Keep an eye out for spotted wing drosophila

By Denise Ruttan

Trap to monitor for the invasive Asian vinegar fly known as spotted wing Drosophila. Photo by Amy Dreves

As backyard fruit ripens, it's time for gardeners to trap spotted wing drosophila. A new, easy-to-make trap design, based on recent research, will help gardeners monitor the invasive vinegar fly's presence in their backyards.

The tiny spotted wing drosophila resembles other small fruit flies but is distinctive for its yellowish-brown body, red eyes and dark unbroken bands encircling its abdomen.

This tenacious invasive pest lays its eggs in ripening stone fruits and berries.

Amy Dreves, a researcher for Oregon State University's College of Agricultural Sciences and entomologist for the OSU Extension Service, is investigating this Asian fly that was first identified in Oregon in 2009.

"Our research is very much detective work because this invasive pest is so new in our area and still has a lot of unknowns," said Dreves. "There is a lot to understand about its biology and behavior, where it likes to hide during the winter months and during which fruit stage it lays eggs. This information is important in order to develop effective tools for managing it."

OSU researchers are working with organic and conventional fruit growers in Oregon to help test new trap designs and attractive baits on farms. Among their findings: traps with bigger entrances and a red color catch more flies.

Gardeners can use this research, too.

A trap can tell gardeners if the flies are present in their yard, and it can help reduce the numbers of the pest, according to Dreves. Now is still a good time to set up traps, as lots of fruit is harvested in August and September. But next year start monitoring for the fly in early April.

To create a homemade monitoring trap, use a red 18-ounce plastic cup with a lid. Drill several holes, each about three-sixteenth inch wide, around the side of the upper third of the cup. Another option is to glue a rectangular piece of plastic canvas mesh to the side of an open area cut into the cup measuring about 1.5 inches by 4 inches.

Then, fill the cup with 1.5 inches of pure apple cider vinegar and a drop of non-fragrant liquid soap.

Data have shown that the adult fly is also drawn to a yeast solution. Instead of the vinegar, you could make a solution with one tablespoon active dry baker's yeast, four tablespoons sugar and 12 ounces water.

Attach a heavy-coated wire, strap, or durable twist tie to the top of the cup. Hang it from the shady, cooler side of the plant near the level of the fruit to capture adults. Refresh the bait once per week. Improved trap designs and baits are also under investigation.

To scout for larvae in the fruit, collect suspect fruit and crush it in a sealable plastic bag. Prepare a solution of salt or light brown sugar-water one day in advance and pour it in the plastic bag. Dissolve one cup salt or 2.5 cups brown sugar per one gallon of warm water. Shake lightly and observe. Small, white legless larvae should float to the top, if present. Good lighting is important.

Dreves also advises against adding infested fruit to your compost pile. You can, however, place infested fruit in a shallow hole in the ground, then cover and seal it tightly with a clear plastic sheet. The heat of the sun will kill the larvae inside the fruit.

Other methods of managing the fly include picking fruit often as soon as it ripens, keeping it cold, and immediately cleaning up fruit that is left on the plant or has fallen on the ground. Dreves recommends these practices to help keep populations of the fly from growing.  

"It's not a quick fix with spotted wing drosophila. A lot of people want a silver bullet. But it will take a combination of strategies to manage this challenging pest," Dreves said.

For the latest updates and resources, check out OSU's website at http://spottedwing.com or the OSU guide, A New Pest Attacking Healthy Ripening Fruit in Oregon.