Farmers growing late-season berries should be on the lookout for a strain of fruit fly that affects barely ripe soft-skinned fruit such as raspberries, strawberries, blackberries — and in some cases, blueberries.
The University of Maine Cooperative Extension monitors the population rates of the insect, the spotted-wing drosophila, and is encouraging farmers still growing these crops to take precautions against possible infestation.
With good weather prolonging the viability of late season raspberries and strawberries, the high populations of spotted-wing drosophila being detected could ravish the berries before farmers get to them, according to David Handley, a Cooperative Extension vegetable and small fruit specialist.
“If you’re still harvesting fruit, you need to stay on the ball here,” Handley said.
The spotted-wing drosophila is different than the common fruit fly you likely have found perusing your kitchen over the last month. This strain of fruit fly is native of northern Asia and was first seen in Maine in 2011. Common fruit flies feast and multiply on over-ripened fruit, but the spotted-wing drosophila targets soft-skinned fruits before they are ripe.
While the common fruit fly utilizes the already decomposing skin of overripe fruit in order to lay its larvae and multiply within the fruit, the spotted-wing drosophila is able to pierce the skin of soft-skinned berries that haven’t fully ripened yet and lay eggs within the fruit, Handley said.
If a swarm of these flies infest your fruit crop, it compromises the berry before you even have a chance to harvest it.
“When you as a farmer go to pick your fruit, even though it’s barely ripe, it is turning to goo. The reason it has turned to goo is because the larvae in there have started to hatch and are feasting on the fruit,” Handley said. “Your fruit starts to melt away from the inside out.”
Infestation can happen quickly, Handley said, since one female fly can lay up to 400 eggs. The eggs only need between 10 and 14 days to become an egg-laying adult fly itself. “Very quickly you can have millions, if not billions of flies roaming around,” he said.
Unfortunately for farmers, there are not many ways to combat against these flies except for spraying an insecticide. Handley said there are a couple of companies that have begun to manufacture a mesh covering fine enough to keep out the flies, but the product is expensive.
Since the flies have only been detected in Maine in recent years, Handley said the problems they cause have “really changed the game” in terms of late-season berries. With the spotted-wing drosophila population normally building up in August, early berries are generally safe, with late-berries bracing the brunt of the infestation.
The flies were first detected in the U.S. in California in 2009, likely coming in from produce shipped in from northern Asia, Handley said. It only took two years for a population of the flies to be found in Maine as well, which he said likely arrived on produce from California or Florida.
While there is already a population of these flies in Maine, during the summer, warm fronts coming up the east coast bring more flies, Handley said.
Unless you’re able to mitigate the fly problem with an insecticide or protective mesh, Handley said it could take two or three good frosts before the flies subside.