Spotted-wing drosophila (SWD)

Description

Spotted-wing drosophila is a small, invasive, vinegar fly. It is native to southeast Asia but has been rapidly spreading around the world. It was seen in Japan as early as 1916, and by the 1930s it had spread throughout eastern Asia. In North America it was first seen on the west coast in 2008 and on the east coast in 2010. It was first recorded in Minnesota in 2012. It has been much studied, written and talked about, and is commonly referred to by the acronym SWD. It can be found from spring through fall in agricultural fields, orchards, and gardens, and year round in kitchens.

SWD is a serious crop pest of summer fruits, including cherries, blueberries, grapes, nectarines, pears, plums, pluots, peaches, raspberries, and strawberries. Unlike fruit flies, which feed on decaying fruit and vegetation, SWD lays its eggs on healthy fruit.

Adults are yellowish, 5⁄64″ to 9⁄64″ (2 to 3.5 mm) in length, and look like fruit flies.

The head is yellowish-brown. There are two large compound eyes on the sides of the head and three small simple eyes (ocelli) in a triangle on top of the head. A pair of bristles (setae) behind the ocellar triangle (postocellar setae) are well developed. The compound eyes are red. They do not meet at the top of the head on either sex. The upper part of the face has sparsely scattered bristles. On each side, near the margin of the compound eye, there are two bristles. One is bent down at the base (reclinate seta), the other is bent upward at the base (proclinate seta). The proclinate seta is larger and is above and slightly outside of the reclinate seta. There are two stiff, well-deveolped bristles (vibrissae) below the antenna bases. There is a ridge (carina) in the middle of the face. The carina is broad and well developed, including on the lower part of the face. The antennae are very short and have three segments. The second segment does not have a longitudinal groove (suture) on top. The third segment has a long, forward-pointing bristle (arista) on the upper side. The arista is branched (plumose), with several branches above and two or three branches below.

The thorax is yellowish-brown above and on the sides, with no stripes or spots. It has three segments. Each segment has four principal exoskeletal plates, one above, one below, and one on each side. The upper (dorsal) plates, from front to rear, are the prescutum, scutum, and scutellum. The prescutum and scutum have several large black bristles and numerous shorter hair-like bristles. There are eight longitudinal rows of small bristles in the middle of the scutum, and two pairs of large bristles near the rear margin. The lower front plate of the middle segment (katepisternum) has three large bristles.

The abdomen has six visible segments on both the male and female. On the female, segments 2 through 6 are pale yellow with an unbroken, dark band on the rear margin. On the male segments 2 through 4 are similar to the female, segments 5 and 6 are completely dark.

The wings are clear. Their is a vein (costa) on the leading edge (costal margin) of the wing. The costa is broken twice. On the male there is usually a dark cloudy spot at the end of each radial vein R₂₊₃ and R₄₊₅. This is the feature that gives the fly its common name. On the female the wings are clear with no dark spots or white markings.

The legs are yellow. The last part of the leg, corresponding to the foot (tarsus) has five segments. The first segment is longer than the second, and is not swollen. On the front leg of the male there is a black comb on the first and second segments. The comb on the first segment has 4 to 6 teeth, the comb on the second has 2 or 3 teeth.

Size

Total length: 5⁄64″ to 9⁄64″ (2 to 3.5 mm)

Similar Species

Habitat

Agricultural fields, orchards, gardens, and kitchens

Ecology

Season

Up to 13 generations per year: Year-round

Behavior

Life Cycle

Larva Food/Hosts

Adult Food