Source: Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org
SWD was introduced into North America around 2008 and has spread from coast to coast in less than 4 years.
Although some strains from different regions of Asia are quite cold tolerant, the U.S. introduction appears to have spent at least 10 years in Hawaii adapting to the warm local conditions before moving to the continental United States. Subsequently, our U.S. strain of SWD appears to suffer from heavy winter mortality.
Discovered in Adams County, Pennsylvania, in 2011, it has since has spread throughout most of the state. Here SWD attacks midsummer crops starting with black raspberries and blueberries, and then populations peak with the most damage in fall crops like blackberries and primocane raspberries.
While in Oregon it is a major pest of cherries and in North Carolina a pest of spring strawberries, it appears numbers are too low for it to attack those crops in Pennsylvania because of the heavy winter mortality. Reports so far of SWD attacking cherries, peaches, plums, and grapes have proved to be from overripe or damaged fruit rather than from attacks initiated on ripening and undamaged fruit. As SWD likely adapts to colder weather, it will be able to survive in higher numbers and attack earlier crops, so crops will have to be continually monitored for changes in susceptibility. Evidence that SWD can adapt to cold weather can be seen in Michigan, where it has become a major pest of blueberries. Certain blueberry varieties have been shown to be less susceptible to SWD because of thicker skins--the same reason most grapes, nectarines, and especially peaches with additional "fuzz" seem to be protected from these tiny flies. "Split-pit" peaches, grapes with black rot infections or yellow jacket damage, and fruit with Japanese beetle damage would obviously be more at risk to SWD damage, but reports of attacks on plums and tomatoes need to be investigated further. SWD eggs are also unique as they have long filament spiracles used to "breathe" in a liquid environment, but generally a microscope is necessary for identification because of the minute size.
Description and life cycle
The basis for the name "spotted wing fruit fly" is one obvious dark spot on each of the male wings. Females do not have spots on their wings. What makes this pest different from other pumice/vinegar flies is that females have a sharp ovipositor that allows them to attack fruit before it is ripe, rather than just attacking overripe or damaged fruit like its close relatives. Many other pumice/vinegar flies can be found in Pennsylvania fruit orchards. A fact sheet providing identification, Spotted Wing Drosophila, Part 1: Overview and Identification, is available from your county extension office and online. Like other vinegar flies, SWD is also attracted to damaged and rotting fruit, where it will reproduce quickly with up to a dozen generations in a year.
Adult flies survive the winter in protected locations. The early season activity in the Northeast starts in May to mid-June. Adults live from a couple weeks up to 2 months during the growing season. Multiple generations are usually observed until October. A single female can lay up to 600 eggs, depositing them under the skin of mature and immature fruit. Larvae can hatch in 2 hours to 3 days after egg deposition. Depending on the food source and temperature, maggots can feed inside of the fruit from 3 to 15 days. Pupation can occur either inside of the fruit or on the ground. Mature females usually start depositing eggs within 2 days after emergence. Female adults exposed to cold temperatures lay only a few eggs, which can be killed by several days of constant exposure to temperatures around freezing. More on the natural history of SWD can be found at in the fact sheet Spotted Wing Drosophila, Part 2: Natural History, available online.
Spotted wing fruit flies are attracted to many volatiles, including those in vinegar, wine, yeast, and fruit. Apple cider vinegar was originally thought to provide a good attractiveness, but recently a yeast attractant (1 tablespoon of dry yeast and 4 tablespoon of white sugar dissolved in 2 cups of water) proved to be very effective in monitoring flies and a commercial yeast lure in a plastic capsule is available with less mess. Using apple cider vinegar with a few drops of soap to make the flies sink increases the attractiveness of the lures and serves as a killing and preserving fluid in the traps. Also a Merlot wine mixed with apple cider vinegar was very attractive in monitoring flies during late season. Inexpensive traps can be made from clear drink cups with lids. A line of small holes (3/16 inch) should be drilled in the upper part of the container. For best results, place traps in the field at least 2 weeks before fruit begin to color, optimally near the suspected overwintering shelter areas. More monitoring information can be found in the fact sheet Spotted Wing Drosophila, Part 3: Monitoring, available from your county extension office and online.
Cultural management includes removal of all ripe and cull fruit from the planting. Frequent harvesting will also help reduce the fly population. Removed fruit should be buried at least 2 feet deep to prevent emergence of flies. Composting or simply placing infested fruit away from the orchard will not stop the development of flies or possibility of another infestation. Insecticides from different groups such as pyrethroids, spinosyns, diamides, organophosphates, and carbamates should provide effective control of spotted wing fruit flies. Unfortunately, frequent in-season insecticide applications may be required to prevent flies from continuously infesting fruit. Management information for this new pest is changing monthly as new insecticides are registered for use and field trials are being conducted. Spray coverage has been almost as important as pesticide selection with even the most effective pesticides failing to give control unless thorough penetration and coverage of the entire crop canopy is achieved. Airblast sprayers used for tree fruit have been the most effective. More management information is available in the fact sheet Spotted Wing Drosophila, Part 4: Management, available online.