After watching and waiting for the past few years, the spotted wing drosophila (SWD), an invasive pest found in neighboring states, has been confirmed in Iowa.
The spotted wing drosophila (Drosophila suzukii) is an invasive insect pest that attacks fresh fruits and is fast becoming a problem throughout the U.S. Drosophila flies, sometimes called vinegar flies, are familiar to producers and homeowners. The household fruit fly is commonly found on or near overripe, damaged and fermenting fruits and vegetables. The new, invasive species is closely related but behaves very differently.
SWD causes damage when the female flies cut a slit and lay eggs in healthy, undamaged fruit, particularly thin-skinned fruits such as raspberries, blackberries, blueberries, cherries, strawberries and grapes. Larvae emerge inside the fruit and begin to feed causing collapse and eventual decay and complete destruction of the fruit.
SWD is native to Asia and has been in Hawaii since the 1980s. It was found in the mainland U.S. in California in 2008 and quickly spread to Oregon, Washington and Canada. It is now widely established in North America with confirmed detections in at least 20 states, including neighboring Wisconsin, Illinois and Minnesota. SWD has been detected along the Atlantic coast from Maine to Florida and in New York, Ohio and Michigan. It appears that SWD is here to stay and has spread very quickly. Human-assisted transport is the likely cause of the recent rapid spread. The small flies cannot fly very far and natural dispersion in unlikely.
SWD is approximately one-eighth inch long and resembles the common household vinegar flies with bright red eyes, a pale brown body, dark horizontal stripes on the abdomen and two transparent wings. Distinguishing SWD from the normal fruit fly (Drosophila melanogaster) requires magnification with a hand lens or microscope. The adult males of SWD have a dark spot along the front edge of the wing near the wingtip. The females have an extended ovipositor (egg laying organ) on the end of the abdomen. The ovipositor has two rows of dark, serrated, saw-like teeth that allow this female to saw open and lay eggs in healthy fruit.
For more on the appearance and biology of SWD see the North Central IPM Pest Alert.
SWD will cause big headaches for fruit growers and home gardeners, especially producers of raspberries, strawberries, plums and other soft skin fruits. Producers in infested states have already been seriously impacted with maggots in the fruit. Some producers have suffered a complete loss of their crop. Management options include the following:
- Use good sanitation to prevent spread and further establishment of SWD.
- Pick all fruit when harvesting and remove and destroy any fallen, damaged and overripe fruit.
- There are insecticides available but options will be limited and reapplication at weekly or bi-weekly intervals may be needed.
- At this point do not treat until SWD has been confirmed in your area.
It is likely that SWD is already spread far beyond the reported incidence (see map). Your help in tracking its distribution in the state will be appreciated. Please report your observations and submit suspected specimens to the Plant and Insect Diagnostic Clinic.
SWD can be monitored by hanging apple cider vinegar-baited traps. Instructions for building and using traps for monitoring SWD in fruit plantings can be found at Michigan State University's website.
Spotted wing drosophila male showing the characteristic spot along the wing tip. Photo by Laura Jesse.
SWD female's saw-like ovipositor (egg laying organ). Photo by Laura Jesse.