Adult male spotted wing drosophila. Photo: Martin Hauser, California Dept of Food and Agriculture
Adult female (left) and adult male (right) on raspberry. Photo: Hannah Burrack, NC State Univ.
Larva inside raspberry. Photo: Hannah Burrack, North Carolina State University, Bugwood.org
Vinegar trap. Photo: Hannah Burrack, North Carolina State University, Bugwood.org
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. It has many hosts, but is most often attracted to grapes, cherries, peaches, blueberries, raspberries, strawberries, and other soft-flesh fruits.
Spotted wing drosophila deserve 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 maggots feeding inside the fruit. After only a few days, the skin will collapse and create craters in the fruit, making it susceptible to decays and rots. It is possible, however, for them to leave no visible impact on the fruit, only detectable once the fruit is picked and prepared for eating.
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 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 small scar on the fruit's surface, but after 5-7 days of the larvae feeding inside, the skin collapses and the fruit may 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 a mixture of yeast, sugar, and water has proven to be very effective as well. A plastic cup and secure lid with several medium-sized holes drilled around the top of the cup filled with an inch or two of either mixture can serve as a trap. Hang these from a tree branch or stake/pole in the ground with a strong but malleable wire in an area where SWD would likely be found. There are also commercial traps available. Traps work best when they are serviced/cleaned at least once per week; by placing a coffee filter or sieve inside a funnel, the funnel over a container to catch the trap liquid, the flies can be retrieved by dumping the trap's entire contents into the funnel. 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, and immediate disposal of infested fruit is good practice for protecting crops.
PA IPM, Spotted Wing Drosophila, Individual sections in PDF format: