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Spotted wing drosophila fly detected in Iowa

By Staff | Sep 15, 2012

AMES (ISU) – The Iowa State University Plant and Insect Diagnostic Clinic confirmed on Sept. 6 that the spotted wing drosophila, an invasive pest that can cause damage to many fruit crops, has been found in Story County.

“This is the first confirmed SWD infestation in Iowa,” said Laura Jesse, entomologist with diagnostic clinic. “Previous specimens had been collected in Dubuque County that appeared to originate in fruit brought from another state.”

The spotted wing drosophila attacks fresh fruits and is fast becoming a problem throughout the U.S., Jesse said. 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 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.

“Producers of raspberries, strawberries, plums and other soft skin fruits in neighboring states have been seriously impacted with maggots in the fruit,” Jesse said. “Some producers are facing a complete loss of their crop this year.”

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, she said. 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 requires magnification with a hand lens or microscope, Jesse said. The adult SWD males have a dark spot along the front edge of the wing near the wingtip. The females have an extended ovipositor, or egg-layimng 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.

“Once established this new pest will cause big headaches for fruit growers and home gardeners,” said Donald Lewis, an entomologist with ISU Extension. “Management options are being refined and will be shared at a later date.”

Until then, Lewis said, growers should consider:

  • 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 heir areaa.

Though this is the first confirmed infestation of SWD in Iowa, it is likely that the insect is already established elsewhere in the state.

“We are interested in documenting the presence of SWD in Iowa,” Lewis said. “Growers’ help in tracking its distribution in the state will be appreciated.”

To report an observation, or to submit suspected specimens to the Plant and Insect Diagnostic Clinic, visit www.ent.iastate.edu/pidc/.

Lewis said property owners can make simple, vinegar-baited traps for surveying their property.

Instructions for building and using traps for monitoring SWD in fruit plantings can be found at www.ipm.msu.edu/SWD/SWD-monitor.htm.

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