Monitoring for Spotted Wing Drosophila
When word of a new drosophila species got out, growers became concerned about not if, but when the invasive pest would arrive in their area.
Like other invasive insects, the spotted wing drosophila (SWD) arrived in the United States as an undocumented passenger from eastern Asia. It was first found in California in the mid-2000s, then spread northward into Washington and Oregon. By 2011, SWD was reported in areas throughout the Northeast, as well as in Florida. Although the SWD (Drosophila suzukii) looks quite similar to the common fruit fly (Drosophila melanogaster) that thrives in rotting fruit, the potential for damage to soft fruits is far more serious.
The characteristic black wing spot of the SWD helps distinguish it from other fruit flies, although it can easily be confused with other fruit flies that have bands or blotches. The black wing spot isn't always present, and can be hard to see. A more accurate identification can be made by the presence of two black bands on each of the front legs of male flies.
"The female has an ovipositor and can saw into the fruit just as it's starting to color," said Kathy Demchak, a senior extension associate at Penn State University who is conducting research on the SWD. "She lays eggs in the fruit, and the white larvae hatch in the fruit."
Demchak recommended that growers check for the presence of the SWD by placing vinegar traps among thin-skinned, soft fruit crops, such as plums, peaches, cherries, raspberries, blackberries and strawberries, that are starting to ripen. Traps can be purchased, but are easy to make. Start with a 16 or 32-ounce lidded deli cup and drill about six to 10 entrance holes around the upper section of the cup. Leave one side of the cup without holes so that flies aren't lost when the trap is emptied. Pour about one inch of apple cider vinegar and a drop of unscented dish detergent in the cup. The vinegar attracts and preserves flies, and the detergent acts as a surfactant so that flies sink to the bottom of the liquid. Put the lid on the cup and hang the trap from a wire threaded through holes at the top of the cup.
Kathy Demchak, senior extension associate at Penn State University, hands out materials for making traps to monitor for the presence of spotted wing drosophila.
Photo by Sally Colby.
"They [the SWD] seem to like cooler conditions," said Demchak. "For strawberries, put the trap on the ground, and for tree fruit, put the traps in the middle of the tree or on the shady side of the rows." Traps should be checked at least weekly, and for best monitoring results, replace the vinegar each time the trap is checked. Used vinegar should be disposed of away from the trap site so that it doesn't attract more flies.
SWD larvae, which are the main cause of damage to soft fruit, may be present in fruit before adults are discovered. To create a trap for larvae, place some mashed fruit in a small ziplock bag and add a small amount (to fill half the bag) of salt solution. The fruit will attract the flies, and the salt solution, made with .25 cup of salt in 4 cups of water, will cause the larvae to float on the surface. If 1/8-inch long white larvae are found in fruit that is close to ripening or ripe, it's likely that they are SWD larvae.
Because of its size, the SWD might be difficult to identify with a hand lens. Demchak suggested that growers capture fruit flies in traps and examine them with the help of a digital camera. "Drain the vinegar from the trap, then add some water to the fruit flies and pour them into a white pan. Then you can use a digital camera to take pictures of the flies, upload them to a computer and look at them on a computer screen." Demchak said that this allows a view of the flies that's similar to the view through a dissecting microscope.
Prevention measures include removing as many culls as possible to eliminate feeding areas. Large populations of the SWD have been found in wild fruit hosts such as blackberries, so whenever possible, eliminate these sources. Research shows that certain crops grown in the vicinity of orchards are more likely to harbor the SWD. Late-season feeding preferences include day-neutral strawberries and fall raspberries, as well as melons, especially those that are overripe or are split. Demchak pointed out that even after a crop is harvested, the SWD will still be feeding on decomposing fruit on the ground.
Growers who are successfully keeping the SWD under control are those who have identified the pest early and taken steps to keep populations low. Fall berries, especially wild species growing in wooded areas near orchards, should be controlled.
Fruit flies that may look like the SWD include Scaptomyza, commonly found in decomposing organic matter. Like the SWD, Scaptomyza males have a black wing spot, but the spot is smaller and located at the tip of the wing. Scaptomyza also lack the characteristic black bands that identify the SWD.
Another SWD look-alike is Leucophenga varia, which also has wing spots. The wing spots of Leucophenga are smaller and more difficult to see than those on the SWD.
The SWD is susceptible to organophosphates such as malathion and pyrethroids such as bifenthrin, but pesticide labeling doesn't always allow application of these products. Check labels carefully for allowed uses, and watch for changes in labels that will allow certain chemicals to be used to control the SWD.
Comprehensive, up-to-date fact sheets on the SWD are available online at http://extension.psu.edu/vegetable-fruit/news/2012/spotted-wing-drosophila-fact-sheets-completed-and-online.
The author is a frequent contributor and freelance writer who farms and raises Great Pyrenees in south-central Pennsylvania.