Figure 1. SWD “sting” on a ripe sweet cherry. Image: University of California Statewide IPM Program, taken by Larry L. Strand.
Unlike other vinegar (fruit) fly species (Drosophila spp.) that lay their eggs in over ripe, previously damaged, rotting, and fermenting fruit; SWD will attack undamaged fruit as it ripens. Adults are small flies about 1⁄16 to 1⁄8 inch long with red eyes and an amber colored body with black stripes on the abdomen. The male flies have a black spot towards the tip of each wing. The females do not have spots on the wings but they have a very prominent, sawlike ovipositor (egg laying structure), larger than other vinegar flies. The female will penetrate the skin of soft-skinned fruit laying the eggs just under the skin leaving a small puncture ("sting") on the fruit surface (Figure 1). Eggs hatch and larvae develop and feed inside the fruit, and this damage can provide an entry site for other vinegar flies and secondary pathogens.
Sampling for SWD in fruit is the best way to determine whether SWD is causing damage in your fields and to evaluate the effectiveness of management practices. Once SWD lays its eggs in a ripe fruit, the fruit begins to degrade, and other vinegar flies may also begin to infest it. Therefore, it is important to sample ripening fruit with no apparent damage rather than degraded or overripe fruit when determining SWD presence in fruit. The more fruit you sample the more confident you can be in your results, and currently it is unknown how many fruit to sample to determine whether SWD is present at market detectable levels. We typically sample at least 30-40 fruit at market ripeness from various locations (including border rows) within the planting. Selecting fruit from the interior portions of the plant may increase your chances of detecting SWD. Recent research in Dr. Hannah Burrack's lab at North Carolina State University suggests that SWD density is higher in fruit in the central part of a plant. Most of the time, these berries also do not receive proper spray coverage, and therefore are not well protected from SWD.
There are a variety of methods to sample for larvae in the fruit, however, we do not know which method is best for finding larvae. Perhaps the easiest is directly looking for larvae in the fruit. One of the earliest signs of larvae in raspberries is evidence of juice on the receptacle when the fruit is harvested. Individual fruit can be crushed or cut open and you can look for larvae. A hand lens might help find some smaller larvae, but this method likely misses the smaller larval stages.
Larger larvae are visible to even the naked eye, remembering that SWD larvae are pointed at both ends and only a little longer than 1⁄8 inch when fully grown (Figure 2). SWD larvae cannot be visually distinguished from other vinegar flies, so selecting ripening good quality fruit rather than overripe fruit is important. Other flies that may be found within fruit, such as blueberry maggot, are larger at around 3⁄16 inch and are only pointed at one end (Figure 3). Other internal fruit feeding pests such as plum curculio, raspberry fruitworm, cranberry fruitworm, and cherry fruitworm (Figure 4) may also be found with some of these methods.
- Figure 2. SWD larva in blueberry. Image: British Columbia Ministry of Agriculture, taken by T. Hueppelsheuser.
- Figure 3. Blueberry maggot in fruit. Image: Michigan State University, taken by Rufus Isaacs.
- Figure 4. Cherry fruitworm on blueberry. Image: North Carolina State University, taken by Hannah Burrack.
Larval flotation methods can be used to extract larvae from fruit and see them floating on the top of a water solution. Oregon State University has a detailed guide with pictures. In this method, fruit should be gently crushed and/or cut into pieces (especially larger fruit like strawberries) in a container. This can be done in a thin layer in a plastic food storage container or in a sealed plastic bag like a gallon sized re-sealable bag. Then a sugar (white sugar or light brown sugar works) water or a salt (non-iodized seems to be better) water solution can be added. Usually 1 cup of white sugar or table salt, or 2.5 cups of light brown sugar is used per gallon of water. Dissolving the salt or sugar in warmer water a day or so beforehand can ensure that it is fully dissolved and will increase the flotation. Sugar solutions tend to keep the larvae alive a bit longer, and live larvae are easier to see because they continue to move.
Agitate the fruit while it is in the solution; and allow 10-15 minutes for the larvae to emerge. SWD larvae (if present in the fruit) should float to the surface and be visible, other internal feeders such as cranberry fruitworm that are larger tend to sink rather than float. It can help to use wire mesh or window screen to hold down the fruit particles when using a tray or plastic food storage container so that only the water and larvae are at the surface. Pouring the water through a fine mesh (a US Standard Mesh Size 12 or 0.06" opening is a good size) sieve can also help locate larvae by sieving out the fruit particles. The water should be collected and larvae counted after sieving. Putting the water (or water and fruit) in a clear container over a dark background can make it easier to spot the larvae (which are white to cream-colored), and a hand lens can also help for viewing smaller larvae.
Heating, slow freezing or refrigeration of fruit held in sealed plastic bags often causes the larvae to leave fruit on their own, after which they are found on the surface of the fruit or on the bag. I often just leave the bags sealed in my truck on a warm day and come back to the larvae abandoning the fruit.
Finally, the only way to be 100% sure that you have SWD rather than other vinegar flies in your fruit samples, is to rear the flies out of the fruit. This can be hard to do with fruit like raspberries that degrade very quickly, but is a very successful method for something like cherries and blueberries. Hold the fruit in a tightly sealed container (to prevent escaping and secondary infestation by other vinegar flies) that is still able to exchange air. Making a hole in a plastic food storage container and then sealing it with fine mesh (like organdy) using caulk works well. Because the fruit will start to degrade, a lining like cotton, paper towels, or sand will be needed on the bottom to soak up the liquid and prevent the larvae from drowning.
Keep the containers at room temperature and then use a sticky card to catch the flies as they emerge, or wait until fly emergence and freeze the container to collect the flies. You can then identify the emerging vinegar flies to see if they were SWD. This method may overestimate market detectable infestation because eggs have time to hatch and develop. Penn State has a fact sheet that distinguishes spotted wing drosophila from other similar species found in fruit plantings.
Identification guides for SWD can also be found at many IPM websites.
Burrack, H. 2012. "Spotted wing drosophila (Drosophila suzukii) larval sampling and post harvest considerations." North Carolina State University.
Dreves, A.J., A. Cave, and J. Lee. 2014. "A detailed guide for testing fruit for the presence of spotted wing drosophila (SWD) larvae." Oregon State University.
Hamby, K.A., M.P. Bolda, M.E. Sheehan, and F.G. Zalom. 2014. Seasonal monitoring for Drosophila suzukii (Diptera: Drosophilidae) in California commercial raspberries. Environmental Entomology 43: 1008-1018.