A tiny, red-eyed fruit fly from Asia is packing a strong punch in Minnesota and upending the economics and growing techniques for raspberries, blueberries, strawberries and even grapes.

The spotted wing drosophila, first detected in the state in 2012, has damaged locally grown crops, shortened pick-your-own seasons, increased insecticide use and reduced incomes for producers.

"It's a game changer," University of Minnesota Extension entomologist Bill Hutchison said.

At only about 3 millimeters or one-tenth of an inch long, the fruit fly is one of the worst insect invasive species Hutchison has worked with in his 25-year career.

From anecdotal evidence, the Extension service estimates that 20 to 25 percent of Minnesota berry growers have suspended production or gone out of business, Hutchison said. The U plans to do a thorough economic analysis in the future.

The fly uses the berries to lay eggs, which hatch within days into tiny white worms that quickly turn the fruit into mush. Nearly all other fruit flies feed on overripe or deteriorating fruit that has little market value, but the spotted wing drosophila lays its eggs in intact, ripening fruit.

Fall raspberries are the most vulnerable fruit, but the pest can also penetrate blueberries, blackberries, midsummer strawberries and grapes that have soft skins or sometimes split open just before harvest.

Like many invasive species from other parts of the world, the fruit fly has no known natural predators in the U.S., Hutchison said, and produces seven to eight overlapping generations each summer. That creates constant pressure on the berry crops from late June through the rest of their growing season.

Jerry Untiedt, whose farms just west of the Twin Cities grow dozens of fruit and vegetable crops, said he's been working with Extension researchers to trap male and female drosophila to determine when they show up and how quickly their populations expand. The information will help to determine how the fly might be controlled, including how early and how often to spray fruit to kill the pests.

Untiedt said most of his raspberries are fairly protected because they're raised in high tunnels, which are long, plastic-covered shelters that can be covered on the ends with netting if necessary. Even so, he said, the fruit fly has gotten into some of the tunnels and spoiled some fruit.

The insect has spread throughout the country in just the past few years, he said, and is already having a profound impact on the industry.

"Especially if you're out of doors, I think that many growers have just given up because many times you don't notice this pest until you notice the white worms emerging from the fruit," Untiedt said. "So you've spent all of this time and used your finances to take care of the plant and irrigate it and fertilize it and prune it, and then all of a sudden your crop is not marketable."

It's difficult to estimate the value of the soft berry crops raised in Minnesota, because the vast majority of them are sold directly to consumers at farmers markets or pick-your-own farms, said Paul Hugunin, director of the agricultural marketing and development division at the Minnesota Department of Agriculture.

The state lists commercial berry farms in its Minnesota Grown directory, which includes about 80 strawberry farms, 41 summer raspberry farms, 33 fall raspberry farms and 31 blueberry farms. Some are duplicate listings, he said, and do not include additional producers that sell only at farmers markets.

Regardless of how growers approach this pest, Hugunin said, it's going to increase their costs and make their business more difficult by adding another layer of work. Scouting to see when the pests show up, applying pesticides or taking other control measures will increase time, labor and likely chemical costs, Hugunin said.

"Farming anything isn't easy, and nobody needs additional pests or things to try to deal with," he said. "This is something relatively new that's going to make this a more complicated industry for folks."

That is true for Kevin Edberg, owner and manager of the Berry Patch, just northeast of the Twin Cities in Forest Lake. Edberg lost nearly all of this year's berry crops in a June hailstorm, but during the previous two years had been battling the new fruit fly. His 20-acre farm includes strawberries, raspberries and blueberries, open for picking from early June to early August.

"I've never had to use insecticides on my raspberries before this [pest] came along," he said. "The yuck factor of finding little worms in your fruit is so damaging to consumers that it caused us to re-examine what level of management we have to take on."

That has included spraying some bug killers, he said, and shutting down his operations to the public earlier than normal once the flies began to infest the berries.

U entomologists started to research the fruit fly soon after it was confirmed in the state in 2012, thanks in part to help from the legislatively funded Rapid Agricultural Response Fund and projects funded by lottery proceeds.

At the Rosemount Research and Outreach Center just south of the Twin Cities, researcher Eric Burkness and others are focused mainly on how the pest affects raspberries. Different rows of berries have been sprayed with different pesticides during the past couple of summers to determine which chemicals seem to be more effective, and how they affect the quality of the berries.

Berries are also growing under more than a dozen plastic-covered high tunnels to test a number of theories, including whether the higher temperatures inside the tunnels make it less likely the fruit flies will enter and remain to lay eggs, and whether netting at the ends of the structures can be effective in keeping them out.

Burkness has also been trapping the fruit flies at Alexis Bailly Vineyard near Hastings, where president and winemaker Nan Bailly said that most local grape varieties on her 13-acre farm seem to be too tough for the pest to penetrate. But Bailly said one type of grape seems to be vulnerable because of its tendency to split open before harvest, enabling the drosophila to enter and spread bacteria.

Bailly said she never had to use pesticides in 40 years of grape production until the new pest arrived, and last year she had to plow under 8,000 pounds of grapes that became infested and unusable.

"We're hoping to be better stewards of the land by finding alternative means of control where we're not out there spraying," she said. This year she has worked with Extension scientists to test how well netting the grapes in a fine mesh before the fruit flies arrive will protect them during the growing season, and whether trimming more leaves from the vine canopy will increase sunlight and make the fruit clusters less inviting.

Netting presents its own problems of expense, labor and maintenance until the fruit is ripe and then removing it all, she said, but it may be worth it on a small scale of an acre or so.

"What we've learned so far is that we're not going to eradicate them," Bailly said. "We're going to have to control them somehow so we can continue to make the wine that we want."

Tom Meersman • 612-673-7388