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Tiny Invasive Pest Wreaking Havoc on Blueberries

by Rick A. Richards

Published: Friday, November 9, 2012

An invasive pest barely 1/16th of an inch long that first turned up in California in 2008 has caused millions of dollars in damage to Michigan's blueberry crop.

Rufus Isaacs, a small fruit entomologist at Michigan State University, said the spotted wing drosophila is expected to cause $27 million in damage to the state's $102 million blueberry crop this year.

"We first saw it in this country in California in 2008 and we found it here in Michigan in 2010," said Isaacs. "It came here from Asia and now it's everywhere. It's been found in New Jersey, Maine and up into Canada. It's in Michigan, Indiana, Illinois and throughout the Midwest."

There are native drosophila, but they are less destructive because they only lay eggs in rotting fruit. The spotted wing drosophila frequently isn't found until after it lays its eggs in healthy fruit, and by that time the damage is done.

According to a Michigan State University Entomology Department bulletin, the spotted wing drosophila is a member of a class of insects called vinegar flies. But unlike native vinegar flies that require damaged fruit in order to lay eggs, the spotted wing drosophila cuts a slit in healthy fruit and lays eggs inside. Adults live only about two weeks, but can lay more than 100 eggs a day.

In addition to blueberries, it has been found in Michigan's cherries and grapes.

Isaacs said no one knows for sure how the pest arrived here, but it likely hitched a ride on imported fruits and vegetables and then traveled the country on its own or on trains and trucks that carried produce.

"There is a lot more international trade now and it's easy for this to happen," said Isaacs.

But as devastating as the spotted wing drosophila can be, Isaac is optimistic that his two years of study has come up with ways for growers to manage and contain the pest.

"With the information we've been able to generate, I think we can manage this," said Isaacs. He said that while the spotted wing drosophila is a pest, it can be controlled much more easily than other invasive pests like the emerald ash borer.

"We're holding a series of educational programs this winter for growers to show them what we've learned," said Isaacs. "We've actively studied this the last two years."

Isaacs and others from the MSU entomology lab have placed traps throughout the blueberry growing areas of the state. The traps are plastic buckets containing fermented yeast. Holes in the bucket allow the pests in to feed, but they aren't able to leave.

Growers can create their own traps to monitor for the presence of the pest. Use a 32-ounce cup with several 3/16th to 3-8th inch holes at least three inches above the bottom of the cup. Pour one to two inches of pure apple cider vinegar into the trap as bait along with a yellow sticky trap to keep them from escaping. Hang the cup in the shade, using a stake or a wire attached to the side of the trap and then fasten it to a branch or trellis wire. The traps should be checked weekly.

Once vinegar flies are trapped, growers will need to separate the spotted wing drosophila from native species. Examine the wings of trapped vinegar flies with a magnifying glass. Some native flies have dark patches on their wings, but will not have distinctive dart dots on both wings as do spotted wing drosophila males; females can be identified by the serrated ovipositor (a hook-like appendage) at the base of the abdomen which is used to cut a slit in fruit in order to lay eggs.

Isaac said traps have been placed in Berrien, Van Buren, Allegan, Ottawa and Muskegon counties, where the highest density of spotted wing drosophila have been found.

"We advise growers of susceptible fruit in all Michigan counties to initiate active monitoring for this pest," said Isaacs, adding that it has been trapped on blueberry farms and near strawberry, blackberry and raspberry farms.

Even though Isaacs is confident the pest can be managed, he said it likely won't be eradicated. He said the spotted wing drosophila is so widespread and numerous, that any thought of eradicating it is years away.

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