Jul 2, 2012
SWD becoming more established in Michigan

The spotted wing drosophila (SWD) is an invasive pest of berries, stone fruit, grapes and some pome fruit crops. It is native to Asia but was detected in North America for the first time in California during 2008, and has spread quickly across the continent. This pest is now found in most of the fruit-producing regions in the nation, and in Europe. Its activity in Michigan during 2012 started even earlier than the previous year, with first captures in early June indicating that this insect is becoming well established.

SWD flies look similar to the many species of small vinegar flies that typically infest fruits and some vegetables in late summer. Unlike other vinegar flies, female SWD have a serrated ovipositor, or egg-laying device, used to cut a slit into the skin of intact fruit to lay their eggs. This makes SWD a more significant pest than the native vinegar flies that require damaged fruit to lay eggs. Larval feeding by SWD causes fruit to collapse and increases the risk of the small white larvae being found at harvest time.

Effective management of SWD consists of these key components: 1) Monitor fields with traps and check them regularly – this is an essential first step. 2) Check trapped flies to determine presence and number of male and female SWD. 3) If SWD is found, apply effective insecticides registered for SWD on that crop. 4) Continue monitoring to evaluate your management program, and respond quickly if needed. 5) If possible, remove leftover fruit to reduce SWD food resources.

Monitor for SWD from fruit set until the end of harvest. This will help identify the start of fly activity, although the most important monitoring period is from fruit coloring until the crop is harvested, when the fruit are susceptible to SWD infestation. Hang traps for SWD in a shaded area of the bush canopy in the fruit zone, using a wire attached to the top of the trap. Make sure the trap is clear of vegetation with the holes exposed, so that SWD can easily fly in.

These flies can be caught using a simple trap consisting of a plastic 32-ounce cup with ten 3/16-inch to 3/8-inch holes around the upper side of the cup, leaving a 3- to 4-inch section without holes to facilitate pouring out of the liquid attractant, or bait. The holes can be drilled in sturdy containers or burned with a hot wire or soldering iron. The small holes allow access to vinegar flies, but keep out larger flies, moths, etc. The traps can be baited with 1 inch of apple cider vinegar. To help ensure that trapped flies do not escape, a small yellow sticky trap can be placed inside, hung on a paper clip that is attached with hot glue to the lid underside. The traps will also work without the yellow sticky insert, but then a drop of unscented dish soap should be added to the vinegar to ensure flies remain trapped in the liquid.

In 2011, we found that SWD traps were more attractive to SWD when baited with a yeast-sugar mix rather than apple cider vinegar. This mix is made by combining 1 tablespoon of active dry yeast (we use Red Star brand) with 4 tablespoons of sugar and 12 ounces of water. This ratio produces a solution that ferments, and the flies are attracted to the odors. More of these traps caught SWD, plus they were found earlier and in greater numbers than when using apple cider vinegar. Although these traps are harder and messier to service, the yeast bait is less expensive than the apple cider vinegar traps, and the benefits of earlier detection are obvious when needing to protect crops from infestation. Traps baited with yeast will collect many flies, so sorting through these traps will take more time.

For blueberry growers in Michigan, we recommend a minimum of one SWD trap every 5 to 10 acres. Traps should be checked for SWD flies once a week at a minimum, by looking on the yellow sticky trap and in the liquid. If you use a yeast trap, checking only the sticky trap can be a way to reduce the amount of time needed to service the trap. At each check, fresh bait should be swapped out and disposed of, away from the trap location.

For more information, visit the MSU IPM website.

By Rufus Isaacs, Michigan State University

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