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Spotted wing drosophila

Quick facts

  • Spotted wing drosophila (SWD) is a member of the “small fruit fly” or “vinegar fly” genus Drosophila.
  • In Minnesota, SWD primarily attacks raspberries, blackberries (and other cane berries), blueberries, strawberries and wine grapes.
  • Native to Asia, SWD is currently found in most of the primary fruit growing regions of the U.S.

How to tell SWD from other fruit flies

Adults

  • Spotted wing drosophila (Drosophila Suzuki) is only 1/12 to 1/8 inch (2-3 mm) long.
  • Yellowish-brown.
  • Dark-colored bands on the abdomen.
  • Prominent red eyes.
  • They can be difficult to distinguish from other species of small fruit flies.
  • Male SWD are relatively easy to identify as they have clear wings and a dark spot along the first vein near the tip of each of wing.
  • Female SWD also have clear wings, but they do not have any spots on them.
    • They can only be identified by their saw-like ovipositor, which has two rows of dark-colored teeth (the ovipositor is the structure used by the female fly to insert eggs into berries).
    • High magnification is needed to see the ovipositor.

Larvae

  • SWD maggots are white with a cylindrical body that tapers on one end. 
  • They do not have legs or a conspicuous head. 
  • This is a small insect, only reaching 1/8th inch long.
Fruit fly with one black dot on the end of each wing, suspended in glossy fluid.
Male SWD. Note the dark spot on tip of the wing. Credit: Sheila Fitzpatrick
Female spotted wing drosophila fly with close-up view of the ovipositor with serrated edge.
Female SWD (note the lack of wing markings), and close-up view of the ovipositor with serrated edge. Credit: Sheila Fitzpatrick
Tiny white larvae inside a strawberry.
SWD larvae in a strawberry

Biology

SWD first appear during late June or early July, and the numbers increase rapidly during the middle of summer, with the populations peaking in August.

  • Adult flies insert eggs into soft fruit where the larvae develop.
  • Larvae leave the fruits to pupate and later emerge as adults.
  • SWD can complete its life cycle in as little as seven days.
  • Multiple generations of SWD can occur in a year, with populations building throughout the summer. August and September are the months of highest numbers.
  • SWD overwinters as an adult.
  • Most SWD seem to migrate into Minnesota from the south, although there is evidence that some SWD can survive Minnesota winters.

Damage

Spotted wing drosophila on a ripe raspberry.
Male and female SWD on a raspberry

SWD larvae feed on healthy, intact, ripening fruits. In particular, SWD will feed on thin-skinned, soft fruits such as raspberries, blackberries, blueberries, strawberries, grapes, plums and cherries.

They do not attack healthy apples, pears or cherry tomatoes. However, if these fruits become damaged, they can successfully infest them. They do not appear to attack cranberries.

  • SWD larvae feed within the fruits causing brown, sunken areas. 
  • It is possible that larval feeding symptoms won't show until after crops are harvested.
  • During egg-laying, the female may introduce Acetobacter, a bacterium that causes the fruit to rot, and infested fruit often develop a fermented or a sour smell.
  • If berries are stored at room temperature, larvae can hatch after the fruit has been picked. Fruit that was normal may be soft and maggot-infested a day or two later.

  • Mature larvae often crawl out of fruit stored on the counter.
  • The feeding makes fruits susceptible to infestation by other insects, rot fungi and bacteria.
  • During minor infestations, infested fruit can be processed into wine or jelly.
  • During severe infestations, fruit is too rotten to be processed.

Managing SWD in home gardens

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Managing SWD on farms

The following information is specific to fruit farms and those growing fruit for commercial sale or use.

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Jeff Hahn and William Hutchinson, Extension entomologists; Annie Klodd, Extension fruit and vegetable production educator; and  Suzanne Wold-Burkness, College of Food, Agricultural and Natural Resource Sciences

Reviewed in 2020

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