Dr. Leonard Perry
University of Vermont Extension
This time of soft fruits such as blueberries, raspberries, and perhaps grapes; if you grow or even buy such from local markets, you may want to read on if you haven’t heard of the Spotted Wing Fruit Fly before. One way it differs from the usual fruit fly is that it can infect fruit already picked, so when you get them home maggots may appear in them. This insect is now spreading through northern states including Vermont and Maine and Michigan. Below is from a news release by Dr,. Vern Grubinger of UVM Extension in late August. Note that late fall may be a good time to trap any adults.
“A new fruit pest, the spotted wing drosophila (SWD), has arrived in Vermont.
This small fly feeds on many different cultivated and wild fruits but is a particular threat to soft fruits that ripen in the late summer and fall. SWD is likely to be a significant problem in small fruits such as blueberries, fall raspberries and grapes as well as tree fruits including peaches and cherries. So far it appears that early season fruit such as strawberries and hard fruit such as apples will be less impacted, but more information is needed.
SWD was first spotted in southern Vermont and parts of New England in fall 2011 and in recent weeks it has become widespread across the state and the region. With this pest being so new to the United States–it was first found in on the west coast in 2008–little is known about how big an impact it will have and what management tactics will be most effective.
The scientific name for this invasive pest from Asia is Drosophila suzukii. It looks very similar to the fruit flies that typically feed on and fly around overripe fruit on a kitchen counter. However, unlike these flies, it feeds on healthy, intact fruits as they ripen. Once a crop has finished fruiting, the flies move on to other crops.
To the naked eye, SWD is hard to tell apart from other species of small flies. A hand lens or microscope is needed to see its identifying features.
Like other fruit flies it is small, only one-twelfth to one-eighth inch long, but it has yellowish-brown coloration and prominent red eyes. The males have a dark spot near the edge of each of their clear wings (thus their common name). However, some other species of small flies also have spots on their wings, so SWD is a bit tricky to positively identify.
If you see fruit flies swarming in the evening around ripening fruit in the garden or on the farm that is a pretty good clue that you have SWD. To date, they have only been found outdoors. Fruit flies found indoors are likely to be a different species.
What makes this insect such a concern for farmers and gardeners is that the females have a saw-like egg-laying structure, which they use to puncture firm, ripening fruit. Once deposited in the fruit, the eggs quickly hatch into small larvae that feed inside the fruit, causing discoloration and decay. Sometimes these symptoms won’t show up until after harvest. In addition to the damage from larvae, infested fruit becomes susceptible to fungi and bacteria that cause softening and rot.
SWD has a short life cycle with many generations per year. After the larvae feed, they pupate and later emerge as adults ready to mate and lay more eggs. Thus, populations can build up over the summer and fall. It overwinters as an adult. Its ability to survive in Vermont is not known, but since it has overwintered successfully in Michigan, it is likely to be able to survive here, too.
Extension specialists suggest several tactics for managing this pest. Timely harvest and sanitation are important to reduce local buildup of SWD populations. This means frequent picking of a crop to ensure ripe fruits are removed from the field as soon as possible and removing and destroying any old fruit remaining on stems.
Fine-mesh row covers may be able to exclude the pest if placed over a crop before any fruit starts to ripen, but this technique needs field research to prove that it works well enough to justify the expense. Certain insecticides will kill SWD, but frequent spraying is needed to adequately protect a fruit crop during the harvest period.
Insecticides must be used according to the label, which often includes a lengthy waiting period until harvest can resume after spraying. For home gardeners and organic growers, alternating sprays of two natural insecticides can protect fruit if the sprays are applied before the insects lay eggs in the fruit.
Both spinosad and pyrethrins have a short waiting period and are available in formulations allowed for organic production. Keep in mind that overuse of an insecticide can cause the target pest to develop resistance to it. Use of any insecticide also poses risks to honeybees and other pollinators if a crop is still in bloom. In that case, sprays should be applied in the evening when pollinators are less active.
Trapping may be an effective means of reducing overwintering SWD populations. In late fall, once all ripe fruit is gone in an area, the adults can be attracted to and trapped in plastic cups or small buckets baited with apple cider vinegar plus a drop of dish detergent so the flies will drown.
For more information check out these four factsheets developed by Penn State University Extension: “