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Invasive and damaging Spotted Wing Drosophila (SWD) flies detected; pose threat to berry crops

Minnesota growers need to monitor maturing crops closely

July 16, 2013

University of Minnesota and Minnesota Department of Agriculture (MDA) Entomologists Mark Asplen and Mark Abrahamson have detected and confirmed the presence of Spotted Wing Drosophila (SWD) flies in Minnesota fruit fields this year.

SWD flies are invasive insect pests that can damage berry crops and officials are encouraging growers to take action to manage the pests as soon as they are detected.

 "We strongly encourage growers of SWD-susceptible fruits (e.g., raspberries, strawberries, grapes, blackberries, blueberries, cherries, and plums) to monitor for SWD activity and begin management practices upon detection," said Bill Hutchison, professor and Extension Entomologist.

The first confirmed report of SWD this growing season happened on June 27 with the capture of a single male SWD in pre-fruiting grape vines in Dakota County.  One week later, on July 3, entomologists used adult trapping and larval detection to verify SWD’s presence in summer raspberries in Rice County. The pest could be present in other metro-area counties as well. Further information on that could be available later this week.

SWD looks similar to small fruit flies found on overripe bananas. However, unlike these other flies, which typically feed on overripe or deteriorating fruit, SWD feeds on healthy, intact, ripening fruit. Often the symptoms won’t show until after the fruit are harvested, possibly not even until the fruit are in possession of consumers. In addition to the damage caused directly by the larvae, the feeding makes the fruit susceptible to infestation by other insects, rot fungi and bacteria. The larvae will then leave the fruit to pupate and later emerge as adults.

"SWD was first officially detected in Minnesota last August by the MDA, and the pest was a real challenge for our berry growers, statewide," Hutchison said. "Everywhere we looked last fall, either flies or maggots (in fruit) were detected – a total of 29 counties were positive last year."

The entomology department was able to secure emergency funding from the U of M’s legislatively-funded Rapid Agricultural Response Fund and hire Asplen, a post-doctorate researcher to work on SWD. "As the most recent invasive insect pest in the state, very little is known about its biology, host range and pest management options," Hutchison said. 

Asplen encourages growers to use this three-pronged approach to manage this pest effectively:

•Use yeast-based or apple cider-vinegar traps for early detection of adult flies.

•Implement clean and timely harvests to avoid leaving over-ripe fruit or fallen fruit anywhere on the farm site.

•Use insecticidal sprays appropriately as needed.

"An Integrated Pest Management (IPM) approach is truly needed for this pest, as the damage potential and economic risk to growers is very high," Asplen said.

The best source of IPM information on SWD in Minnesota is the U of M’s "VegEdge" web page (for fruit and vegetable growers) at  http://z.umn.edu/swd. Home gardeners can find information at http://z.umn.edu/swdgarden.

The Department of Entomology in the university’s College of Food, Agricultural and Natural Resource Sciences (CFANS) and the MDA have been conducting SWD monitoring programs statewide since mid-May. These programs will continue into the fall, ending with the harvest of fall raspberries in mid-October.

SWD is a small fly, only 2 to 3 mm long, with yellowish brown coloration and prominent red eyes. Male SWD have dark spots on the wing tips. SWD larvae are white with a cylindrical body that tapers on both ends. The adult flies are difficult to distinguish from other small flies; however, if growers find an abundance of small, white maggots in what were apparently healthy fruits at the time of harvest, contact the Minnesota Department of Agriculture "Arrest the Pest" hotline by email at Arrest.the.Pest@state.mn.us or leave a voicemail at 1-888-545-6684. 

Also, a county map of SWD activity is available at the MDA website: http://www.mda.state.mn.us/plants/insects/swd.aspx.

Given the early detections this year, it is possible that SWD may be able to overwinter in Minnesota. SWD comes from a rather broad geographic range across Asia, but researchers need to learn more about what it may do here.

"The pest may be able to overwinter in the state; however, this again is unknown territory…so to speak," Hutchison said. "More research is needed and we are initiating that this fall."

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