SWD Pest Profile
Eric Burkness, Suzanne Wold-Burkness, Jeff Hahn, and Bill Hutchison
Department of Entomology, University of Minnesota
Spotted Wing Drosophila (SWD), Drosophila suzukii, is an invasive insect pest of raspberries, blueberries, blackberries, grapes, stone fruit and some other fruit crops. SWD is native to Asia and was first found in California in 2008 and is currently found in Florida, Michigan, and most of the primary fruit growing regions of the U.S. In August of 2012, SWD was confirmed in Ramsey, Hennepin, Anoka, and Olmsted Counties, and also several unconfirmed reports have been made covering several more counties (Fig. 2). To date, the primary crop attacked has been raspberry but grapes and wild blackberries were also found to be infested.
Figure 1. SWD adults on raspberry (S. Wold-Burkness, Univ. of MN)
Figure 2. SWD distribution in Minnesota (MN Dept. of Ag.)
The SWD looks very similar to small fruit flies you might occasionally see flying around overripe bananas on your kitchen counter. However, unlike these other flies, which typically feed on overripe or deteriorating fruits, the SWD feeds on healthy, intact, ripening fruits. In particular, the SWD will feed on thin skinned, soft fruits such as raspberries, blackberries, blueberries, strawberries, grapes, plums and cherries. Female SWD use a saw-like egg laying structure to lay their eggs in ripening fruits. The larvae of the SWD then feed within the fruits causing brown, sunken areas. It is possible that larval feeding symptoms won’t show until after the crops are harvested and sometimes not until the fruits are in possession of the consumers. In addition to the damage caused directly by the larvae, the feeding makes the fruits susceptible to infestation by other insects and rot fungi and bacteria. The larvae will then leave the fruits to pupate and later emerge as adults. Multiple generations of SWD can occur in a year, with populations building throughout the summer. The overwintering stage of the SWD is the adult;consequently, its ability to survive Minnesota winters remains unknown. More research in Minnesota will be necessary to determine the extent of overwintering success in Minnesota; regardless of future impacts, however, this pest has caused devestating losses in numerous crops since 2012.
Figure 3. SWD larvae in grapes (E.C. Burkness, University of Minnesota)
Because larval feeding by SWD may not show up until after harvest, there is an increased risk that consumers may purchase infested fruit and makes implementing management programs in raspberry critical to minimizing the impact of SWD. As mentioned by researchers at Michigan State University, effective management of SWD consists of several key steps:
- Monitor fields before fruit ripening begins by using traps and check them regularly.
- Identification of SWD flies is critical (see Michigan State Univ. SWD identification guide).
- If SWD is found in traps, immediately apply effective insecticides registered for raspberries to protect the fruit (see below).
- Critical: Also remove overripe, old, diseased, or missed fruit and maintain a high level of sanitation in the field to reduce SWD breeding sites and food resources. This must be done every two days during harvest, and particularly in "pick your own" operations, following harvests by customers
- Continue trapping after insecticide applications to verify reduction in fly populations and to monitor if and when populations of SWD return to the field.
- Stay up to date on the latest recommendations for management of SWD. Recommendations are subject to change based upon new information.
Critical to implementing an Integrated Pest Management (IPM) program for SWD is development of an effective monitoring program on your farm. Monitoring should occur from fruit set until the end of harvest. This allows the grower to identify the start and end of fly activity, although the most critical time period to monitor is when fruit color first starts to develop until the crop is harvested. This is when the fruit are susceptible to SWD infestation.
Figure 4. Trap for capturing SWD adult flies.
Adult SWD flies can be trapped using a plastic 32oz cup with several 3/16”-3/8” holes around the upper side of the cup, leaving a 3-4 inch section without holes to allow pouring out of the liquid attractant, or bait (Fig. 3). Holes can be made using a drill in sturdy containers or if in softer material, burned with a hot wire or soldering iron. The small holes allow access to vinegar flies, but keep out larger flies and other insects. Pour one inch of apple cider vinegar into the trap as bait. To help capture the flies, a small yellow sticky trap can be placed inside. The traps will also work without the yellow sticky insert, but then a drop of unscented dish soap should be added to the vinegar to ensure flies remain trapped in the liquid.
Traps should be hung in the fruit zone, in a shaded area of the canopy, using a wire attached to the top of the trap (Fig. 4). Make sure the trap is clear of vegetation with holes exposed so SWD can access the trap.
Traps should be checked 1-2 times/week for SWD flies - more often if fruit is harvestable, by looking on the yellow sticky trap and in the liquid. Fresh vinegar should be placed in the trap at each check and disposed of away from the trap location. Records should be kept for SWD captures to document the periods of fly activity and the corresponding stage of fruit ripeness should be noted as well.
Identification of Flies
Vinegar flies are small (2-3 mm) with rounded abdomens (see Fig. 4). Traps baited with apple cider vinegar catch both male and female SWD flies. Native species of vinegar flies (and other insects) will also be attracted to the traps; therefore, proper identification of SWD is important. Identification of SWD flies will become easier with practice, and it is suggested to use a hand lens or other magnification to examine the wings of trapped flies. Some native flies have dark patches on the wings, but will not have the distinctive dark dot that is present on the wings of SWD males (Fig. 5).
Figure 5. Male SWD fly with distinctive wing markings (2 mm long)
Female SWD do not have dots on the wing (Fig. 6), so their ovipositor needs to be examined closely in search of its serrated characteristics (the ovipositor is used by the female fly to insert eggs into berries). A 30X magnification hand lens or microscope is needed to detect the distinctive saw-toothed ovipositor on female SWD. It is important to check traps for female flies as these often emerge first from overwintering sites.
Figure 6. Female SWD fly (note the lack of wing markings)
If flies suspected of being SWD are trapped in counties where this insect has not yet been reported, we encourage growers, scouts, and consultants to place flies trapped on the sticky traps into another container (or pull those floating in the vinegar out of the liquid and place in a small vial) and contact the Minnesota Department of Agriculture’s “Arrest the Pest” hotline at 1-888-545-6684 or Arrest.the.Pest@state.mn.us. Please note the location and date of collection for the specimen.
Identification may also be aided by use of the online dichotomous key provided by Oregon Department of Agriculture.
Sampling Fruit for Larvae
If you suspect fruit may be infested, larvae can be sampled using a fruit dunk flotation method. Fruit can be selected in 2 ways, either by collecting fruit at random, or by collecting only fruit you suspect is infested (i.e., the presence of oviposition scars and/or soft spots on the fruit).
Place fruit in a plastic “ziplock” bag and crush lightly to break the skin. Then add a sugar-water mixture (4 cups water to every 1/4 cup sugar). SWD larvae will float in the liquid and the fruit will sink. Detection of small larvae may require the use of a hand lens, and this works well with a light behind the bag to create backlighting.
Another method developed is to dissolve 1 Tbsp. of salt in 1 cup of water. Suspect berries are placed on a tray and the salt solution is lightly poured over the fruit. Watch the fruit closely for 10-15 minutes to see if larvae exit through egg-laying holes. Larger SWD larvae will be visible as they emerge from the berries.
SWD Management Options
With the potential for rapid population increase by SWD, close monitoring throughout the ripening period is necessary. This allows for a rapid response to detections of SWD. Given that SWD has only recently been detected in Minnesota, what follows are recommendations based on experiences in other states. Recommendations will be updated as new options are developed and tested.
The relationship between the level of trap catch and the level of fruit infestation is unknown. In addition, there is not an established action threshold for SWD in raspberry. Therefore, to minimize infestation risk, a conservative approach has typically been adopted where once a SWD fly is captured and berries are at a susceptible stage; it is suggested to begin implementation of control options in the field. If fruit are ripening (color change has begun) and SWD flies are trapped, growers should: 1. Increase monitoring intensity to assess fly distribution (check traps at least twice per week); 2. Use cultural controls where possible (mainly removing old, infested, or damaged fruit from the field), 3. Use of a registered insecticide from detection of the fly until harvest is completed.
This is particularly important if female flies are found on traps. Female SWD are able to lay eggs into fruit from the start of color development until harvest. If spotted wing Drosophila are found, the available management options and best strategies will depend on the scale of infestation, whether the field is certified organic or not, and the time of the season. SWD populations tend to increase in the later part of the summer; we therefore expect late-harvested fruit such as fall raspberries or blackberries to experience higher infestations from SWD than fruit crops that are harvested earlier in the summer.
Cultural controls may be very helpful in reducing reproduction and survival of flies. Several practices can be used include scheduling timely harvests and removing over-ripe fruit from fields as soon as possible to minimize SWD egg lay and larval development. Growers in other regions of the country have sent pickers through fields with one container to collect good fruit and another container to collect over-ripe fruit, again, to minimize egg laying and larval development sites. This practice may better in small scale situations vs. large scale commercial operations. A final cleanup picking to remove the last berries from the bushes may be worthwhile, but this approach has not yet been evaluated and again would likely depend on the scale of the operation. The removal of wild host plants near production fields that could support SWD populations could be another potential option but has not been tested. Wild hosts include plants such as wild grape, pokeberry, black raspberry, and blackberry.
Recent research in Oregon has compared various ways to cause mortality of SWD in infested berries, i.e., how best to dispose of infested fruit. Two methods that worked well at eliminating SWD were 1. bagging fruit inside clear or black plastic bags and 2. solarizing, where 1-2 ml clear plastic sheeting is placed over the fruit in a sunny location and sealed around the edge using soil. Important Note: simply burying infested fruit was not effective.
Some of the insecticides already registered for caneberries in Minnesota will provide some protection against SWD. However, members of the neonicotinoid class (e.g. Provado, Assail, Actara) are considered weak on SWD and are not recommended for its control (see Table 1).
It is important to realize that SWD females can start laying eggs one day after adult emergence. This places a high priority on monitoring to detect the flies as early as possible to allow management decisions to be made. SWD will complete multiple generations and there will likely not be distinct generations but rather a continuous increase in activity once the flies become active. For this reason, if SWD are detected, spray intervals should be relatively short (4-5 days) to prevent crop infestation before and during harvest. In high pressure situations in western states in the U.S., a 5-7 day spray interval has been required. Sprayers should be calibrated to provide thorough coverage of fruit, especially in the center of the canopy, where flies like the shade. Application equipment that attempts to cover several rows in a single pass are unlikely to achieve good enough coverage of the fruit in all the rows.
A number of registered insecticides have been very effective against SWD in laboratory trials, including some recent trials done at Michigan State University on flies from a colony that was collected in Michigan. The most effective chemicals are organophosphate, pyrethroid, and spinosyn classes of insecticides. Pyrethrum class, organic based insecticides were less effective, but can still be a valuable tool for organic growers. Under field conditions, insecticides with fast knockdown activity have performed well at protecting fruit.
Organic growers in the Pacific Northwest have used 2-3 applications of Entrust (spinosyn) effectively to protect fruit in the pre-harvest period, and this should be alternated with Pyganic (pyrethrum) to extend the period of control and also to reduce the chance of resistance development. It is important to note that Entrust provides ~5 days residual control and Pyganic provides ~2 days of control. Note also that Entrust has a 9 oz/acre seasonal maximum (see below for more details).
The table below provides a list of insecticides registered for use in caneberries that have also shown high efficacy against SWD. Selection of insecticides for SWD control should take into account other pests present, harvest date, re-entry restrictions, and potential impacts on existing IPM programs. Most of these insecticides are also active on other insects and most will have some activity on Japanese beetles (another emerging pest in Minnesota) that may be active at the same time. Always follow the specific label guidelines and use restrictions for caneberry on the product label. There may be supplemental labels available that have been developed specifically for SWD. Details of label updates can be found at www.cdms.net (look under Services). The level of control achieved will depend on the SWD population, timeliness of application, coverage of fruit, and product effectiveness.
When selecting an insecticide for SWD control in caneberries, consider the REI, PHI, and especially the MRL restrictions in the destination country if exporting fruit. Remember that most insecticides labeled for caneberries will be Restricted Use and will require a MN pesticide applicators license to purchase. See the label for restrictions on distance to surface water and safety to pollinators and other beneficial arthropods. Remember to rotate classes of insecticides to delay development of insecticide resistance. This is especially critical in organic production where there are only two classes of insecticide registered for use against SWD.
Organic caneberries: Organic fruit growers should be aware that the insecticidal control tools available to them are less effective than conventional insecticides against SWD, and will require more timely application. Experience in the west coast states indicates that SWD can be controlled in organic production through more intensive monitoring, timely application if flies are detected, and shorter intervals between sprays. In organic production systems, cultural controls will be important to help reduce the overall SWD population level. Organic insecticide options are limited but the experience so far in California and Oregon indicates that Entrust and Pyganic are the two most effective options for SWD control in organic production.
Future Developments for SWD Management and Research
Spotted winged drosophila is a new pest to North America and has only recently been detected in Minnesota in August of 2012. We will continue to work on developing the best possible recommendations for the fruit crops affected. We will make this information available through factsheets and/or web updates on FruitEdge and VegEdge and we will continue to update these fact sheets with new information as it becomes available.