Insect of the Week: Spotted Wing Drosophila
Spotted Wing Drosophila (SWD) is a small vinegar fly which has the potential to damage many fruit crops. This insect is a pest of most berry crops, cherries, grapes and other fruits, with a preference for softer-fleshed fruit. The Spotted Wing Drosophila was first discovered in the western United States in 2008 and moved quickly through the Pacific Northwest. It was first detected in Michigan in 2010 and has quickly expanded its range across the United States. The fly is rather small and cannot fly very far, it is assumed that human-assisted transportation rather than natural dispersion is the most likely cause of the rapid spread of this insect.
Spotted Wing Drosophila appear similar to other vinegar flies. Adult flies are 2-3 mm in length, with red eyes, tan colored body with darker bands on the abdomen. Males have characteristic single spots at the leading edge of the tip of the wing and two dark spots on the front legs. Females lack wing spots and leg spots, but are distinguished by a robust, saw-toothed ovipositor. Larvae are white, nondescript and legless. Unlike most other vinegar flies that require damaged fruit to attack, SWD causes damage when the female flies cut a slit into healthy fruit. The female fly accomplishes this task by using a serrated ovipositor to inject eggs under the skin. As a result, the larvae of the SWD can be present during ripening, which may allow the larvae to be detected in ripe fruit after harvest. During the egg laying process, fugal pathogens may be introduced, which will fruit quality.
The adult SWD lives for about two weeks, with the female having the potential to lay more than 300 eggs. The life cycle is very short, they can go from egg to adult in as little as 8 days.
Monitoring for this insect if very important, SWD traps should be present before fruit starts to ripen and become susceptible. A simple trap can be made using a clear plastic 32 oz. cup with several 3/16” diameter holes around the sides of the cup, leaving a 3” to 4” section without holes. The traps should be baited with apple cider vinegar, add about 1” to 2” inchesof bait per trap. To insure that trapped flies do not escape, a small yellow sticky trap can be placed inside the trap. Traps should be monitored weekly for SWD flies, and the bait should be replaced with fresh liquid. Continue to monitor the traps through harvest and post-harvest.
Some native species of vinegar flies and other insects may be attracted to the traps. For positive identification of SWD, please contact your local Extension Educator.