The distinctive damage from blister mites is ugly, but the damage almost never has an impact on grapevine performance. The new presence of this mite, seen here in a Riesling block in the Horse Heaven Hills actually shows how biocontrol and species diversity are working well in Washington vineyards, said entomologist David James of Washington State University. (Kate Prengaman/Good Fruit Grower)

In Washington vineyards, mites generally look like a biocontrol success story. Since growers cut back on broad spectrum pesticides more than a decade ago, predatory mite populations have grown enough to keep common pest mites, such as the two spotted spider mite, largely in check.

“We’re not causing the problems we were causing 18, 19 years ago,” said Washington State University entomologist David James.

He presented the results of ongoing mite surveys to the Washington State Wine Commission’s Research Advisory Committee in January, showing that the state’s vineyards have a greater diversity of mites today — and for the most part, that’s a good thing.

“Our grape system now has more potential pests, but they are all being well controlled by predators,” James said. “What we’re doing in agriculture now is trying to get to be a balanced ecosystem, and the closer you get to a balanced ecosystem, the fewer problems you’ll have.”

That means growers scouting for pests may encounter new things, like blister mites, which produce distinctive leaf damage but rarely cause economic impacts, or spider mites more common in California that have recently been found in Washington vineyards.

“There’s lots to look at and keep tabs on with this diversified portfolio of pests, but they are all at low levels,” James said, adding that growers should continue monitoring for mites, because flare-ups do still occur, especially if growers have used certain pesticides that can upset the balance of native predators. “They do have the potential to be a serious problem.”

But last summer, for the first time in a long time, several grape growers in Eastern Washington encountered significant mite damage that they linked to weather conditions, not pesticide issues.

“I haven’t had to control for mites in years,” said grower Dick Boushey. However, he said he had to spray vineyards in both the Yakima Valley and Red Mountain late last season. “We really resist spraying for mites because it’s nonselective and you’ll kill the predator mites, too, plus it’s expensive. But this was kind of a wake-up call.”

The problem mites in Boushey’s vineyards could have been Pacific spider mites, which are relatively new to Washington vineyards but a common pest in California, or McDaniel spider mites, which are native to Washington and look so similar that even entomologists like James have to send samples to mite identification experts who can spot differences in the tiny insects’ genitals under powerful microscopes.

James’ survey didn’t capture the mite boom in Boushey’s vineyards, because his sampling wraps up in the midsummer since the pest population typically drops quickly in late August and September.

“Mites have always been good at disappearing before harvest,” he said. The survey found these particular mites in 19 percent of the vineyards but rarely at infection levels significant enough to cause damage. Predatory mites important for biocontrol were found in about two-thirds of the vineyards.

Boushey said the problems in his vineyard really began in the late summer when the region’s weather was altered by wildfire smoke and the predators couldn’t keep up.

“I think the sustained heat and smoke initiated another batch” of spider mites, he said, adding that the smoke created humid conditions and protected the mites from direct sun. “They were there in small numbers until this year, when they just blew up on us late.”

The theory that smoky weather could have benefited the mites is intriguing, James said. There’s no research on the topic, but it’s well established that spider mites do well in dusty conditions. Smoke particles in the air could definitely have a similar effect.

Rick Hamman, viticulturist for Hogue Ranches, said he also saw McDaniel or Pacific spider mites building in August during the smoky conditions and ended up having to spray blocks of Pinot Gris and Riesling he hadn’t had to spray before. Spider mites “have been fairly quiet, but not eliminated,” in recent years, he said.

The challenge is that most miticides have a pre-harvest interval of at least two weeks, and, in this case, the pressure built up right before harvest, complicating timing for picking, he said.

“They aren’t hard to control, you just want to stay on top of them,” Hamman said. “Usually, it’s during late summer when it’s been dusty and hot. If your predators don’t build up enough to keep up, pretty soon it’s blowing up.”

Many miticides also knock out the predator mites that provide biocontrol under normal conditions, so growers who used them last summer should be extra vigilant this coming season, James said. If mites are a concern, it’s also important to avoid imidacloprid for other pests, since it has been shown to stimulate spider mite reproduction.

Hamman said he tries to use chemicals that don’t kill the predators whenever possible, and that cultural practices such as keeping the canopy open and healthy and performing good dust control can go a long way to reduce spider mite populations as well.

Whether last season’s mite flare-up was just a coincidence or a sign that hot, smoky conditions in the late season could be changing the pests’ behavior remains to be seen.

James said he’s planning to do more surveys later in the season this year, especially if conditions are similar to last year, and growers would like to see that. His research is funded by the commission, for which Hamman is the chair of the Research Advisory Committee.

“Until this year, we thought the only thing that upset mites was pesticides that upset the balance of predators, so this is a new thing to look at the climate,” James said. “The good news is that mites really are not the problem that they used to be, but changing climate and changing complex of species could alter that, so it’s good we are looking at the situation to identify possible problems for the future.” •

by Kate Prengaman