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The spotted wing drosophila is easy to catch using cheap and readily available baits.

Apple cider vinegar is being recommended for homemade traps in the Pacific Northwest. A cheap red wine can be used, also.

But what is it about wine or vinegar that the flies find so attractive?

This is a question that Dr. Peter Landolt, research leader at the U.S. Department of Agriculture’s laboratory in Yakima, Washington, hopes to answer so that more powerful and consistent baits can be developed.

The spotted wing drosophila (Drosophila suzukii) originated in Japan and was first seen in California in 2008. It has since been found in a wide range of crops on the West Coast, with the largest populations in coastal areas. The spotted wing drosophila is threatening to orchardists because, unlike the common vinegar fly Drosophila melanogaster, it attacks fruit on the tree before it is fully mature. The fly was found in eastern Washington orchards in 2010. To monitor for the pest, growers have been using either commercial fruit fly traps or homemade traps containing either vinegar or wine, all of which are effective.

Acetic acid is the main ingredient in vinegar, while ethanol is the main chemical in wine. Previous studies that Landolt did with houseflies showed that acetic acid and ethanol are strongly synergistic—meaning that a combination of the two is far more attractive to the flies than either product alone.

Landolt is cooperating in his research with Dr. ­Helmuth Rogg, supervisor of the insect pest prevention and management program at Oregon State Department of Agriculture in Salem, where the spotted wing drosophila is established in high numbers. Rogg is doing the field tests of attractants, while Landolt’s lab is ­analyzing their chemical composition.

Although the spotted wing drosophila showed up in parts of Washington last season, Landolt said he doesn’t yet consider it established in Yakima. “So I’ve been a little reluctant to bring flies into the lab with the potential to escape before the fly is here on its own,” he said. “I need people to do field work where the numbers are high enough to get good data.”

The Salem area in western Oregon area fits the bill. Last season, Rogg and colleague Todd Adams set out the vinegar and wine traps in an area near orchards, blue­berries, and wild blackberries, and were finding as many as 15,000 flies per trap as late as October. The flies are active over a long period of time in that area. By March this year, large numbers of flies were active again, and the entomologists are continuing the field trials.

“It’s a huge advantage to me when we can work for so many months of the year and get a quick turnover to the experiments,” Landolt said.

The field trapping confirmed that a combination of acetic acid and vinegar is more attractive to the spotted wing drosophila than either product separately, so a ­mixture of the two would work best.

“The standard protocol was using the apple cider vinegar, but a combination of wine and vinegar is certainly much better than the apple cider vinegar,” Rogg reported.

The experiments also showed that wine and vinegar were several times more attractive to the flies than ethanol and acetic acid, suggesting that there are other components in wine and vinegar that attract the flies.

Landolt is now conducting lab experiments to analyze the odor chemistries of wine and vinegar. The goal is to identify the attractive components so that chemical lures can be developed for trapping flies.

This approach would have several advantages:

First, chemical lures could be used in dry traps, which would require less labor and be easier to handle than traps containing the liquids.

Secondly, chemical lures could have controlled and consistent release rates, whereas there’s a lot of variance in food products.

And thirdly, chemical lures would allow for more powerful traps. This would be of little benefit in areas where the number of flies is high, because there would be no reason to catch more flies, but a more powerful trap could be very important for monitoring the arrival of the pest in areas where there are few flies or for monitoring emergence after the winter when the numbers are still small. In eastern Washington, where populations are likely to be set back by severe winter weather, it might even be possible to use the chemical attractants in an attract-and-kill scenario to control the first flies to emerge in the spring before they’re able to reproduce, Landolt said.