Master Gardeners: Raspberries and SWD


How are your raspberries growing? Now is the time to place traps for spotted wing Drosophila and check the traps regularly throughout the growing season. This column has had information about the SWD before, but information bears repeating. More about this later.

Late July remove dying floricanes to improve air movement through the bed and reduce spread of fungal cane blights.

August – monitor for fruit-eating beetles in fall bearing raspberries by walking rows. Dispose of any overripe or rotting berries. Picnic beetles can be a problem. You can uses rinds of cantaloupe, baiting them with Garden Guard. This attracts the picnic beetles, kills them and keeps them off the ripening raspberries.

Have you heard about the dreaded spotted wing Drosophila? This insect was first detected in Minnesota in August 2012 and has become a major pest in summer and fall bearing red raspberries and black raspberries. It is closely related to the common fruit flies that feed on decaying fruit but SWD larvae will infest raspberries before they ripen on the canes.

SWD are about 1/8th inch long, have a tannish body, red eyes and brown bands on their abdomens. The male has a distinct black spot near the tip of each wing. The female can only be distinguished by looking at the tip of their abdomen under a dissecting microscope. The larvae (maggots) are white and blend in easily with the seeds and white fibers of a raspberry.


The best way to check for SWD larvae is to place four or five ripe berries in a water and salt solution (1 tablespoon salt per 1 cup water) in a plastic zip bag. Gently crush the fruit to break the skin. Any larvae that are present will float to the surface.

During picking SWD are easier to detect. Infested fruit are soft to the touch when picked. Fruit with large larvae fall apart when picked, causing the pickers’ fingers to become covered in juice.

Damage from SWD makes the fruit soft and unappealing. During egg laying the female may introduce fungi that cause the fruit to rot and infested fruit often develop a fermented or sour smell.

If berries are stored at room temperature, larvae can hatch after picking causing raspberries that looked normal during picking to deteriorate a few hours later. So get them in the refrigerator right away.

During minor infestations, the fruit can be processed into wine or jelly. During severe infestations, the berries are difficult to harvest and should not be processed.

Management of SWD is best achieved through a combination of detection, sanitation and insecticides. The way to make a trap is to take a large clear plastic cup with a cover and make holes 3/16 in diameter on the sides of the cup. Heat a small (8 or 10 penny) nail which can melt the right size hole in the cup. Put apple cider vinegar in the bottom of the cup. Add either a yellow sticky card slightly above the vinegar or a little bit of dish soap. Check the trap several times a week, especially early in the growing season.

Insecticides can kill adult SWD but tend to be ineffective on larvae in the fruit. They should be used only in the evening to avoid killing honeybees and other pollinators. These include carbaryl, malathion, spinosad and pyrethrum. Spinosad and pyrethrin are approved for organic production. Always read and follow labels and follow the pre-harvest interval for all products. Be sure the product you intend to use is labeled for raspberries.

Good luck.

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