Spotted Wing Drosophila

The Spotted Wing Drosophila (SWD), Drosophila suzukii, is a small vinegar fly with the potential to damage many fruit crops. In the North Central region, it was first detected in Michigan in late September 2010.

Spotted Wing Drosophila has also been observed occasionally attacking other soft-flesh fruit such as plums, plumcots, nectarines, and figs when conditions are right. Adults and maggots closely resemble the common vinegar fly, Drosophila melanogaster, and other Drosophila species that primarily attack rotting or fermenting fruit.

Adults are small (2-3 mm) flies with red eyes and a pale brown thorax and abdomen with black stripes on the abdomen. The most distinguishable trait of the adult is that the males have a black spot towards the tip of each wing. Larvae are tiny (up to 3.5 mm), white, cylindrical maggots that are found feeding in fruit. One to many larvae may be found feeding within a single fruit. After maturing, the larvae partially or completely exit the fruit to pupate.

Life cycle:Drosophila suzukii prefers a moderate climate but can also survive in cold conditions . The flies are most active at 20°C (68°F). Activity becomes reduced at temperatures above 30°C (86°F) or below freezing. However, D. suzukii is firmly established on the island of Hokkaido in Japan where winters average -4 to -12°C , suggesting the possibility of its establishment in cooler climates. Preliminary research from Oregon suggests that D. suzukii larvae, pupae, and adults have the potential to survive fluctuating overwintering conditions for periods up to 60 days. Adults are able to withstand longer periods of cold conditions than larvae or pupae .

Drosphila spp. are very sensitive to desiccation. However, sensitivity to desiccation depends upon climatic conditions and flies could develop desiccation resistance over time .Like most Drosophila spp., reproduction in D. suzukii is rapid. The total life cycle may be completed within one to two weeks depending upon the climatic conditions . Adult life span is about three to nine weeks. However, flies emerging late in the season overwinter and may live longer. In Japan, the fly produces 10 to 13 generations .

Flies could survive up to 10 generations per year under California climate conditions . The egg, larval and pupal stages last from 1–3, 3–13, and 4–5 days, respectively . Degree-day models on D. suzukii suggest that the entire life cycle (egg to egg laying female) can be completed within 12 to 15 days at 18.3°C (65°F) or a little more than a week at 21.1°C (70°F).A single female can lay one to 60 eggs per day and 200–600 eggs in her lifetime. A female lays approximately one to three eggs per oviposition site. The eggs are laid singly and are randomly distributed on fruits. Multiple clutches of larvae may be present on the same fruit because females may oviposit on the same fruit.

Damage:Unlike other vinegar flies that occur in California, spotted wing drosophila attacks healthy ripening fruit as well as damaged or rotting fruit. The female ovipositor is very large and serrated, so it is able to penetrate the skin of soft-skinned fruit and lay eggs just under the skin, creating a small depression on the fruit surface. Each clutch of eggs is from one to three, and the female will oviposit on many fruit. Multiples of larvae within a single fruit are quite possible because many females may visit the same fruit to oviposit.

Once fruit integrity is compromised by spotted wing drosophila’s activities, common vinegar flies may also oviposit in the damaged fruit.Eggs hatch and the maggots develop and feed inside the fruit, causing the flesh of the fruit to turn brown and soft; sunken areas that exude fluid often appear on the surface of smooth skinned fruit like cherries and blueberries. Damage can provide an entry site for infection by secondary fungal and bacterial pathogens, but this is not always the case.

Control:The organophosphate insecticide malathion will control SWD, but malathion is very toxic to bees and natural enemies of other pests in the garden, so care must be taken to keep the application on the target plant and to avoid drift and runoff. Improper application can also result in injury to cherry trees. If monitoring indicates a need to spray, the application should be made as soon as the fruit just begins to turn from yellow to pink. This should be about 2 to 3 weeks before cherry or berry harvest. In the case of indeterminate fruiting berries such as raspberries or strawberries, sprays may need to be repeated to keep SWD populations low during their prolonged fruiting period in summer and fall. Because of the potential negative impact of malathion in the garden, use it only where you are certain you will have a SWD infestation, either because you had a problem last year or from trapping and positively identifying insects as SWD this season.

An alternative to malathion with fewer negative environmental effects is spinosad however, it is not quite as effective against the fruit fly adults as malathion. To get satisfactory control two sprays may be required; the first applied as the fruit just begins to turn pink and the second applied 7 to 10 days later. Additional sprays may be needed for berries with a prolonged fruiting period. Be sure to wait the interval specified on the label before harvesting the fruit. Some spinosad products are sold to be applied with a hose-end sprayer, but a compressed-air sprayer will give more reliable coverage.

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