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Spotted wing drosophila in home gardens

Quick facts

Spotted wing drosophila (SWD), Drosophila suzukii, is an invasive small fruit fly (sometimes called vinegar fly).

In Minnesota, SWD is a pest that primarily attacks raspberries, blackberries (and other cane berries), and blueberries but may also infest strawberries, grapes and stone fruit.

Native to Asia, SWD was first found in California in 2008, and is currently found in most if not all of the primary fruit growing regions of the U.S.

The Minnesota Department of Agriculture monitors this invasive species. Please report any SWD you spot at Arrest the Pest.

How to tell SWD from other fruit flies

SWD is a small fly, only 2 - 3 mm (1/12 - 1/8 inch) long, with yellowish-brown coloration, dark colored bands on the abdomen, and prominent red eyes. They can be difficult to distinguish from other species of small fruit flies.

spotted wing drosophila on its side, with 1 visible translucent wing with a dark spot at the top  edge of each wing, and large red eye
Male spotted wing drosophila. Note the dark spot on tip of wing.

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.

However, while female SWD also have clear wings, they lack any spots on them. They are more difficult to identify. 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.

tiny white larvae inside a strawberry
SWD larva in a strawberry

SWD maggots (larvae) 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.

Life cycle

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.
  • The 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.
  • SWD overwinters as an adult.
  • There is evidence that some SWD are able to survive Minnesota winters.

Damage

tiny fruit fly on a black raspberry
Female SWD on a blackberry

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.

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 fungi that cause 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 then may be soft and maggot-infested a day or two later. Mature larvae often crawl out of fruit stored on the counter.

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.

During minor infestations, infested fruit can be processed into wine or jelly. During severe infestations, fruit is too rotten to be processed.

Managing SWD

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Jeff Hahn, Extension entomologist and Suzanne Wold-Burkness, College of Food, Agricultural and Natural Resource Sciences

Reviewed in 2018

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