Spotted wing Drosophila
The spotted wing Drosophila (Drosophila suzukii) (SWD) is closely related to the common fruit flies that feed on decaying fruit. Whereas most fruit flies feed on decaying fruit, SWD lay their eggs in ripening berries that are still on plants. SWD larvae have been found in many different types of fruit, including raspberries, cherries and blueberries. SWD are a new pest in Minnesota, and were first detected in August, 2012. In 2013, SWD were detected in blueberries across southern and central Minnesota.
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. They only grow as large as 1/8 of an inch long. The larvae blend in easily with the light colored flesh of a blueberry. Small larvae are especially difficult to see. The first indication that fruit have SWD is if they are soft to the touch when picked. The best way to see if SWD larvae are in a batch of blueberries is to place four or five berries in a water and salt solution (one Tablespoon of salt per one cup of water) in a small container or plastic bag. Gently crush the blueberries to break the skin. After 30 minutes, any larvae that are present will float to the surface. Infested berries typically have multiple larvae.
SWD larvae 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 the fruit has been picked, Fruit that was normal then may be soft and maggot infested a day or two later. Mature larvae often crawl out of berries stored on the counter.
During minor infestations, infested fruit can be processed into wine or jelly. During severe infestations, the berries are too rotten to be processed.
SWD overwinter as adults in brush near the fields. SWD have a wide host range, and are known to attack other soft-skinned fruit, including raspberries, blackberries, cherries, plums, strawberries and grapes. They infest a number of wild fruit, including buckthorn. SWD first appear during late June or early July, and the numbers increase rapidly during the middle of summer, with the populations peaking in August. SWD mature extremely rapidly. During warm weather, SWD can go from egg to mature adult in seven days. The females can lay several eggs on each berry.
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 several holes, 3/16 in diameter, near the top on one side of the cup to allow for easy liquid disposal. 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.
Pour 1 to 2 inches of 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 liquid soap, such as dish soap. Hang traps on branches in a shaded location near fruit. Replace the sticky card and apple cider vinegar bait at least once a week when checking for SWD, disposing of the old apple cider vinegar away from the trap location. Checking traps more often can be beneficial for early detection of adult SWD, 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 berries or place fruit in a compost pile.
Insecticide applications should be considered once adult SWD are detected, as infestation can progress very quickly. These chemicals can kill adult SWD, but are ineffective on larvae in the fruit. If started at first detection and maintained on a regular schedule, insecticide sprays can help prevent larval infestation of fruit before harvest. Insecticides should be applied in the evening to avoid killing honeybees and other valuable pollinators. Readily available insecticides that kill adult SWD include permethrin, carbaryl, malathion, spinosad and pyrethrin. Spinosad and pyrethrin are compatible for organic gardens. Always read and follow labels when spraying pesticides, and follow the pre-harvest interval for all products.
For more information on SWD see: http://www.extension.umn.edu/garden/insects/find/spotted-wing-drosophila-in-home-gardens/index.html