The male spotted wing drosophila, right, has the distinguishing brown spots on its wings. Credit: Courtesy of Griffin Dill

This summer, pest experts are warning of a spotted wing drosophila population explosion in the making that should have home gardeners and commercial berry growers concerned. Unlike the common fruit fly drosophila, which is considered a simple nuisance, the spotted wing destroys ripening berry crops.

“This is a whole different species than the fruit fly people are used to if they leave bananas out too long on the counter and then start flitting around and driving you crazy,” Jim Dill, pest management specialist with University of Maine Cooperative Extension, said. “The spotted wing drosophila is out in the fields or in your garden laying eggs in your fruit.”

The female spotted wing drosophila has a sawlike structure she uses to cut into ripening fruit on the bush or vine to create a cavity in which she will lay her eggs. It’s that behavior that makes the insect such a danger to fruit. Once those eggs hatch, the larvae will start eating and render the berries mush. If the hatch occurs soon after the berries are picked, it can be a nasty surprise to the consumer who has unknowingly purchased fruit containing the already laid eggs.

“You get home with a nice quart of fresh, just picked raspberries or blueberries, pop one in your mouth and then leave them out for a day,” Dill said. “Before you know it those berries have turned to mush and there are larvae crawling all over the place.”

Cooperative Extension sets up and monitors traps for the flies around the state to track the population and this week noticed an increase in those numbers. What’s worse, those numbers are only going to go up until the first frost of the fall kills the insects.

“Anybody who grows berries that ripen later in the season should be concerned,” said Dr. David Handley, vegetable and small fruit specialist with University of Maine Cooperative Extension. “These include wild or high bush blueberries, late summer raspberries or everbearing raspberries and strawberries.”

Handley has already started seeing as many as 1,000 of the spotted wing drosophila flies in a single trap that had been left out for several days.

“Those numbers can get really high in our traps,” Handely said. “We expect populations to increase further in the coming weeks as more food becomes available for the flies, especially if conditions remain warm and humid.”

The spotted wing drosophila is a relatively new invasive species in Maine. It was first spotted in the state in 2011, Dill said. The flies can be found throughout Maine, but right now the highest numbers are in southern and coastal areas.

Though the fly is a destructive pest on cultivated berries, Handley said people who live and garden far from a commercial crop should not be complacent as it will find them.

“This insect has found quite a few suitable hosts in the wild including blackberries, chokecherries and even honeysuckle,” Handley said. “So if you are thinking you are far enough out it will never find you, it will by following the wild berries.”

The best defense against the spotted wing drosophila is picking fruit a little earlier than normal in the ripening process. That will hopefully give it less time to lay its eggs.

Once the fruit is picked, it should immediately be refrigerated to at least 34-degrees Fahrenheit to halt the development of any eggs or larvae already in the berries.

“The eggs or larvae are so small you won’t really see them and they won’t hurt you if you eat them,” Dill said. “They also won’t breed in your house, so that’s one good thing.”

Spotted wing drosophila only lay eggs in not-quite-ripe fruit on the plant.

The best way to find out if you have spotted wing drosophila in your garden is to set up some simple traps. Handley recommends using a red plastic solo cup with two rows of one-eight inch holes — or smaller — punched around the top. Pour four-ounces of apple cider vinegar with a pinch of yeast and a drop of dish soap into the cup and then cover it. Pound a 12- to 14-inch stake into your garden and attach a round pipe clamp as a cup holder. Put the solo cup in the holder and check it every day.

“You are going to get a lot of flies other than spotted wing drosophila,” Handley said. “You are looking for the fly with the single spot on its wings and if you see them, you know you have a problem.”

Only the male has the spots, but Handley said the ratio of males to females is one-to-one, so if there are males that means there are an equal number of egg-laying females out there.

There are chemical sprays approved for controlling adult spotted wing drosophila and a list of those agents can be found on the New England Small Fruit Management Guide website. It’s important to follow all instructions and safety precautions with any chemical spray used around growing food.

The chemical pyrethrin, which is extracted from perennial daisies, is toxic to drosophila. It is approved for use on organic crops and is non-toxic to humans.

Unfortunately, biological pesticides do not work on the spotted wing drosophila.

The good news is that nature will take care of the pests this fall, with the first frost killing most of them off. But until then, both Dill and Handley say their numbers will continue to climb, with trap counts already above the levels considered damaging to crops.

Julia Bayly

Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.