Male Spotted Wing Drosophila.
Spotted Wing Fruit Fly: Pest Profile
The spotted wing fruit fly, also known as Spotted Wing Drosophila (Drosophila suzukii) or SWD is a new pest problem that originated in Asia and was only first identified here in the United States in California in 2008. It was first seen in South Dakota in 2013. As of 2016 it was known to exist in nine counties in South Dakota, but populations of the insect can be quite scattered. It has been found in many types of areas, but particularly where fruit is available for the fruit flies to feed on.
Unlike the much more common vinegar fruit fly, that is attracted to damaged or decaying fruit, this particular fruit fly has the ability to infest healthy fruit. The female has a saw-toothed ovipositor, which she can use to slice into the outer skin of nearly ripe fruit to lay her eggs. Soon the eggs hatch and the tiny white larvae begin to feed inside the fruit, eventually causing the fruit to get mushy and collapse.
Damage to Fruit
Spotted Wing Drosophila like to feed on all types of berries, stone fruits like cherry and plum and some pome fruits like pear. These insects will also feed on the fruit of buckthorn, honeysuckle, hackberry, and mulberry, which often harbor he highest adult counts from summer to fall. Common buckthorn and honeysuckle are very common woody weed species in much of Eastern S.D. so there is a ready food source in many locations. This indicates that late season fruit crops or varieties could be at risk from SWD fruit injury in this area. Such late fruit crops such as fall-bearing raspberry are generally harvested when SWD populations still exist in this region. Traps are used to monitor populations of the adults. The number of captured adult insects is positively related to the amount of damage seen in these fall fruit crops. There is also concern that SWD may also begin feeding on tomatoes if other fruit are not available. Overripe and fallen apples may also provide a food source for overwintering adults.
South Dakota Impact
Ordinarily SWD has not been a problem this early in the growing season but they have been captured in traps in Michigan and Minnesota for the last few weeks already. We received reports of mushy strawberries over the last two weeks and have confirmed SWD in those berries. Historically, we have not seed SWD until the middle to the end of July where they are most often found in raspberry fruit. However, the high temperatures earlier this spring may have pushed development earlier in the season.
Spotted Wing Drosophila in strawberry. 10 miles East of Brookings, S.D.
Raspberry fruit infested with Spotted Wing Drosophila.
Management of SWD is difficult for a number of reasons. However, the first step in management should be monitoring to see if you have SWD in your yard or near your fruit crops.
A simple monitoring trap can be made from a 32 oz. plastic cup with lid. Make several 3/16” to 3/8” holes around the sides of the cup, leaving a 3” to 4” section without holes to facilitate pouring out liquid. Drill the holes in sturdy containers or use a hot wire or wood burner to melt the holes in thinner plastic cups. Pour 1” to 2” of yeast-sugar solution into the trap as bait. Add 1/2 tsp active dry yeast to 2 tsp sugar in 2 oz. of water to make the bait. Add a drop or two of dish detergent so that the flies will drown instead of float. You can also place a small, yellow sticky card inside the trap to help attract flies and ensure that trapped flies do not escape. The card can be hung on a small, plastic-covered paperclip that is poked through the lid. Rain or irrigation on these traps can enter through the hole in the lid, thereby diluting the bait. Sealing the hole with a small dab of hot glue or duct tape keeps the water out. Premade 3”x 5” yellow sticky cards can be obtained from online gardening supply stores.
Place the traps in areas where the flies most like to spend their time - in the shade (usually the north side of the plant), in the fruit zone, using a stake or a wire attached to the sides of the trap, and fastened to a branch or trellis wire. Place the traps near the ground so they are in the fruit zone for strawberries whereas for bush fruit or tree fruit, place the trap up in the plant. Traps are most likely to catch flies if placed in areas that remain in the shade or have ripe fruit remaining on the plants. Check traps weekly to determine the presence of the first SWD flies. The black wing spots on the males are relatively easy to see with a hand lens or magnifying glass. Change bait every 1-2 weeks and the yellow sticky card weekly. Dispose of the bait away from the trap location. When you find the first SWD, you will need to begin your management program for this pest.
One of the most important practices for managing SWD in the home garden is sanitation. Pick fruit as soon as it is ripe, and clean up and dispose of overripe or rotted fruit by sealing it in plastic bags and putting it in the trash. Don’t allow it to remain on the ground or on the bushes, to contribute to building the populations of SWD. It is not a good idea to put this damaged or overripe fruit in the compost pile either. Keep picked fruit stored in the refrigerator. Refrigeration will stop further development of larvae or eggs if they are present. Freezing fruit if it can’t be used within a day or so of picking will kill any larvae or eggs in the fruit. Small plantings may be protected with fine netting to exclude SWD using 1 mm (1/32 inch) mesh. Wait until pollination has taken place to apply the netting so that bees and other pollinators have access to the blossoms.
Several insecticides that are effective on SWD are labeled for use on fruits in home garden plantings. Before applying a pesticide, always read and follow all directions on the pesticide’s label. The crop (e.g. blueberry, strawberry, raspberry, cherry) must be listed on the label in order to legally use the product on it. Be sure to check the label and follow any restrictions on how long you must wait after you apply the insecticide until harvesting the fruit. Use protectant insecticide treatments starting when fruit first begins to color and continuing to harvest, according to the label instructions. They will protect fruit from infestation, but will need to be applied before the eggs are laid in the fruit. Choose the most effective insecticides, when known, and check with local Extension staff to learn about the available options. Alternating the use of insecticides with different active ingredients will reduce the chance of insecticide resistance developing in SWD. Do not apply any insecticides during bloom or when bees are active. The most effective products for use against SWD in home fruit plantings contain the active ingredient spinosad or malathion. Pyrethrins, insecticidal soap, neem oil and horticultural oils have very low and short-lived activity against SWD.
Finally, getting rid of non-crop, fruit producing like weedy common buckthorn and honeysuckle bushes from your yard and nearby shelterbelts can help to reduce the prevalence of this pest. This can be a difficult task however, since many shelterbelts and even home landscapes are heavily infested with these species.
- Spotted Wing Drosophila, MSU
- Spotted Wing Drosophila Management in Home Fruit Plantings, MSU
- Spotted Wing Fruit Fly, SDSU Extension
- Common Buckthorn, SDSU Extension
- Tatarian Honeysuckle, SDSU Extension