Spotted Wing Drosophila in home gardens
Spotted Wing Drosophila (SWD), Drosophila suzukii, is an invasive small fruit fly (sometimes called vinegar fly) in the U.S. In Minnesota, SWD is a pest that primarily attacks raspberries, blackberries (and other cane berries), and blueberries but may also infest strawberries, grapes, and stone fruit. Native to Asia, SWD was first found in California in 2008, and is currently found in most if not all of the primary fruit growing regions of the U.S.
In August 2012, the first confirmation of SWD was made in Minnesota in Ramsey, Hennepin, Anoka, and Olmsted Counties and has since been found in the majority of counties in the state. Follow this link to view the current distribution of SWD in Minnesota.
Martin Hauser, California Dept. of Food and Agriculture
Figure 1. Male SWD with distinctive wing markings
SWD is a small fly, only 2 - 3 mm (1/12 - 1/8 inch) long, with yellowish-brown coloration, dark colored bands on the abdomen, and prominent red eyes. They can be difficult to distinguish from other species of small fruit flies. Male SWD are relatively easy to identify as they have clear wings and a dark spot along the first vein near the tip of each of wing (Figure 1).
However, while female SWD also have clear wings, they lack any spots on them. They are more difficult to identify. They can only be identified by their saw-like ovipositor which has two rows of dark-colored teeth (the ovipositor is the structure used by the female fly to insert eggs into berries). However, high magnification is needed to see the ovipositor.
SWD larvae (also called maggots) are white with a cylindrical body that tapers on one end. (Figure 2). They lack legs and a conspicuous head.
Adult flies insert eggs into soft fruit where the larvae develop (Figure 3). The larvae eventually leave the fruit and drop to the ground where they pupate. They later emerge as adults. SWD can complete its life cycle in as little as 7-10 days, although in cooler weather it could take up to 25 days or longer.
Trap catches have first found SWD in Minnesota in mid to late June. In the upper Midwest, it is believed about 10 - 12 generations occur during the growing season creating essentially continuous activity. Populations build through the growing season, peaking in late summer. SWD overwinters as an adult; however, its ability to survive Minnesota winters is unknown at this time.
SWD larvae feed on healthy, intact, ripening fruits. In particular, SWD will feed on thin-skinned, soft fruits such as raspberries, blackberries, blueberries, strawberries, grapes, plums and cherries (Figure 4). SWD larvae feed within the fruits causing brown, sunken areas.
It is possible that larval feeding symptoms won't show until after the crops are harvested. In addition to the damage caused directly by the larvae, the feeding makes the fruits susceptible to infestation by other insects, rot fungi, and bacteria.
Monitoring for SWD adults helps determine whether SWD is in your area and when it is first present. This information helps to time insecticide applications so they can be most effective. The best method for monitoring SWD is the use of traps.
Traps should be set out as soon as fruit are noticeable. Traps should be monitored from just after blossom petals have fallen (fruit set) until the end of harvest. This allows the home gardener to identify the start and end of fly activity. The most critical time to monitor is when fruit color first starts to develop until the crop is harvested. This appears to be when fruit are most vulnerable to SWD infestation.
Adult SWD flies can be monitored with a homemade plastic trap filled with apple cider vinegar. To construct the trap you will need a 32 ounce plastic cup with a top (like a deli container), apple cider vinegar, and a small piece of yellow sticky card. Drill or poke several 3/16'-3/8" size holes around the upper side of the cup, leaving a 3-4 inch section without holes to hold the vinegar (Figure 5). The small holes allow access to SWD, 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, place a small yellow sticky card inside. Yellow sticky cards can be purchased from local garden supply companies and from Gempler's. The traps will also work without the yellow sticky insert, but if this is done make sure to add a drop of unscented dish soap to the vinegar to trap the flies 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 (Figure 5). Make sure the trap is clear of vegetation with holes exposed so that SWD can easily enter the trap. Traps should be checked every other day, or as often as possible.
If it is desirable to test fruit for the presence of maggots, there are several sampling methods to test for larvae. For more information, see Sampling berries for spotted wing drosophila larvae (Michigan State University).
Sanitation is important to reduce the local buildup of SWD populations. The best sanitation practice is to frequently harvest crops to ensure ripe fruits are not in gardens for extended periods of time. It is also important to remove and destroy any old fruit that remains on stems or that has fallen to the ground.
Once infested or fallen fruit has been collected, place it in a plastic bag and seal it tightly. Fruit in clear bags can be left outdoors where the heat from the sun will kill any flies in the bag. Plastic bags can also be placed in the trash. Do not compost this material as that method is unreliable in killing SWD. Also, do not bury infested material as research has found that SWD can survive being buried as deep as 18 inches.
If your fruit looks intact, but you still suspect an infestation, place it in your refrigerator. Chilling can help slow and even stop the development of larvae. Note that these fruit are safe to eat. There is no known risk to human health posed by ingesting SWD.
Another cultural control method that can be used to manage SWD is exclusion through the use of netting or floating row covers. Netting can be used over a more permanent structure, such as a small high tunnel (Figure 6), or placed directly over the row similar to a floating row cover. The cover prevents SWD access to the developing berries, and can potentially reduce the infestation.
One drawback is that the covering would have to be opened at each harvest, which could provide SWD access to the berries. However, for raspberries, the covers should not need to be removed to allow for pollination because raspberries are primarily wind pollinated. A fine mesh netting should be used and an 80 gram insect netting has been shown to provide good results in University testing. Coarser 60 gram insect netting will allow SWD to pass through and infest fruit. Netting can also provide protection from birds and hail.
Once SWD is detected in your garden, an insecticide is necessary to protect susceptible fruit. More than one application will likely be necessary to protect fruit throughout the growing season. Insecticide resistance is a concern with this pest and you should rotate classes of insecticides if possible. Keep in mind that insecticides are targeting the adults, before they lay eggs, and will not control larvae already in the fruit. Once fruit is infested, there is not any effective control other than using sanitation to prevent SWD from emerging.
There are several insecticides approved for organic use that are available for treating SWD. Home gardeners still need to be judicious in rotating between insecticide classes, if possible. Intervals between applications may need to be shortened, but you still need to follow the label and be certain that all instructions, including PHI (pre-harvest interval) are allowed.
|*Very short residual lasts less than 1 day. Short residual lasts 3-5 days. Medium residual lasts 7-10 days.|
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