Spotted-wing drosophila (SWD; Drosophila suzukii; Family: Drosophilidae) is a key pest that targets a wide variety of susceptible fruits including tree stone fruits (e.g., cherries) and berries (e.g., strawberry, blueberry, raspberry, blackberry, and wine grapes). SWD is widespread throughout all the important production regions in the U.S., Europe and South America and originates from Asia.
Pest description, fruit damage, and life cycle Adult SWD flies resemble common vinegar (pomace) flies and sometimes are referred to as fruit flies. SWD can damage ripe-to-overripe fruit by depositing eggs directly beneath the surface. Larvae develop from these eggs, resulting in unmarketable fruit. Drosophila suzukii can survive on a range of non-crop hosts in surrounding vegetation, which makes control more difficult.
Identification At 0.125 inch (2-3 mm) in length, SWD flies can be confused with common vinegar flies. Because SWD and vinegar flies are closely related, identification can be difficult. Both have red eyes and a yellowish-brown amber colored body. Two key characteristics distinguish SWD from other common vinegar flies: 1) a black spot (sometimes dark, sometimes faded) near the leading top edge of adult male wings (females do not have wing spots); 2) the prominent saw-like ovipositor (used to insert eggs into fruit) on the female’s posterior end; and 3) the pupae of both sexes have two respiratory horns sticking out of the anterior end; each horn with a complete whorl of 6-8 fingers/points. Several dark, continuous bands are visible around the abdomen of both the male and female. A hand lens and good lighting are useful for seeing these characteristics. Note: if the female gets caught in a liquid bait (e.g., apple cider vinegar or yeast) within a monitoring trap, her ovipositor may extrude from the body (because of the liquid), thus aiding in the identification of female SWD. See references below.
Symptoms from damage To recognize SWD damage, look very closely for scarring or spotting on the fruit surface; liquid exuding (when squeezed) out of scar/hole where eggs were laid; softening, collapsing and/or bruising of fruit at damage site; small white larvae and pupae that can be seen with naked eye if fruit is opened; and, under a microscope, two hair-like filaments sticking out of fruit where they are connected to a white egg within the fruit can sometimes be seen. See OSU Extension Bulletin EM 9021, Recognize Fruit Damage from Spotted Wing Drosophila (https://catalog.extension.oregonstate.edu/em9021).
Life cycle A single female can lay several hundred eggs. Adults can live 3-4 weeks and several months in summer and winter respectively. Larvae feed inside the fruit for about 5 to 7 days, until they are ready to pupate. Most larvae pupate outside of fruit. Non-feeding pupae are brownish yellow with star-like (6 to 8 points) respiratory horns at one end. Pupation lasts for ~3-10 days and emerging adult flies will mate, producing additional offspring. Adult SWD use alternative hosts in surrounding vegetation to survive the winter period. Several generations commonly occur in Oregon, depending on environmental conditions during the cropping season.
Sampling
Higher numbers of SWD are captured by traps, which have an increased fly entry area on the trap side (mesh screen or many small holes), large bait surface area, and small headspace (space between the bait surface and the exit holes).
To construct a simple homemade trap, use any plastic container (18 oz to 32 oz) with a removable lid. Drill small holes (0.188 inch to deter larger insects) around the middle of the cup to allow SWD to enter. Leave an undrilled space on one side to change the liquid bait solution easier and without spillage. Add a liquid bait such as apple cider vinegar to the cup so that the liquid bait is two inches from the bottom. Add a drop of liquid soap to the vinegar to break the surface tension of the liquid so that captured flies sink more easily. An alternative bait that catches even more flies can be made from a mixture of yeast, sugar and water (2 teaspoons of dry baker’s yeast, 4 teaspoons of sugar, and 1.5 cups of warm water).
Effective traps and lures (e.g., Scentry and Tréce) are commercially available. Attach a wire or string to hang the trap from a plant or stake within the shaded side and at the fruiting level in the plant canopy. Select trap locations where flies are likely to be intercepted (e.g., field or garden edge within the shaded portion of the canopy; near adjacent non-crop plants; near fence borders shared with other fruit growers). For strawberries, secure a trap on the ground within the plants. It is recommended that traps be strategically placed on borders of susceptible plants before the fruit starts to ripen and darken in color. More traps increase the likelihood of intercepting SWD, thus a minimum of three traps should be placed to increase chances of SWD detection. Check and refresh traps once a week. To see SWD in a trap, hold trap in the light and look for flies with a dark spot on the male’s wings. It is difficult to confirm SWD and see the female’s ovipositor in a cloudy yeast bait without a microscope. For a closer look, filter the contents from your traps using a strainer into a pan with a solid white background. Spread the flies out (using a small paintbrush or tweezers) and examine contents with a magnifying lens or under a microscope. Numerous species of Drosophila and other insects will also be attracted to these traps, especially if the yeast bait is used. Damage thresholds have not yet been established for this pest.
Fruit inspection
Several methods have shown positive results for recovering larvae by using a salt or sugar solution over crushed fruit. Collect ripening fruits in a plastic bag, which are suspected to be infested with SWD. See “A quick, 7-step guide for detecting larvae in fruit” (OSU Extension Bulletin, EM 9097) and “A Detailed Guide for Testing Fruit for the Presence of Spotted Wing Drosophila (SWD) Larvae” (OSU Extension Bulletin, EM 9096).
Preparation of extraction solution: Dissolve 1 cup plain salt in 1-gallon warm water (10 BRIX); –or– 2.5 cups of brown sugar in 1-gallon water (16 BRIX). Solutions must be thoroughly dissolved to help larvae float on top of solution for easy viewing. Prepare the solution in advance, if possible.
Extraction method: Place a layer of crushed fruit in a shallow white pan. Pour solution (salt or sugar) over crushed fruit. A good proportion of larvae will exit the fruit after a few minutes, looking for air at the top of the liquid. A majority will float to surface, unless stuck in or under pulp. Wait for at least 15 minutes to get a majority of larvae out of infested fruit. Look for moving white larvae on the surface of liquid. Eventually, however, they will die and sink to the bottom of the pan. The sugar solute will keep them alive longer than salt. Do not mistake SWD larvae for plant parts, fruitworm, thrips, aphid skins, other Drosophila larvae, etc.
Fruit dunk bag method: Place suspect fruit in a large sealable plastic bag. Crush fruit by using a rolling pin over bag or squeeze/crush fruit with hands through bag. Add solution to the bag of crushed fruit. Shake bag lightly to promote penetration of solution into fruit. If fruits are infested, white SWD larvae will float to the top, and fruit should settle on the bottom (note: some fruit floats, depending on fruit weight/amount and sugar levels in fruit). It may take over 15 minutes or so for larvae to float and fruit to separate. If larvae are small, a hand lens or scope and good lighting are useful to see their presence. Hold the clear bag in light and small larvae may be seen moving and floating among fruit.
Management—cultural and physical methods
Harvest in a timely manner—Pick fruit at regular intervals to prevent egg-laying opportunities and SWD infestations. Avoid leaving overripe fruit to hang on host plant.
Clean up infested fruit—To avoid SWD populations from increasing, clean up overripe hanging, fallen and SWD-infested fruit.
Create a barrier—If a monitoring program detects SWD, cover fruiting clusters (e.g., blueberries) or entire fruiting plants (e.g., caneberries) with a fine netting (less than 1 mm in size if feasible) to reduce egg-laying. Use weed fabric, and control weeds as they provide a more suitable habitat for SWD. Weed fabric reduces survival of pupating larvae.
Protect fruit from damage—Protect fruit from rain and sun to reduce fruit splitting and to improve fruit quality. SWD can be attracted to damaged fruit.
Select early season fruit cultivars to reduce SWD pressure.
Non-crop hosts from areas surrounding your fruiting crop may be an issue—Potential perimeter, wildland and backyard uncultivated plants used by SWD may include berries from: dogwood, elderberry, Himalayan blackberry, laurel, sweet box (Sarcococca spp.), flowering cherry, honeysuckle, and dozens of other fruiting species. See OSU Extension publication EM9096: Noncrop Host Plants of Spotted Wing Drosophila in North America. (https://catalog.extension.oregonstate.edu/em9096). Some of these fruits may not be affected by SWD under certain environmental conditions or because of specific management practices being used.
Canopy and irrigation management— SWD prefer shady and humid habitats. Maintain an open and aerated plant canopy that is less attractive to SWD adults and minimize leaky irrigation lines and overhead irrigation. Use drip irrigation, as it increases temperature and reduces water availability to adult flies. Drip irrigation also results in increased host feeding of pupal parasitoids, ultimately resulting in higher mortality of SWD.
Cooling fruit—Chill fruit (less than 34°F) immediately after harvest for extended time periods (greater than 4-8 days), retaining fruit quality to slow or kill eggs and young larvae.
Management—biological control
Research is underway to determine the specific predators and parasitoids (wasps) that attack SWD larvae and pupae. Field observations suggest that ants, spiders, predaceous bugs (e.g., minute pirate bugs, big-eyed bugs), yellow jackets, lacewing larvae and parasitoid wasps may be important biological control agents.
Management—chemical control
Chemical controls should be coupled with monitoring efforts. Rotate chemical families to avoid resistance and follow the label for each crop. See specific fruit for recommended chemicals.
Pesticide families that help control SWD include: spinosyns, pyrethroids, carbamates, and organophosphates. These chemicals kill SWD adults, and some may have an effect on larvae that are developing within and are protected by the fruit. Follow the label for appropriate rates and risks. Do not apply when bees and other pollinators are present, such as when plants are flowering or when pollinators are active.
For further information:
Mermer, S., L. Brewer, D. Dalton, R. Nieri, K. Park, F. Pfab, M. V. Rossi-Stacconi, and V. Walton. 2019. Improved Chemical Control Strategies for Spotted-wing Drosophila. Oregon State University Extension Service EM 9265.
Mermer, S., G. A. Hoheisel, H. Y. Bahlol, L. Khot, D. Rendon, L. Brewer, D. Dalton, R. Nieri, K. Park, F. Pfab, M. V. Rossi-Stacconi, and V. Walton. 2019. Optimizing Chemical Control of Spotted-wing Drosophila. Oregon State University Extension Service EM 9266.
Rendon, D., S. Mermer, L. Brewer, D. Dalton, C. B. D. Silva, J. Lee, R. Nieri, K. Park, F. Pfab, G. Tait, N. Wiman, and V. Walton. 2019. Cultural Control Strategies to Manage Spotted-wing Drosophila. Oregon State University Extension Service EM 9262.
Rossi-Stacconi, M. V., L. Brewer, D. Dalton, J. Lee, R. Nieri, K. Park, F. Pfab, G. Tait, and V. Walton. 2019. Host Range and Characteristics Affecting Fruit Susceptibility to Spotted-wing Drosophila. Oregon State University Extension Service EM 9263.
Rossi-Stacconi, M. V., L. Brewer, B. Miller, D. Dalton, J. Lee, K. Park, F. Pfab, V. Walton, and C. B. D. Silva. 2019. Biocontrol of Spotted-wing Drosophila. Oregon State University Extension Service EM 9229.
Silva, C. B. D., B. E. Price, D. Dalton, D. Rendon, K. Park, L. Brewer, V. Walton, and M. V. Rossi-Stacconi. 2019. Potential Impacts of Irrigation and Biocontrol on Spotted-wing Drosophila Populations. Oregon State University Extension Service EM 9268.
Tait, G., D. Rendon, L. Brewer, D. Dalton, J. Lee, R. Nieri, K. Park, F. Pfab, M. V. Rossi-Stacconi, and V. Walton. 2019. Noncrop Host Plants Used By Spotted-wing Drosophila. 3.
Tait, G., M. V. Rossi-Stacconi, B. Miller, D. Dalton, J. Lee, K. Park, V. Walton, T. Peerbolt, and L. Brewer. n.d. Monitoring Techniques for Spotted-wing Drosophila. Oregon State University Extension Service EM 9267.
Walton, V., L. Brewer, D. Dalton, S. Tochen, R. Nieri, K. Park, F. Pfab, D. Rendon, G. Tait, N. Wiman, and M. V. Rossi. 2019. How Seasons Affect Population Structure, Behavior and Risk on Spotted-wing Drosophila. Oregon State University Extension Service EM 9261.