Q: I’ve found those tiny white worms inside my raspberries this summer. What are they and is there any way I can get rid of them?
A: From your description, it is most likely Spotted Wing Drosophila (SWD) larvae that you found in your raspberries. The SWD is a small vinegar fly that lays its eggs inside of ripening soft-skinned fruit. The raspberry is one of its favorites. It was first discovered in Minnesota in 2012, and it’s quickly creating havoc in our raspberry crops. More recently, it’s also been seen in blueberries, cherries, strawberries and grapes. Some of our ever-bearing varieties are still being affected this season.
Control is multifaceted. Daily monitoring of your fruit is recommended. The first SWD this year were found in Hastings, Minn., on May 23. Here in the Northland, we generally do not see it until later in June to early July. Homemade apple-cider traps can help with early detection, but there are also some commercial traps (e.g. Scentry.com) used that are more specific to SWD.
When fruit is infested, it may be mushy, or it may look normal but be mushy on the inside. You can test your fruit for the larvae by putting the fruit in a salt water bath. If there are larvae, you’ll see them float up to the surface of the water. If your fruit is infected, you can still eat it as long as you process the fruit immediately upon picking by refrigeration, freezing or canning.
Harvest as soon as fruit is ripened. Hygiene in your garden is very important. Do not leave any decaying fruit lying on the ground. Fruit that you believe to be infested should be placed into a clear plastic bag and tightly sealed. You can place it in the sun to kill the SWD and/or place it still sealed into the garbage. You can decrease SWD habitat by more aggressive pruning of the canopy and by mowing, trimming and laying black plastic around your plants. Exclusion netting can help, but there have been mixed results with high tunnel netting. It has been found that that the flies can still get in when the net is opened for harvesting or through small openings. Also, shaded, forested areas around the fruit area are being found to be “reservoirs,” as the flies do not like high heat.
Once you've detected SWD, your choices are essentially to tolerate the larvae or spray pesticide. Two organic sprays are spinosad and pyrethrin. Synthetic sprays include carbaryl and malathion. Spraying needs to be done following manufacturer’s direction, usually every few days. Insecticides should be rotated to prevent resistance. Spraying in the evening hours is recommended to diminish harm to pollinators. More information is available at extension.umn.edu/yard-and-garden-insects/spotted-wing-drosophila.