Growers of raspberries, strawberries, grapes and other Minnesota summer fruits are on guard for a newly arrived pest that could spoil some late-season delights.

Spotted wing Drosophilia, an Asian fruit fly that arrived in the United States only five years ago, wiped out some raspberry crops in Minnesota last year and has growers and researchers developing defenses against it as harvest time approaches.

The tiny fruit fly has a nasty habit of feeding and laying eggs on healthy fruit, after which its larvae develop under the fruit's skin, even as it ripens. The larvae emerge sometimes only three days later.

Nobody has reported being surprised by a mouthful of maggots, and infested fruit probably would be too far gone to make it to market anyway, said Mark Asplen, a University of Minnesota research associate who has been studying the fruit fly and its effects. But the bug's development doesn't paint a pretty picture.

"From what I've heard, it's pretty gross," said Bill Jacobson, a strawberry production manager at Pine Tree Orchard Inc. in White Bear Lake, where that early-season crop was unaffected this year. "It almost looks like the berry's alive. It's a real turnoff for the consumer."

Jerry Untiedt of Untiedt Vegetable Farms in Waverly, Minn., a familiar figure at the Minneapolis Farmers Market, said the insect isn't really an problem for consumers, because growers won't bring ruined fruit to market. He said that he was able to detect and eradicate some spotted wing Drosophilia last year before the fruit flies damaged his strawberry or raspberry crops, but that those steps are likely to add costs to production, and ultimately to the cost of fruit.

University of Minnesota research associate Mark Asplen checked traps near rows of strawberry plants that contained the spotted wing Drosophilia and other flies Wednesday at the U’s St. Paul campus. He described the insect as a “manageable pest ” if commercial and home growers monitor and trap adult flies early in the season, report findings to the state Department of Agriculture, and apply insecticides.

"I'm most concerned about it, I'll tell you that," he said.

The fruit fly was detected in Minnesota for the first time last year, and in a big way. Infestations were found in 29 counties, from the Iowa border to Clearwater County in ­northwest Minnesota.

So far this year the fruit fly has been confirmed only in Dakota, Rice and Ramsey counties, with tentative appearances in Olmsted, Washington and Wright counties. The Dakota County detection, on wild raspberries June 27, was the first this year. But the Drosophilia population increases as the season wears on.

Asplen described the insect as a "manageable pest" if commercial and home growers monitor and trap adult flies early in the season, report findings to the Minnesota Department of Agriculture and apply insecticides, some of which are acceptable for organic crops. But he said those steps are not now on most growers' to-do lists, since Minnesota fruit pests tend to damage blossoms and not fruit.

Jacobson said he believes that the fruit fly is another "next great pest" that over time will become a controllable ­irritant.

"But for the next few years, until we get a handle on it, for soft, small fruits, it's going to be a big deal, I'm afraid," he said.

Bill McAuliffe • 612-673-7646