Extension > Garden > Yard and Garden > Fruit > Integrated pest management for home raspberry growers > Spotted wing Drosophila
Spotted wing Drosophila (SWD)
The spotted wing Drosophila (Drosophila suzukii) (SWD) was first detected in Minnesota in August, 2012 and has since become a major pest in summer red raspberries, fall-bearing red raspberries and black raspberries. SWD 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 larvae have been found infesting many different types of fruit, including raspberries.
SWD adults look very similar to the fruit flies that accumulate near overripe fruit during late summer. They are about 1/8th inch long, have a tannish body, red eyes, and brown bands on their abdomens. Male SWD can be recognized by a distinct black spot near the tip of each wing. The female SWD can only be distinguished from other species by looking at the tip of their abdomen under a dissecting microscope; you need to examine their distinctly serrated ovipositor to properly identify them.
The larvae (maggots) are white, with a body that tapers to one end. The largest larvae are only 1/8 of an inch long and blend in easily with the seeds and white fibers of a raspberry. Smaller larvae are difficult to see, especially in a raspberry that is not fully ripe. The best way to check for SWD larvae is to place four or five ripe raspberries in a water and salt solution (one tablespoon salt per one cup of water) in a plastic zip bag. Gently crush the fruit to break the skin. Any larvae that are present will float to the surface. Infested raspberries typically have multiple larvae.
Evidence of SWD on raspberry receptacles that were recently picked.
Thaddeus McCamant, Central Lakes College
SWD are difficult to detect in raspberries before the fruit is picked. The oviposition scars where the female laid eggs are small and difficult to see in raspberries, and there are no entrance holes. During picking, SWD are easier to detect. Infested fruit are soft to the touch when picked. Even fruit with larvae too small to see with the naked eye will be softer than uninfested fruit. Fruit with large larvae fall apart when picked, causing the pickers' fingers to become covered in juice. Fruit that are infested "bleed" juice onto the white receptacle. When the fruit are picked, the normally white receptacle that stays on the plant will be stained with red raspberry juice.
SWD burrow through the berries, making 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 a 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. SWD populations are highest between the middle of July and the first frost, so both summer-bearing raspberries and fall-bearing raspberries can be destroyed. During severe infestations, nearly all the fruit will have larvae.
During minor infestations, infested fruit can be processed into wine or jelly. During severe infestations, the berries are difficult to harvest and should not be processed.
SWD overwinter as adults in raspberry fields or in brush near the fields. SWD have a wide host range, and are known to attack other soft-skinned fruit, including strawberries, cherries, blueberries, plums and grapes. They particularly like to infest raspberries. They are also known to infest buckthorn fruit. SWD first appear during late June or early July, and the numbers increase rapidly during the middle of summer. The populations appear to peak in August. SWD mature extremely rapidly. During warm weather, SWD can go from egg to mature adult in seven days. The female lays several eggs on each raspberry.
Management of SWD can be challenging but is best achieved through a combination of detection, sanitation, and insecticides.
Gardeners who are concerned about SWD should monitor for the presence of adults. Take a large clear plastic cup with a cover. Make holes, 3/16 in diameter on the sides of the cup. Larger holes will allow larger flies and other insects such as sap beetles to enter the trap, making detection of the SWD more difficult.
The easiest way to make the holes is to 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. You can then either add a yellow sticky card slightly above the vinegar or a little bit of soap (e.g. dish soap). Check the trap several times a week for SWD adults, especially early in the growing season.
Keep the patch picked clean to keep SWD numbers low. When picking, put good fruit in one container and the soft, infested fruit in another container. Be careful to keep fruit from falling on the ground. Infested fruit should be disposed of in a manner that kills the larvae. The larvae can be killed by microwaving the fruit, cooking the fruit or placing the fruit in a sealed plastic bag that will be put in the trash. Do not bury infested fruit or place fruit in a compost pile.
Insecticides can kill adult SWD, but tend to be ineffective on larvae in the fruit. Insecticides should be applied in the evening to avoid killing honeybees and other pollinators. Readily available insecticides that kill adult SWD include carbaryl, malathion, spinosad and pyrethrin. Spinosad and pyrethrin are approved for organic production. Always read and follow labels when spraying pesticides, and follow the pre-harvest interval for all products. Be sure the specific product you intend to use is labeled for raspberries.
For more information on SWD see: http://www.extension.umn.edu/garden/insects/find/spotted-wing-drosophila-in-home-gardens/index.html