Spotted wing drosophila in home gardens
Spotted wing drosophila (SWD), Drosophila suzukii, is an invasive small fruit fly (sometimes called vinegar fly).
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.
How to tell SWD from other fruit flies
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.
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). High magnification is needed to see the ovipositor.
SWD maggots (larvae) are white with a cylindrical body that tapers on one end. They do not have legs or a conspicuous head. This is a small insect, only reaching 1/8th inch long.
SWD first appear during late June or early July, and the numbers increase rapidly during the middle of summer, with the populations peaking in August.
- Adult flies insert eggs into soft fruit where the larvae develop.
- The larvae leave the fruits to pupate and later emerge as adults.
- SWD can complete its life cycle in as little as seven days.
- Multiple generations of SWD can occur in a year, with populations building throughout the summer.
- SWD overwinters as an adult.
- There is evidence that some SWD are able to survive Minnesota winters.
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.
SWD larvae feed within the fruits causing brown, sunken areas. It is possible that larval feeding symptoms won't show until after crops are harvested.
During egg laying, the female may introduce fungi that cause the fruit to rot, and infested fruit often develop a fermented or a sour smell. If berries are stored at room temperature, larvae can hatch after the fruit has been picked. Fruit that was normal then may be soft and maggot-infested a day or two later. Mature larvae often crawl out of fruit stored on the counter.
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.
During minor infestations, infested fruit can be processed into wine or jelly. During severe infestations, fruit is too rotten to be processed.
Start monitoring your garden for adult SWD as soon as you see fruit set until the end of harvest. This allows home gardeners to identify the start and end of fly activity. Though the most critical time period to monitor is when fruit color first starts to develop until the crop is harvested.
Make your own SWD trap
Adult SWD flies can be trapped using a plastic 32-ounce cup.
- Poke several 3/16" holes around the upper side of the cup (small holes let SWD in, but keep out larger insects)
- Leave a 3-4 inch section without holes to hold the bait
- Pour one inch of apple cider vinegar into the trap as bait
- Place a small yellow sticky card inside (purchase from a garden supply store or Gempler's). If you can't get the sticky cards, add a drop of unscented dish soap to the vinegar to trap flies in the liquid.
Hang traps from branches in the shade near growing fruit, and begin monitoring before fruit begins to ripen. Replace the sticky card and apple cider vinegar bait at least once a week when checking for SWD. Dispose of the old apple cider vinegar away from the trap location. Checking traps more often can help with early detection of adult SWD, especially early in the growing season.
If you'd like to test fruit for the presence of maggots, there are several sampling methods you can use. 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.
- Frequently harvest crops to ensure ripe fruits are not in gardens for extended periods of time.
- 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.
- 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 from eating 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, 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 fruit.
For raspberries, the covers should not need to be removed to allow for pollination because raspberries are primarily wind pollinated.
Use a fine mesh netting: 80 gram insect netting has been shown to provide good results. Coarser netting will allow SWD to pass through and infest fruit. Netting can also provide protection from birds and hail.
It is important to remember that SWD females can start laying eggs one day after adult emergence. This makes it very important to monitor to detect whether SWD is present and when they first appear. The sooner the flies are discovered, the quicker management decisions can be made.
SWD will complete multiple, overlapping generations so there will be continuous activity once the flies become active. 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.
Insecticides should be applied in the evening to avoid killing honeybees and other pollinators. Readily available insecticides that kill adult SWD include carbaryl, malathion, spinosad and pyrethrin. Spinosad and pyrethrin are approved for organic production.
Insecticide resistance is a concern with this pest and you should rotate classes of insecticides if possible.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
Be sure that the plant you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Reviewed in 2018