Spotted Wing Drosophila

A destructive pest of ripening fruit.

Spotted Wing Drosophila Biology

Drosophila suzukii, commonly known as the spotted wing drosophila or SWD, is a member of the family of so-called vinegar flies or fruit flies. The SWD is native to Southeast Asia but has spread to many other regions alongside imported fruits and vegetables. There are about 3,000 Drosophila species, and most of them only infest overripe fruit that has begun to rot and ferment, hence the name vinegar flies. However, the spotted wing drosophila is one of two known species that infests and lays its eggs in ripening fruit, which can cause significant damage to crop yields. Damage caused by SWD often leads to secondary damage by bacterial or fungal infection as well as other pest insects.

SWD are most active at 68 F, but they are able to mature and reproduce in a fairly wide range of temperatures. A spotted wing drosophila can have a life cycle as short as 8-14 days, so it is estimated that in the PNW climate, SWD may have up to 10 generations per year. A female can lay up to 300 eggs in her lifetime, so provided an adequate food source, the population can grow exponentially and travel from crop to crop in response to which fruit is currently in season.

The most affected crops seem to be berries, which are one of the most important crops in the PNW. As of 2009, Oregon had already confirmed infestations of SWD in blueberries, wild blackberries, red raspberries, Marionberries, cherries, strawberries, plums, peaches, grapes, figs, hardy kiwis, and Asian pears.

Disposing of Infested Fruit

Immediately removing and destroying infested or waste fruit is a key principle in controlling SWD. Simply composting the fruit may not be enough to reduce the population. A study found that placing all culled fruit into clear plastic bags and letting them sit in the sun for 2-3 days killed 99% of SWD larvae and pupae, whereas simply burying or composting the fruit did not significantly reduce SWD pressure.

Photo of a cherry with damage caused by Drosophila suzukii. Image from Wikipedia.

Photo of an adult spotted wing drosophila. Image from Flickr.

Identifying SWD

The adult SWD has red eyes and a yellow to brown body with darker bands on the abdomen. The male has a distinct dark spot near the tip of each wing, but females do not have spotted wings. Setting fly traps and frequently checking for SWD can be a good way to catch an infestation as early as possible.

An infested fruit displays small speckles and indented soft spots where a female fly has deposited her eggs. These soft spots will grow as the larvae feed on the fruit, and the fruit often collapses around the deposition site. Because only one other species of Drosophila lays eggs on ripening fruit, any ripening fruit that appears to have larval feeding sites should be properly disposed of.

Managing SWD

Culling and disposing of infested fruit immediately should be the number one priority when SWD is a concern. A great strategy to significantly reduce fruit loss to SWD is to increase harvest frequency, and cull infested fruit during harvesting. A study of raspberries found that when harvested every two days rather than every three days, the number of SWD eggs and larvae in harvested fruit was significantly lower, and harvesting every two days resulted in the highest marketable yield. In the long-term, more diligent harvesting and culling will lower SWD populations in the area.

Most commercial farms manage SWD with chemical pesticides, but organic farms have fewer options. Spinosad is the only organic pesticide that has shown to be somewhat effective at managing adult SWD. From 1-10 days after treatment, spinosad applied to blueberries caused 40-70% mortality in adult SWD. However, the larvae and pupae were not significantly affected. Therefore, spinosad can somewhat effectively be used as a preventative measure to prevent adult SWD from laying eggs on fruit, but fruit that is already infested will be unaffected.

There is research showing that certain essential oils can be used to deter SWD. While they are not toxic to the flies, the volatile organic compounds may disrupt the flies’ olfactory senses. When applied to raspberries, Ecotrol (a mix of rosemary (10%), geraniol (5%), and peppermint (2%) oils) had less SWD infestation than the control (6% vs. 17%), which was comparable to a Spinosad treatment (6%). By this logic, companion planting with rosemary, mint, garlic, and other plants with volatile oils may reduce SWD pressure.

Further Reading

Goodhue, R. E., Bolda, M., Farnsworth, D., Williams, J. C., and Zalom, F. G. (2011). Spotted wing drosophila infestation of California strawberries and raspberries: Economic analysis of potential revenue losses and control costs. Pest Management Science.

Gullickson, M., Hodge, C. F., Hegeman, A., and Rogers, M. (2020). Deterrent effects of essential oils on spotted wing drosophila (Drosophila suzukii): Implications for organic management in berry crops. Insects.

Leach, H., Moses, J., Hanson, E., Fanning, P., and Isaacs, R. (2017). Rapid harvest schedules and fruit removal as non-chemical approaches for managing spotted wing drosophila. Journal of Pest Science.

Oregon State University Extension Service. (2009). EM8991: A new pest attacking healthy ripening fruit in Oregon.

Sial, A. A., et al. (2019). Evaluation of organic insecticides for management of spotted‐ wing drosophila (Drosophila suzukii) in berry crops. Journal of Applied Entomology.

Van Timmeren, S. and Isaacs, R. (2013). Control of spotted wing drosophila, Drosophila suzukii, by specific insecticides and by conventional and organic crop protection programs. Crop Protection.