Theresa Riege was grabbing a cluster of grapes during harvest a couple of years ago when a wasp stung her hand. 

Riege, who owns Lucky Stone Vineyard in Chilton, Wisconsin, with her husband, Scott, learned two important lessons that day: First, take off your jewelry before picking; and second, wasps can be a serious problem for grape producers, in more ways than one.

German yellow jackets, Vespula germanica, on a cluster of grapes in a Wisconsin vineyard. Yellow jackets and other social wasps are causing problems in Midwestern vineyards, eating grapes and stinging pickers during harvest. (Courtesy Abby Lois/University of Wisconsin)

Riege initially shook off the wasp sting and kept picking, but she noticed her rings were getting tighter and her fingers were starting to swell. She went inside for ice and Benadryl, but it was too late. She had to go to the emergency room. They treated the swelling but had to cut her rings off in the process. 

“It would have been fine if I had not had my rings on,” Riege said.

So-called “social wasps” — including yellow jackets, paper wasps and hornets — seem to be causing more problems in vineyards in Wisconsin, and cause problems in other Midwestern states, too, said Christelle Guédot, an associate professor and entomologist at the University of Wisconsin. Guédot discussed the wasp problems during the Great Lakes Fruit, Vegetable & Farm Market EXPO in December. 

Wasps are usually considered a beneficial insect — they often feed on fruit pests and even pollinate occasionally. But in grapes, especially, they can pose multiple challenges, stinging people or eating fruit during harvest. Guédot increasingly hears from growers that it’s a problem, she said, though a very sporadic one. Some growers consider it their top pest — some have suffered a total vineyard loss to wasp damage — while others aren’t affected at all. 

In response to the growing alarm, Guédot and her technician, Abby Lois, started studying wasps in 2015. The most prominent species caught in their traps were Eastern and German yellow jackets, as well as bald-faced hornets. They found that wasp activity ramped up in late August and early September, during harvest. This makes them difficult to manage with insecticides — on top of the fact that no insecticides are specifically labeled for wasps. 

Guédot’s data shows that mass trapping is the best management option, but she’s still studying the best ways to go about it, she said.

Rufus Isaacs, a professor and entomologist at Michigan State University, said social wasps have always been a sporadic problem for Michigan grape growers, but he’s been hearing about more incidents in the last few years. He said yellow jackets caused a lot of late-season cluster damage in 2017. 

“It’s not a new issue, but it’s something we’re paying more attention to now,” he said. “I don’t know if populations are getting bigger in the long term. Wasp populations are cyclical, so we might just be on an upswing right now.”

Isaacs has found several species of yellow jacket and hornet in his traps, but the main species affecting grapes is the Eastern yellow jacket. Yellow jackets like eating ripe fruit, and they really like grapes, both juice and wine. And just as in Wisconsin, Michigan wasp activity ramps up during the grape harvest.

“They’re after sugar,” he said. “As the fruit becomes sweeter, it becomes more attractive to them.”

Isaacs said spraying can work in the short term, but there always seem to be more wasps coming from the nest. If you find a yellow jacket nest, you can try to control it directly — but that’s a dangerous management technique. 

Isaacs recommended perimeter trapping in midsummer, to get rid of as many wasps as possible before harvest. He said tests where he and his team placed traps around the perimeter of a vineyard — about 10 traps per acre — reduced the number of wasps by an average of 50 percent. 

The traps were simple plastic containers with holes in them, baited with protein at first, then apple cider or apple juice later in the season. Chemical baits might work well, too, as well as water and soap, since cider and juice tend to get sticky, he said. 

—by Matt Milkovich

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