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The spotted wing drosophila is a vinegar or fruit fly of East Asian origin. It has been in Hawaii since the 1980s, but was first discovered in California in 2008, and Florida, Utah, the Carolinas, and Michigan in 2010. By 2013, it was reported from most of the continental US, except Arizona, Nevada, New Mexico and South Dakota. It has many hosts, but is most often attracted to raspberries, blueberries, day-neutral strawberries, grapes, cherries, peaches, plums, and other late-season, soft-flesh fruits.
First trap catch in New York
Spotted wing drosophila deserves notice because, unlike other fruit and vinegar flies, which lay their eggs on past ripe or rotting fruit, they lay their eggs inside fresh fruit, often before harvest. Aside from the superficial scars left by the female's ovipositor (their egg-laying device), most damage is done by the larvae feeding inside the fruit. After only a few days, the fruit skin becomes dimpled or wrinkled, forming craters in the fruit, and making it susceptible to decays and rots. It is possible, however, for early-stage larvae or eggs to leave no visible impact on the fruit.
Just as one could imagine from the insect's common name, male spotted wing drosophila have a single black spot on the tips of their wings. Females lack this particular trait, making them more difficult to identify, but both genders have distinct red eyes. What sets female spotted wing drosophila apart from other fruit flies is the dark brown to black, saw-tooth edges that line either side of their ovipositor. While spotted wing drosophila generally have striped abdomens like so many other fruit flies, females tend to have a wider black band at the very end. Spotted wing drosophila are a medium sized fruit fly, generally about 0.08-0.12 inches long.
Females use their ovipositors to cut through the surface of the fruit into the flesh, where they then lay approximately 1-3 eggs per fruit, 7-16 eggs per day. Damage is initially a tiny pinhole on the fruit's surface, but after 5-7 days of the larvae feeding inside, the skin collapses and the fruit may leak juice, turn soft and begin to rot. The larvae then exit the fruit to pupate, taking anywhere from 3-15 days for adult flies to emerge. As adults, the lifespan of spotted wing drosophila can be as short as 8-14 days or, in mid-season at optimal conditions, as long as 3-9 weeks. In its native land of Japan, spotted wing drosophila have roughly 13 generations per year, and upwards of 10 per year are predicted to occur in the United States, depending on the climate.
The simplest means of monitoring for spotted wing drosophila is with a basic red wine/apple cider vinegar trap, although more sophisticated traps are being developed. Please see Spotted Wing Drosophila (SWD) Monitoring Traps. A plastic cup and secure lid with forty 1/8-inch holes drilled around the top of the cup filled with an inch or two can serve as a trap. Hang these, in the shade, on a tree branch or stake/pole in the ground with a strong but malleable wire in an area where SWD would likely be found. Traps work best when they are serviced/cleaned at least once per week. Because spotted wing drosophila have such small defining features, the only real way of knowing whether or not there are any in the trap is to look at all of the specimens under a dissecting microscope. A combination of trapping to know when they've arrived at the fruit planting, immediate disposal of infested fruit, and treatment of ripening fruit with insecticides is good practice for protecting crops. Please see Cornell Fruit: Spotted Wing Drosophila (SWD) Management.
Authored by Juliet Carroll and Kelsey Peterson, New York State Integrated Pest Management Program, Cornell University