October 7, 2012

Maine Gardener: To growers, this fly comes on wings of evil


When gardening news escapes the features sections and ends up in the hard news sections of newspaper and television reports, you know it is big – and probably bad.

Such is the case with the spotted-wing drosophila, which was first detected in Maine last year and has made a full-scale invasion this year.

“We have found it every place we have looked for it,” said David Handley, a vegetable and small fruit specialist working out of the University of Maine Cooperative Extension’s Highmoor Farm in Monmouth. “We don’t have any traps set up in the farther north parts of the state.”

The spotted-wing drosophila is an Asian fruit fly that, unlike the fruit flies with which most of us are familiar, will lay its eggs in healthy, developing fruits and vegetables with soft outer skins. Other fruit flies will bother only fruit that is already ripe.

The drosophila overwinters as an adult, and nature kills a lot of them, Handley said. They first lay their eggs in the spring, but damage then is minimal. It was only when this year’s crop of fruit flies came out that the fly took its toll on crops.

“It depends on the location, but for 80 percent of the state, it took out anything you had growing that ripened after the second or third week of August,” Handley said. “If you have a pint of raspberries, you might see a few fruit flies around it, but the next morning, it looked like someone ate half of it – it melted from the inside out. After two days, it would be a pint of red soup with larvae in it.”

With the insect’s arrival in mid-August, a lot of important Maine crops were unaffected. Traditional strawberries, spring raspberries and much of the blueberry crop were spared.

“This year, if the crop came in before the second or third week of August, it was spared,” he said, “but we can’t guarantee that will happen every year.”

He said the arrival time will probably depend on how hard the previous winter is and a few other factors.

A lot of concern has been with commercial fruit growers, who have large amounts of inviting fruit for the pests. But even backyard gardeners with one or two bushes will not be spared – the spotted-wing drosophila will find them.

“Just for the fun of it, we put a trap right out next to a corn field to see what would happen,” Handley said. “It ended up catching flies pretty quickly. We ended up finding some on honeysuckle bushes in the area. So if you have fruit, they will find it.”

Handley stressed that the arrival of the spotted-wing drosophila will change how people grow soft-skinned fruits that come late in the season, but it won’t mean people will have to stop growing those fruits.

“We can control it with pesticides,” Handley said, “including some pesticides that are approved for use by organic growers. But they will have to apply those pesticides one or two times a week.”

He said that for crops affected by this insect, the state is going to have to go against its preferred practice of fighting pests. The principals of Integrated Pest Management say that you should wait until a pest shows up on your crops, identify the pest and use the correct insecticide for that pest.

But Handley said because the spotted-wing drosophila strikes so quickly, the state is recommending that growers do preventative spraying before the fly even shows up so the crop won’t be decimated.

He did say that home growers comfortable with spraying their crops can do so, but that some might prefer to switch to earlier-ripening fruits.

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