Monitoring and Managing Spotted Wing Drosophila in Fruit Crops

Spotted wing Drosophila (SWD) (Drosophila suzukii) is an invasive vinegar fly that attacks otherwise healthy, ripening soft-bodied fruits. SWD is native to southeastern Asia and arrived in other countries via overseas trade of infested fruit (Rota-Stabelli et al., 2013).

The first U.S. detection occurred in Hawaii in 1980, and in 2008, it was found in the continental United States in California (Lee et al., 2011). This pest had spread rapidly to all states except Nevada and Arizona by 2016. It was detected in Ohio raspberries in 2011 and in a variety of small fruit and grapes in 2012 (Welty, 2013). Given that SWD is well adapted to temperate climates, is capable of overwintering in many regions—including those where freezing temperatures are common—and has no natural enemies in North America, it has quickly become a prolific pest of the small fruit industry (Werner et al., 2018).

SWD Distribution in Ohio

The initial detection of SWD in Ohio occurred in Van Wert County, in northwestern Ohio, in September 2011. Since 2012, a statewide SWD monitoring network consisting of baited traps has been established to detect adults in raspberries, blackberry, blueberry, grape, peach, and strawberry plantings. The network ranges between 12 and 20 counties per year, with one to several monitoring sites inside each county. The network relies on Extension educators, state specialists, and research station managers to supply trap catch data collected from growers’ fields.

During 2013, in addition to the data from the 20 counties monitoring this pest, unofficial confirmations from 15 other counties showed SWD was present in a much larger area of Ohio. A 2018 map including positive detections and suspected but unconfirmed detections is shown in Figure 1. In 2024, it is likely that SWD can be detected in all 88 Ohio counties.

Why Is SWD a Problem?

Unlike other common fruit and vinegar flies, SWD targets intact fruits while they are still ripening on the plant; most common vinegar flies attack only injured or overripe fruits. Furthermore, SWD attacks a wide variety of cultivated fruit hosts, including cherries, strawberries, raspberries and blackberries, blueberries, grapes, and peaches, as well as a broad array of non-cultivated wild hosts such as honeysuckle, wild grapes, mulberry, elderberry, and buckthorn (Dreves et al., 2015). A single female fly is capable of laying hundreds of eggs in her lifetime, which contributes to rapid population growth and infestation of fruit.

Identification

To the naked eye, SWD looks similar to an ordinary vinegar fly: it is small (less than 4 mm), its body is light yellowish brown, and it has red eyes. However, SWD has some unique features that visually set them apart from other common vinegar flies (Van Timmeren et al., 2013).

• Males have a characteristic spot at the front tip of each wing, which is visible to the naked eye (Figure 2A). While other flies may have spots on their wings, the spots are not in the same position. SWD males also have two hairy, comb-like structures on the first and second segments of the front legs that require magnification to see; other species do not have these leg combs.

• Females lack the black spot at the tip of each wing, but diagnostically feature an enlarged, serrated ovipositor, which is the egg-laying organ that is located at the tip of the abdomen. This feature can only be seen under magnification (Figure 2B). The serrations on the ovipositor of the SWD are darker and sturdier than serrations on other species.

Life Cycle

The entire SWD life cycle is completed in 10–25 days depending on temperature (Figure 3). Female SWD use their serrated ovipositor to puncture and deposit eggs into fruit, and depending on ambient temperatures, eggs hatch within 12 to 72 hours. The newly hatched larvae feed inside the fruit and progress through three successive instars, each larger than the last, over the course of five to seven days (Mann & Stelinski, 2011). The larvae then transform to the pupal stage on either the inside or outside surface of the fruit, and after four to 15 days, a new generation of adults emerges. Adult females are capable of laying eggs as little as one to four days after emerging, and a single female can lay over 350 eggs in her lifetime. Adults typically live 20–30 days, but flies that emerge later in the season live longer and overwinter in the adult stage. All these factors mean a huge infestation can develop quickly.  

Monitoring SWD

Monitoring adult and larval SWD is essential for efficient management. The best method for monitoring adult SWD is the use of baited traps, which consist of two parts: an attractive bait (commercial lure, apple cider vinegar, fermented bait, yeast, sugar, etc.) and a drowning solution (full-strength or diluted apple cider vinegar, or water with dish soap). Vigilant monitoring allows for early detection of SWD in an area, which in turn allows for better planning of insecticide applications. The action threshold is detection of one SWD adult in a trap.

Monitoring traps can be homemade or purchased commercially. Homemade traps are inexpensive and easy to build with deli containers but may begin to break down after exposure to direct sunlight during the summer season. Commercial traps are more expensive but may be used for several seasons with only minor maintenance (Isaacs et al., 2016; Hahn & Wold-Burkness, 2016).

Homemade Trap

• Use a transparent plastic 1-quart cup with a lid to easily see and secure trapped flies (Figure 4).
• Using a soldering iron or paper punch, make several 1/8-inch diameter holes around one side of the container near the top of the cup. This is where the flies enter the trap. If the holes are too big, large numbers of non-target insects may enter the trap. An option is to cover the holes with drywall mesh tape, which reduces the size of the opening, as shown in Figure 4.
• Cover the lower portion of the cup with red duct tape to further attract flies.
• Add 1 inch of full-strength apple cider vinegar (ACV) to the container and add a drop of unscented dish soap to break the surface tension. This solution both attracts the flies and drowns them when they fall in.
• Hang one to two traps on the host plant near flowers or fruit clusters; it is best to place one trap at the edge and another trap in the interior of the field.
• Check traps on a weekly basis by pouring contents through a fine strainer to separate insects from liquid solution (do not dump the old solution in the field), then transfer the contents to a container where captured insects can be taken to be magnified and identified.
• Replace the drowning solution of ACV weekly.

However, traps using ACV as bait exhibit a delay in detecting SWD (about one to two weeks) compared to commercial lures. Depending on the fruit being monitored, these are key factors to consider in choosing your bait and trap.

Pros and Cons

ACV serves as both the bait and drowning solution in homemade traps and offers some benefits:

• inexpensive
• easy to purchase specifically attracts SWD compared to other vinegar flies
• serves as a short-term preservative

Commercial Trap

Several trap options can be purchased online from greatlakesipm.com and other vendors. Although trap designs and baits are continuously being changed and improved, the current recommendation is to use a Scentry trap and Scentry commercial lure. Each Scentry lure lasts approximately one month before it needs to be replaced.

Pros and Cons

Compared to ACV bait, these lures can significantly improve early detection of SWD in fruiting systems by one to two weeks, which is critical for management decisions; however, they can also attract many more non-target insects, which requires additional time during the identification phase of monitoring.

The proper setup and deployment of a commercial Scentry trap for SWD adults has been recorded in this video hosted on the Ohio State University IPM YouTube channel: go.osu.edu/scentrytrap.

OTHER TRAPS AND LURES

In addition to the homemade and commercial traps and lures mentioned above, there are several other trap designs and lures available to monitor for SWD. For any of the traps listed below that require a liquid attractant for the flies, an alternative to ACV attractant and drowning solution can be apple juice concentrate (150 mL), yeast bait (1 tablespoon dry yeast, 4 tablespoons white sugar and 2 cups water) or a mixture of red wine and ACV (ratio of 60:40). The benefits of specific trap and lure combination are detailed below.

Timing of trap placement is dependent on the crop but should start at least two weeks before fruit begins to ripen and color (Demchak, 2016). Collect samples and identify adults at least once a week throughout the season. Monitoring can be halted two weeks after harvest ends (Stanley, 2012). For example, if trapping for SWD in a cherry orchard, place the trap in the orchard in early May and remove the trap after harvest is complete, in late July. Monitoring throughout the season allows you to estimate the efficacy of control treatments such as insecticide spray applications based on the relative number of SWD adults found in the traps.

One straightforward design for monitoring SWD adults that requires no liquid attractant and therefore is easier to maintain, is a yellow sticky trap (Figure 5), which is available for purchase online at sites like Abrico Organics, Great Lakes IPM, Gemplers, and others and in home improvement and department stores. It is deployed in the field by either being hung on wiring or tied to an object such as a post, stake, bush, or tree branch. This trap does not need a lure to attract the flies but relies instead on color. Adding a Scentry or Trece lure to the sticky trap will increase the number of flies caught. Traps need to be changed when the sticky surface is covered with insects, typically every two weeks.

Red sticky traps can be used for SWD trapping as well. The red sticky cards, which may be more attractive to the flies because of their color, have been used in studies where a bait is positioned above the trap hanging on either a branch or wire (Baker, 2022). The same technique of adding a lure can be applied to yellow sticky traps as well. Research comparing both Scentry-lure-baited jar traps and Trece-lure-baited red sticky card traps placed in 20 locations found that 40% of the red sticky cards caught their first SWD adult a week earlier, 45% caught their first SWD adult a week later, and 15% caught their first SWD adult at the same time as the Scentry lure baited jar trap. Based on this study, the baited red sticky traps seem to monitor adult SWD comparable to the sensitivity of the standard Scentry lure baited jar traps, giving homeowners and growers confidence that the window of detection is the same for both types of traps, allowing for proper management once detected.

Identifying male and female flies on the sticky card is more difficult due to not being able to move the insects around without damaging them. The males are easier to identify given the large dark spot on their forewing, but the females are especially difficult to identify by their tiny, serrated ovipositor at the end of the abdomen. To have the best success of identifying these flies on a sticky card, a hand lens or a magnifying glass is recommended.

Another trap design to consider is the Victor Yellow Jacket and Flying Insect Trap, which is not specifically designed for SWD trapping but can trap SWD. When researchers used this trap with standard ACV bait, they found a much higher rate of males compared to females or other species. This finding is interesting but insignificant in terms of management because the threshold for treatment is one adult male or female SWD. Comparisons monitoring how early this trap catches SWD adults when baited with ACV versus other trap designs such as the jar or sticky traps have not been conducted.

Both the Scentry SWD and Victor traps are fairly easy to use and are simple to clean and maintain due to their basic design. They are also reusable for several seasons. Any of the liquid baits listed above can be used with either design. The standard Scentry SWD trap is about the same cost as the Victor Yellow Jacket and Flying Insect trap. Again, these products are available online and in retail home improvement and department stores.

Monitoring SWD Larvae in Fruit

Baited monitoring traps capture only adult SWD, which is critical for determining the start time for a spray schedule. However, for fresh market production, it is important to also monitor fruit for SWD larvae, which helps determine if the spray program is providing adequate control of the pest. Monitoring larvae can be done easily using the saltwater test method. With this method, fruit is simply added to a container with warm salt water and left to sit for 10–15 minutes. Any larvae inside the fruit will try to escape the salt water and float to the top where they can be seen by the naked eye. If many larvae are detected in fruit, this suggests that current protective control measures are not effective. It is prudent to check your spray equipment for plugged nozzles and normal function, and it may be necessary to tighten insecticide applications from a seven- to 10-day schedule to every five days or consider choosing alternative insecticides. Please consult the Midwest Fruit Pest Management Spray Guide 2023–2024 (Beckerman et al., 2022) for additional information and recommendations.

Saltwater Test

1. In a small container, zip-top bag, or plastic dish, combine ¼ cup of salt with 4 cups of warm water; this is a ratio of 1 tablespoon salt to 1 cup warm water.
2. Add fruit for inspection to the bag. Note: 2 cups of small fruit (grapes, blueberries, etc.) is suggested.
3. After 15–30 minutes, check for larvae that have floated to the top of the container (Figure 6).
4. Water containing larvae can be poured over a coffee filter for easier collection and viewing. This is especially helpful for spotting the smallest larvae.

The saltwater test to find SWD larvae has also been recorded in this video hosted on The Ohio State University IPM YouTube channel: go.osu.edu/saltwater.

Managing SWD: Sanitation

Keeping your fruiting system free of attractive food resources can improve SWD population management. Harvests should occur frequently to reduce the buildup of ripe fruit. Also, old overripe fruit not harvested, along with any fruit that fell to the ground, should be removed and destroyed. Some guidelines for destruction are as follows:

• Place culled fruit in a well-sealed plastic bag.
• Leave sealed bags in the sun, and the larvae will die from the heat.
• Sealed bags can also be placed in the trash.
• Do not compost the fruit as larvae might still survive.

• Do not bury infested material unless the burial is at a 24-inch depth. SWD can survive as deep as 18 inches underground.

Managing SWD: Exclusion Netting

Netting or floating row covers that exclude or limit SWD access to fruits can be very effective; however, this may be costly and impractical for large vineyards or fruit plantings. In addition, because nets are opened and closed frequently during harvest, there may still be opportunities for SWD to enter and establish infestations. For smaller systems where netting is practical, 80-gram insect netting is recommended. Care must be taken to introduce pollinators into the crop area under the netting.

Managing SWD: Insecticides

Insecticide application is the most direct way to address an infestation. Follow spraying guidelines for best results:

• Application is recommended upon first detection of a single SWD adult in bait traps.
• Protection by sprays should begin when the fruit begins to ripen and should continue until final harvest.
• Spraying every seven days with insecticides that provide seven-day residual activity is suggested. Insecticide recommendations are listed in Table 1 below. Details about insecticide options for SWD are available in the Midwest Fruit Pest Management Guide 2023-2024. Physical copies of this spray guide can be ordered online for $15, and digital copies of the guide are free at the Purdue Education Store at mdc.itap.purdue.edu.

Insecticides should be rotated during the season to minimize the likelihood of SWD resistance to any particular insecticide mode-of-action group.

Table 1 (click image to download Word document). Insecticide options for control of spotted wing Drosophila on Ohio’s commercial fruit crops (based primarily on trials in OR, WA, CA, MI, NJ, NC, FL).

Additional Resources

Washington State University has a webpage devoted to information on the spotted wing Drosophila at
treefruit.wsu.edu/crop-protection/opm/spotted-wing-drosophila.

References

Baker, C. (2022, February 8). Red sticky card traps for SWD monitoring [Blog post]. Cornell Cooperative Extension.
blogs.cornell.edu/swd1/2022/02/08/red-sticky-card-traps-for-swd-monitoring

Beckerman, J., Bessin, R., Welty, C., Athey, K., Wahle, E., Lewis, D., Long, E., Joshi, N., Guedot, C., Meyer, S., Strang, J., & Gaulthier, N. (2022). Midwest fruit pest management guide 2023-2024. Midwest Fruit Workers Group, Purdue Extension.
ag.purdue.edu/department/hla/extension/_docs/id-465.pdf

Demchak, K. (2016, December 29). Spotted wing drosophila, part 3: Monitoring. Penn State Extension.
extension.psu.edu/spotted-wing-drosophila-part-3-monitoring

Dreves, A. J., Lee, J., Brewer, L., Isaacs, R., Loeb, G., & Thistlewood, H. (2019). Noncrop host plants of spotted wing Drosophila in North America, EM 9113. Oregon State University Extension. extension.oregonstate.edu/catalog/pub/em-9113-noncrop-host-plants-spotted-wing-drosophila-north-america

Hahn, J., Hutchinson, W., Klodd, A., & Wold-Burkness, S. (2016). Spotted wing drosophila in home gardens. University of Minnesota Extension.
extension.umn.edu/yard-and-garden-insects/spotted-wing-drosophila

Isaacs, R., Wilson, J., & Rothwell, N. (2016). Michigan spotted wing Drosophila report for June 28, 2016. Michigan State University Extension.
canr.msu.edu/news/michigan_spotted_wing_drosophila_report_for_june_28_2016

Lee, J. C., Bruck, D. J., Dreves, A. J., Ioriatti, C., Vogt, H., & Baufeld, P. (2011). In Focus: Spotted wing Drosophila, Drosophila suzukii, across perspectives. Pest Management Science, 67(11), 1349–1351.
pubmed.ncbi.nlm.nih.gov/21990168

Mann, R., & Stelinski, L. (2011). Common name: spotted-wing drosophila scientific name: Drosophila suzukii (Matsumura) (Insecta: Diptera: Drosophilidae). Featured Creatures. University of Florida.
entnemdept.ufl.edu/creatures/fruit/flies/drosophila_suzukii.htm

Rota-Stabelli, O., Blaxter, M., & Anfora, G. (2013). Drosophila suzukii. Current Biology, 23(1), R8–9.
pubmed.ncbi.nlm.nih.gov/23305672

Stanley, C. A. (2012, January). Monitoring for spotted wing drosophila in Utah [Drosophila suzukii (Matsumura)]. Utah State University Extension.
extension.usu.edu/pests/research/spotted-wing-drosophila-monitoring

Van Timmeren, S., O’Donnell, K., & Isaacs, R. (2013). Spotted wing Drosophila identification guide. Michigan State University Extension.
canr.msu.edu/resources/spotted-wing-drosophila-identification-guide

Welty, C. (2013). Spotted wing Drosophila: A new pest in Ohio’s fruit crops. The Ohio State University.
ohiograpeweb.cfaes.ohio-state.edu/sites/grapeweb/files/imce/pdf_factsheets/SWD_Ohio_handoutV9%283%29.pdf

Werner, T., & Jaenike, J. (2018). Drosophilids of the Midwest and Northeast. River Campus Libraries, University of Rochester.
hdl.handle.net/1802/34909

Originally written March 4, 2019, by Rosalie Sepesy, Department of Entomology, The Ohio State University; Elizabeth Long, Adjunct Assistant Professor, Department of Entomology, The Ohio State University; Celeste Welty, Professor Emerita, Department of Entomology, The Ohio State University; and Jim Jasinski, Integrated Pest Management Program Coordinator, Ohio State University Extension.