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Keeping an Eye on Spotted Wing Drosophila as Fruits Are Ready to Harvest

Summer raspberries are beginning to ripen and become vulnerable to infestation by SWD in southern Michigan. Photo by Julianna Wilson, MSU Entomology.
Summer raspberries are beginning to ripen and become vulnerable to infestation by SWD in southern Michigan. Photo by Julianna Wilson, MSU Entomology.

MSU Extension has been monitoring spotted wing Drosophila (SWD) in 17 counties throughout the state. As blueberries and raspberries are ripe enough to be susceptible. Dr. Julianna Wilson, MSU Extension’s tree fruit outreach specialist, says this year’s numbers are on par with 2017, which was a bad year for SWD.

“Over the last week or so during the hot, dry period, their activity kind of slows,” she said. “We know they go into a short-term hibernation. They sit still until conditions are good again.”

On Wednesday, some parts of the state received rainfall, which could wake up the flies and increase their numbers. Because SWD attacks fruits as they ripen through harvest, it’s a challenge to keep it from infesting fruit.

“They really love humidity and moisture, and other times of the day where it’s not over 90 degrees where they can be active, especially in the morning and later in the afternoon if it’s humid enough. They insert the egg into the fruit, so it’s an internal feeder. Cherries are impacted, blueberries, raspberries, and fall raspberries especially.

So what can be done right now? Wilson said producers should know that it takes eight to 10 days for the pest to reproduce, and females can lay up to 300 eggs in her lifetime.

“The number of adults out there mating and reproduction is tremendously high,” said Wilson. “Most commercial growers have insecticide programs, and they’re gong in every seven days to apply a pesticide that we know is excellent against that pest that’s going to kill the adult. We can’t kill the larvae once it’s in the fruit, so what we’re trying to do is prevent the adults from laying their eggs in the fruit.”

There are two ways to keep track of SWD on your operation. The first is to create a trap, but Wilson says at this point in the season, it’s likely they are around. The second is to put fruit in a gallon bag with saltwater to see if larvae are in the fruit.

To learn how to make a trap and other resources, click here.