ALEXANDRIA, Minn. (07/21/21) ~ As I watch the berries form on my primo cane, Joan J raspberries, I am inspecting for any signs of that new invader, Spotted Wing Drosophila (SWD). This tiny vinegar fly was first found in Minnesota in August of 2012. Since then it has been found in 40 Minnesota counties. SWD flies resemble common fruit flies found near overripe fruit, but unlike the common fruit fly, female SWD have the potential to damage otherwise healthy, intact fruit. This damage can also make fruit susceptible to secondary damage from other pests or diseases. Fruits most affected by SWD include raspberries, blueberries, cherries, and other softer-fleshed fruits.
Female SWD pierce ripened fruit and inject their eggs. Tiny larvae (1/8” long) develop within the fruit, and then drop to the ground to pupate and emerge as adult flies. Multiple generations of this fly can occur in one season. An adult SWD lives for about two weeks and can lay more than 100 eggs a day. Since these fruit flies only have a week between emergence and egg-laying, they can soon infest your whole crop. SWD larvae feed within the fruits causing brown, sunken areas. It is possible these symptoms won’t appear 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.
A Three-pronged approach is necessary to manage SWD:
A)—Monitor for the SWD:
Although the pest has been found in many areas of the state, you really need to know if SWD adult flies are present at your location. If the flies are present, there are likely to be females actively laying eggs in fruit. Monitoring should occur from fruit set until the end of harvest, 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. The sooner the flies are discovered, the quicker management decisions can be made.
B)— “Clean-Harvest” berries as frequently as possible:
All ripe berries should be harvested 3-4 times per week, or as frequently as possible, depending on the weather. SWD flies are most attracted to ripening, or over-ripe fruit; thus, all over-ripe fruit should be removed immediately (including older berries on the ground).
C)—Apply insecticides carefully:
Pyrethrum, Spinosad, Malathion, and Bifenthrin are the common names of insecticides proven to be effective in controlling SWD in your home garden. Follow label directions exactly as stated, paying particular attention to rate of application and post-harvest intervals. Rotate the use of these pesticides to minimize the risk of SWD developing a resistance to any particular pesticide.
For more information on how to build a vinegar fly trap, identification and management strategies, visit:
Until next time, happy gardening!
Robin Trott is a Horticulture Educator with University of Minnesota Extension.