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 most likely arrived in other countries via overseas trade of infested fruit. The first U.S. detection occurred in Hawaii in 1980, and in 2008, it was found in the continental U.S. in California. This pest has since 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.
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.
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 raspberry, 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 mainly 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, suggesting the infestation may be nearly statewide. A current map including positive detections and suspected but unconfirmed detections is shown in Figure 1.
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 over-ripe fruits. Furthermore, SWD will attack 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, buckthorn. A single female fly is capable of laying hundreds of eggs in her lifetime, which contributes to rapid population growth and infestation of fruit.
To the naked eye, SWD looks very 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.
- 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 which require magnification to see; other species do not have these leg combs.
- Females lack the dark 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.
The entire SWD life cycle can be 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 will 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. 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 emerge. 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 of these factors together mean that a huge infestation can develop in a short time.
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.
- Use a clear 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 will 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.
PROS and CONS: Apple cider vinegar serves as both the bait and drowning solution in homemade traps and offers some benefits: it is inexpensive and easy to purchase and appears to be more specific in attracting SWD compared to other vinegar flies. It also serves as a short-term preservative. 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 important factors to consider in choosing your bait and trap.
Several trap options can be purchased online from greatlakesipm.com or 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 OSU IPM YouTube channel: go.osu.edu/scentrytrap.
Monitoring SWD Larvae in Fruit
Baited monitoring traps will 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 to determine whether or not 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 that are inside the fruit will actively try to escape the salt water and, as a result, 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 for additional information and recommendations.
- In a small container or 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.
- Add fruit for inspection to the bag
- Note: 2 cups of small fruit (grapes, blueberries, etc.) is suggested.
- After 15–30 minutes, check for larvae that have floated to the top of the container (Figure 5).
- 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 OSU 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. Additionally, old overripe fruit that has not been harvested, along with any fruit that has fallen 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 burial is at a 24 inch depth. SWD are capable of surviving as deep as 18 inches belowground.
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. Guidelines for spraying are as follows:
- 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. Physical copies of this spray guide can be ordered online for $15, and digital copies of the guide are free: mdc.itap.purdue.edu/newsearch.asp?subCatID=308%20&CatID=12
Insecticides should be rotated during the season to minimize the likelihood of SWD resistance to any particular insecticide mode-of-action group.
|Table 1. 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).|
|Efficacy on SWD||Mode of action group||Product||Residual activity (days)||Pre-harvest interval (PHI)|
|Very effective||5||§ Delegate||5-7||1 day||3 days||X||7 days||7 days||1 day||1 day|
|5||§ Radiant||5-7||X||X||1 day||X||X||X||X|
|28||§ Exirel||5||1 day||3 days||1 day||X||3 days||3 days||3 days|
|3A||! Mustang Max||7-10||1 day||1 day||X||1 day||14 days||14 days||14 days|
|3A||! Brigade||7-10||3 days||1 day||0 days||30 days||X||X||X|
|3A||! Hero||7-10||3 days||1 day||X||30 days||X||X||X|
|3A||! Danitol||7-10||3 days||3 days||2 days||21 days||3 days||3 days||3 days|
|3A||! Asana||7-10||7 days||14 days||X||X||14 days||14 days||14 days|
|3A||! Baythroid||7-10||X||X||X||3 days||7 days||7 days||7 days|
|3A||! Warrior||7-10||X||X||X||X||14 days||14 days||14 days|
|3A||! Pounce||7-10||X||X||X||X||3 days||14 days||X|
|1B||Imidan||7||X||3 days||X||7/14 days||7 days||14 days||7 days|
|1B||!§ Diazinon||7||7 days||5 days||5 days||X||21 days||21 days||21 days|
|1A||! Lannate||3-6||X||3 days||X||X||X||4 days||X|
|Effective||1B||Malathion||5-7||1 day||1 day||3 days||3 days||3 days||7 days||X|
|5||Entrust [OMRI]||3-5||1 day||1 day||1 day||7 days||7 days||1 day||1 day|
|Moderately effective||1A||Sevin||10||7 days||7 days||7 days||7 days||3 days||3 days||3 days|
|4A||§ Assail||1-3||1 day||1 day||1 day||3 days||7 days||7 days||7 days|
|-||Grandevo [OMRI]||1-3?||0 days||0 days||0 days||0 days||0 days||0 days||0 days|
|-||Venerate [OMRI]||1-3?||0 days||0 days||0 days||0 days||0 days||0 days||0 days|
|Slightly effective||3A||Pyganic [OMRI]||1-3||0 days||0 days||0 days||0 days||0 days||0 days||0 days|
|Not effective||4A||Actara||1-3||3 days||3 days||3 days||5 days||14 days||14 days||14 days|
|4A||Admire Pro||1-3||3 days||3 days||7 days||0 days||7 days||0 days||7 days|
|! Restricted-use pesticide.
X means the product is NOT ALLOWED for use on that crop.
|§ Not allowed in greenhouses or high tunnels.
OMRI means allowed for use in organic production.
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