Monitoring traps for catching spotted wing Drosophila
Using the right bait can help improve your spotted wing Drosophila trap’s sensitivity.
With the first catches of spotted wing Drosophila (SWD) detected in Michigan over the past few weeks, it is important to monitor for this pest to know when it is becoming active in berry fields and cherry orchards. Recent research at Michigan State University and elsewhere can guide how to best monitor for this pest, and there have been various developments on trap design and lures in recent years that should be incorporated into trapping programs. Additionally, trap checkers need to have good identification skills so that SWD can be separated from all the other small flies as well as detecting females that do not have the black spot on the wings. MSU has developed a guide with photographs, titled “Spotted Wing Drosophila Identification Guide.”
Traps and lures can be made at home, or they can be purchased from commercial suppliers, such as Great Lakes IPM supplies traps and lures. MSU staff are using a simple monitoring trap that consists of a plastic, 32 ounce cup with 10 holes 0.1875-0.375 inch in size around the upper side of the cup, leaving a 3-4 inch section without holes to facilitate pouring out the liquid. The holes can be melted into the plastic with a hot wire or soldering iron. The small holes allow access to vinegar flies, but keep out larger flies, moths, bees, etc.
To help ensure that trapped flies do not escape and to facilitate checking traps, a small, yellow sticky trap can be placed inside hung on a paper clip, but this is not necessary. A drop of unscented dish soap in the liquid will ensure flies remain trapped. It is important to have many points for entry by the flies into the trap, and to have holes that are not too big, otherwise lots of other insects will be trapped. Some trap designs include red or black coloration and this will help increase captures, but the bait is the most important component of the trap for getting the flies to be attracted and enter the trap.
For baits, there are some different options. A homemade approach is to use a yeast-sugar mix that ferments and attracts the flies. This trap is made by combining 1 tablespoon of active dry yeast (we use Red Star brand, available online or in stores) with 4 tablespoons of sugar and 12 ounces of water. Although these traps are harder and messier to service, the yeast bait is less expensive.
If you prefer the convenience and cleanliness of a pre-made commercial lure, some of those are now available for SWD. This year, we are using the Scentry pouch lure that is pictured in the photo, and in our 2015 trials in Michigan this lure worked as well as the yeast-sugar mix bait. This Scentry lure is hung in the trap over 1-2 inches of water with unscented dish soap, and the liquid can be checked each week for SWD. The lure should be changed monthly to maintain maximum activity. To prevent the development of molds in the liquid and to help preserve the SWD flies for identification, add 32 grams of borax per gallon of water.
Traps for SWD should be hung in a shaded area in the fruit zone, using a wire attached to the top of the trap. Growers should be sure the trap is clear of vegetation with the holes exposed so that SWD can easily fly in. MSU Extension recommends a minimum of one baited trap for SWD every 5-10 acres, with an additional trap in a wooded field margin if present to see when SWD is becoming active. Traps should be checked for SWD flies once a week at a minimum; the yellow sticky trap and liquid inside of the trap should be observed for SWD, and the SWD captures should be recorded each week in a log book. With this method, you can track the number of male and female SWD as the season progresses.
Once fruit are ripening and SWD flies are present, fruit protection will be needed to minimize the risk of infestation. We typically get a sharp increase in captures in July, and when SWD populations reach high levels, it is a very important time for protection of later blueberry varieties, fall raspberry crops and cherries.
For more information about monitoring and management of SWD in Michigan fruit crops, please visit the MSU Spotted Wing Drosophila website.