Monitoring Spotted Wing Drosophila (SWD) with Traps

Spotted wing drosophila (SWD) attacks ripening and ripe raspberries, blackberries, blueberries, grapes, cranberries, late cherries, fall strawberries, plums and peaches, plus fruit of many wild hosts. If you wish to protect your fruit from becoming infested, it is critical to set up traps for the insects, and to monitor those traps weekly when you have ripening crop present. The first flies will probably be trapped between July 2 and 12, and numbers will peak in September or October.

These traps ARE NOT TO CONTROL the flies! Once you detect flies, the crop should be protected with insecticides. Most of the good insecticides should give about seven days of protection, but can be washed off by rain. After you spray, set out fresh traps to determine when crops become at risk again.

Commercial vs. Home-Made Traps

When SWD first appeared in NH, the commercial traps that existed were not effective in comparison with home-made traps. However, effective commercial traps and baits are now available from both Scentry and Trécé, that are hung in jar traps full of soapy water as drowning solution.

In our 2018 UNH Cooperative Extension monitoring program, we are now using Trécé traps and lures for SWD. These traps are reusable, and are available from standard suppliers of IPM products (e.g. Great Lakes IPM).

When to set up the traps? Do this as soon as fruit starts to ripen. Fruits that ripen before July 1 are not likely to be at risk. You can stop monitoring when harvest of susceptible crops is over for you.

Where to set the traps? Set the traps IN the crop, in the shade, AMONG THE FOLIAGE near fruit. We recommend checking traps every 5-6 days at first, moving to every 3-5 days in August (hot weather). As the weather cools, you can lengthen the check interval. Keep checking as long as you have ripe fruit to protect.

Which crops need protection? The most susceptible crops seem to be raspberries, blueberries and strawberries that ripen in August-September; thin-skinned, dark-colored grapes; and some peaches and plums (especially white-fleshed peaches). We don’t know how heavily currants and cranberries are attacked. 

How many traps do I need?  For most plantings, two well-placed traps should be enough. For very large plantings, three may be better. If you have different crops, you’ll want traps in each crop, because the pattern of attack varies crop-to-crop. If you have different varieties of the same crop, begin monitoring in the first variety to ripen, and move the traps to others with ripe fruit when harvest wanes on that first variety. If your crop is in several blocks that are managed (e.g. sprayed) separately, you will need traps in each block.

How long do I monitor? Keep checking until harvest of susceptible fruits ends for you.

How do I check the traps?  Check traps at least once per week. To check the trap, remove the lid, and pour the liquid bait into a shallow white pan or saucer. In bright light, look for the male flies (they are about 2mm long, with light tan body, red/orange eyes, and have a dot near the tip of each wing) with a 2X magnifying glass. When done counting the male SWD’s, write the number down. If you find male flies, your crop is at risk. Then collect the old liquid bait in a waste container, and add fresh bait to the trap. Don’t pour the old bait on the ground in your fruit planting, or it will compete with your traps for the flies’ attention.

Making Your Own Traps

From 2012-2016, we and many colleagues across the Northeast set out hundreds of traps, and learned what combinations worked well enough to make trapping a useful tool. Poorly set or designed traps do not work well enough to predict when you need to protect crops. If you prefer to make your own traps rather than purchase traps, the following tips may help.

This trap design works well: We use red plastic 18 oz Solo cups with transparent lids. We placed a black band of electrician’s tape a bit below the rim. We used a heated nail to melt 1/8 inch holes in the cup, to allow the insects to get in, and the odor to escape. We don’t make those holes too large, or wasps will get in. We placed the entrance holes in and around that band, about 30 to 35 holes per trap. We leave one sector of the cup without holes, to make it easy to pour out and examine the liquid bait, without spilling any.

Bait Mixtures for Spotted Wing Drosophila Traps                     

The bottom line is that apple cider vinegar works very well as a bait. It is inexpensive and simple to use. The odor strongly attracts all drosophilid flies, including SWD. Adding other components, such as apple cider or cheap red wine has been shown to improve attractiveness.

Add just a drop or two of liquid soap to the bait. Flowery-scented soaps might decrease bait effectiveness. The soap decreases the surface tension of the bait, making it easier for the small flies to fall in and drown.

In 2012 and 2013, UNH Cooperative Extension used Cowle’s SWD Bait (recipe below), and found that it worked well. In 2013, we also found that using a yeast/ flour bait in a separate container (described below) worked well. After several years of refining our methods, as of 2018, we are now using a commercial bait/drowning fluid called Suzukii Trap, made by BioIberica.

Other Bait Recipes

Cowle’s SWD Bait Solution – 56% grape juice (reconstituted Welch’s Frozen 100% White Grape Juice Concentrate, 37% apple cider vinegar (Hannaford Cider Vinegar), and 6% “95% ethanol”.

 “Superbait” - 88 fl oz water,  32 fl oz cheap red wine, 6 fl oz molasses, 3 fl oz apple cider vinegar, 1 teaspoon unscented dish soap.

Landolt Bait - 51 fl oz apple cider vinegar, 77 fl oz red wine

Baker’s blended bait - 90 fl oz white vinegar, 2 lbs sugar, 1 fl oz phenyl ethanol, 2 teaspoons lab ethanol, 1 small drop Fly nap (triethylamine).

Yeast/Dough Baits - Active fermentation is very attractive to SWD, and as a result, many of our early trapping efforts involved yeast and flour combinations.  Because the baits containing yeast and/or bread dough make a slimy mess, and are somewhat difficult to manage, we have stopped using these methods, even though they were very effective. If using yeast/flour baits, they must be made fresh every week.

When we used these baits, the solid yeast/dough bait was placed inside a smaller 4-oz lidded cup within the larger 18-oz trap. We cut a hole (1-inch diameter) in the lid. Over the hole we placed fine insect netting, and fastened it with a hot glue gun. The netting is to allow the yeast odor out, but not let flies in. So the assembled trap is a large red cup. Inside that is 2 oz of liquid bait, and floating upright in the liquid bait is the smaller cup with the netted lid. 

Yeast bait - 2 teaspoons active dried yeast,  1 teaspoon sugar,  2 cups water. Mix and let stand overnight.

Whole wheat bread dough - 12 fl oz water, 2 tablespoons vegetable oil, 1 ½ teaspoons salt, 2 tablespoons sugar, 4 cups whole wheat flour, 2 teaspoons active dry yeast.