Issue link: https://read.dmtmag.com/i/366055
A survey of spider mite populations i n W a s h i n g t o n State vineyards last year revealed that, for the most part, spider mites are not an economic problem for growers and are being controlled by benefi cial arthropods. But the survey confi rmed a new spider mite species in the state. Until last summer, it was believed that only a few spider mite species with an appetite for grapes were residents in Washington—two-spotted spider mite, McDaniel spider mite, bud mite, and rust mite. But in a survey of more than 25 vineyards last year, Willamette spider mite was detected for the fi rst time in Washington. The species was found in a vineyard on Red Mountain near Benton City, a region noted for hot summer tem- peratures. Willamette mite was previously identifi ed in a Walla Walla Valley vineyard located on the Oregon side of the valley. "We knew about the Walla Walla Valley vineyard detection, and the grower had been spraying for the mite for a season or two," said Dr. David James, Washington State University entomologist. "But Willamette spider mite had not been recorded before in Washington grapes. The Red Mountain fi nd was a surprise," he said, adding that the species is known to reside in cooler, coastal climates, like Oregon's Wil- lamette Valley and northern California. "We thought it unlikely it would turn up in the Columbia Basin. We didn't expect to fi nd it in a hot region like Red Mountain." He suspects that it has adapted to warmer tempera- tures. In both locations, the mite appears to be doing quite well. Detection of a new mite species is no reason to panic, James said. "Willamette spider mite is less of a problem than two-spotted spider mite because it doesn't seem to do as much damage in large numbers as two-spotted." Don't panic He noted that in California, some growers encourage Willamette spider mite outbreaks early in the season because it allows the plant to toughen up and better fend off mites later in the season. "I'd put it in the same cate- gory as bud and rust mites," he said to Good Fruit Grower. "It can be a problem but shouldn't be the scare that two-spotted spider mite brings." But it is more than just a curiosity. "If it has adapted to our climate, it could go on to become a problem," James said. That's why he's expand- ing his research this summer to learn more about the newcomer. James has been funded by the Wine Advisory Com- mittee, the research arm of the Washington Wine Com- mission, to expand his spider mite work from last year and do another comprehensive vineyard survey this summer. He is also working in the laboratory this sea- son to learn about the life cycle of Willamette spider mite under Washington conditions. His project includes lab experiments for side-by-side comparisons of Willamette spider mite collected from Washington vineyards with those from cooler regions. "We need to know if it has truly adapted and changed its tolerance to high and low temperatures. Or maybe, it's just an ephemeral thing," he said. James will also be looking for effects of predator control of Willamette spider mite. Willamette spider mite is yellow and paler than the two-spotted mite, which is easily recognizable by its two dots. He encourages growers to contact him if they think they have Willamette spider mite in their vineyard. Minor issue James believes that growers should be able to treat Wil- lamette mite as any other spider mite. Generally, spider mite population levels in Washington wine grape vine- yards do not warrant treatment. In California, where Wil- lamette mite is common, it's not been a problem, he said. "At worst, Willamette spider mite can make grapevine leaves turn brown, but it shouldn't impact yields or dam- age the crop, unlike bud and rust mites," he said, adding that if bud and rust mites go undetected, they can cause crop loss and shoot dieback. 10 SEPTEMBER 2014 GOOD FRUIT GROWER www.goodfruit.com New spider mite Willamette spider mite has been detected for the fi rst time in Washington grapes. by Melissa Hansen Grapes MITE sightings G rowers are asked to report suspected sightings of Willamette spider mites to Dr. David James so he can better assess distribution and adaptation to Washington's climate. He can be contacted by phone (509) 786-9280 or e-mail: david_james@wsu.edu. Willamette spider mites are pale yellow in color and prefer the cooler, shady parts of grapevines. PHOTO COURTESY OF DAVID JAMES, WSU (continued on page 12)