UW-Madison: University helps state growers cope with voracious, invading fruit fly

BARNEVELD, Wis. – It’s a beautiful summer day with a hint of fall in the air at Bures Berry Patch, southwest of Barneveld, Wisconsin. On the edge of a quarter acre of raspberry canes, research assistant Andi Nelson is counting fruit fly pupae into small plastic tubs. 

Nearby, rows of raspberry plants emerge from ribbons of white, black or silvery films that work as mulch. The mulches are being tested, with guidance from Christelle Guédot, associate professor of entomology at the University of Wisconsin-Madison, as a way to manage an insect that’s earned a reputation as unmanageable.

The brown marmorated stink bug made quite a stink a few years ago when it invaded Wisconsin houses and crops. But the spotted wing Drosophila may be the worst insect pest you’ve never heard of. But if you grow fruit in Wisconsin, you definitely know it. As one in a ceaseless series of insect invasions, spotted wing Drosophila infests berries, cherries and other soft fruits. When the fly is really active, says owner Ed Bures, he has to throw away half of his raspberry harvest.

Insects are constantly seeking new territory, but if you grow something that interests them, how do you protect your crop against a new menace?

Spotted wing Drosophila became the topic on a visit to the UW-Madison Department of Entomology. The fly has been a pest in the continental United States since 2008, and was first seen in Wisconsin in 2010, say Patrick Liesch, Extension entomologist. 

“It affects all growers of small fruit, from big commercial farms to someone with a few plants in the backyard,” Liesch says. “It’s surprising how quickly it took off in the state, and how dramatic the damage can be. Growers have to spray regularly who did not have to do nearly as much, or did not even spray at all.”

The spotted wing Drosophila could shutter Door County’s tart cherry industry, Guédot adds. 

Typical fruit flies lay eggs in overripe or damaged fruit, but spotted wing Drosophila’s sawtooth egg depositor can pierce ripening fruit skin. The eggs develop into larvae that liquefy the fruit, making it squishy and worthless.

Insecticide control is uncertain and expensive, says Bures. “We’ve been working with Christelle’s group, through the Wisconsin Berry Growers Association, trying to figure what to do besides spraying all the time. Insecticides are limited, and consumers have a right to be concerned about what is on their food. So we’re trying to minimize stuff, but at same time, manage a crop that is profitable.”