Spotted wing drosophila, Drosophila suzukii
Spotted wing drosophila (SWD) is a small, light brown fly that is a member of the “small fruit fly” or “vinegar fly” genus, Drosophila. Spotted-wing drosophila is distinct from common indoor fruit flies as it develops within ripening fruit rather than on yeasts. Males are distinguishable from other species of Drosophila by their dark spot towards the tip of the wing. The young stage is a maggot that feeds within the berries and causes the fruit to rapidly soften. This pest is particularly damaging to berries and stone fruits, especially raspberries, blackberries, and strawberries.
- Spotted-wing drosophila is a small fly that develops within many kinds of fruits. It is particularly damaging to late fruiting plantings of raspberries, blackberries, and strawberries.
- The young stages are tiny maggots that feed within berries and cause them to rapidly soften.
- Regular picking of all ripe fruit at least twice a week is useful in reducing damage by spotted-wing drosophila.
- Insecticides are available to help control spotted-wing drosophila, but they can only be applied during times of the day when pollinating insects are not actively visiting the crop.
SWD larvae in a strawberry. Image credit: Hannah Burrack, North Carolina State University, Bugwood.org.
SWD trap in blackberry planting. Image credit: Hannah Burrack, North Carolina State University, Bugwood.org.
Adult male SWD. Image credit: Hannah Burrack, North Carolina State University, Bugwood.org.
SWD eggs on strawberry. Image credit: Hannah Burrack, North Carolina State University, Bugwood.org.
Life history and habits
Flies overwinter in the adult stage, remaining semi-dormant during cold months. During this time, they may use skins and husks of decaying fruit as protection. As ripening fruits become available in late spring and early summer, females lay eggs in inside the fruit. A single female can lay up to 350 eggs in her lifetime. Eggs hatch in 12-72 hours and the tiny maggots which emerge feed inside the fruit. Optimal conditions for development are around 68-77 °F (20-25 °C) and several generations are produced annually. The highest numbers of insects are usually present in August and September.
Spotted wing drosophila can breed and develop within a wide variety of fruits, including various berries, tree fruits, and fleshy fruits, see factsheet for a list of host plants in Colorado. They can also develop inside apples and crab apples after they have become overripe. Larvae feed within the host fruit and cause them to rapidly soften. This pest is particularly damaging to late fruiting plantings of raspberries, blackberries, and strawberries.
This insect can easily be captured in home-made traps baited with apple cider vinegar and other attractants. Although traps can be used to monitor and provide some control, when used alone, trapping has not proved an effective method of control. For effective monitoring, traps should be placed in a shaded area, hung within the crop near the fruit. Spotted wing drosophila can be monitored by identifying and counting male SWD flies, see factsheet for more detailed instructions.
Cultural and mechanical control
Shifting to cultivars that bear fruit earlier in the season can largely avoid SWD damage. As fruit ripens, it should be regularly and thoroughly picked. If done every 3 or 4 days, this can prevent insects from successfully developing in the crop, reducing their numbers. Harvested fruits should be either immediately consumed or stored in a refrigerator. Refrigeration to 35 °F (2 °C ) will prevent any further development of insects and kill those present within about three days.
Any damaged fruits should be removed and placed in a plastic bag in the sun to kill developing insects. Putting infested fruit in a compost pile is not a reliable way to kill developing SWD. This pest also prefers dense shade and cooler areas in the canopy, so pruning crops in a way that opens the crop canopy can deter insects.
Nets of fine mesh (1 mm) can be used to exclude flies from plants. Such coverings need to be in place before ripening fruit is present that will attract adult SWD. It is worth noting that these coverings will also exclude other beneficial insects, including pollinators
Insecticides applied to kill the adult flies can be effective for control of spotted-wing drosophila. However, in home gardens, options of available products are quite limited. In cases where the crop flowers over an extended time, special care must be taken to prevent killing bees and other pollinators visiting the crop. See factsheet for further recommendations.
Oregon State University. (n.d.). Spotted Wing Drosophila. Oregon State University-Extension. Available https://extension.oregonstate.edu/collection/spotted-wing-drosophila-swd
Spears, L., C. Cannon, D. Alston, R. Davis, C. Stanley, R. Ramirez. 2017. Spotted Wing Drosophila. Utah State University-Extension. Available https://extension.usu.edu/pests/research/spotted-wing-drosophila
University of Minnesota. 2020. Spotted wing drosophila. University of Minnesota Extension. Available https://extension.umn.edu/yard-and-garden-insects/spotted-wing-drosophila