Tests found that the Haviland trap (a food storage container trap designed by David Haviland, University of California extension advisor) performed best (probably because of its large bait surface area). Additionally, red and yellow were found to be the most attractive colors in a national study, although this varied by crop. The real game changer in attracting spotted wing dro- sophila to traps was the development of synthetic lures. Two lures have been commercially developed by Trécé Betsy Beers

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