Spotted wing drosophila (Drosophila suzukii) was first found in Canada infesting blueberries in the Fraser Valley in September 2009 and trapped soon after in a cherry orchard near Kelowna.
This pest of fruit crops is native to Southeast Asia where it feeds on a range of soft-skinned fruit. Unlike most species of vinegar fly (fruit fly) that feed on very ripe, damaged or rotting fruit, the spotted wing drosophila (SWD) is able to attack ripening, firm fruit.
Unlike other Drosophila species, SWD do not prefer rotting fruit, and their relationship to the spread of sour rot and other pathogens has not been demonstrated.
Adult SWD are slightly larger (2-3 mm) than other vinegar flies, and males have a dark grey to black spot toward the outer margin of each wing.
Females lack the wing spots and have a saw-like ovipositor visible with the aid of a good hand lens or microscope.
The elongate, creamy white, legless larvae, up to 3 mm long, are difficult to distinguish from other species of vinegar fly.
SWD overwinter as adults. Beginning in spring, several generations develop throughout the year on a range of available fruit, such strawberry, blackberry, cherry, etc., including grapes in the late summer and fall.
Females lay large numbers of eggs that hatch into larvae that mature at a rapid rate. A generation can develop in as little as 1-2 weeks when temperatures are warm, resulting in a rapid increase in numbers.
Larvae of several species of vinegar fly develop in damaged or decaying grapes late in the season, causing little apparent damage. A 2020 laboratory and field study conducted in the Okanagan Valley by Acheampong, Lord & Lowery found that SWD cause little direct economic damage to wine grapes. They were not found in intact, healthy fruit and formed a very small proportion of all vinegar fly species in damaged or rotting wine grapes.
SWD were found, however, infesting a small proportion of apparently healthy table grapes of several varieties. The presence of vinegar fly larvae and pupae in fresh table grapes is a major concern. Feeding by larvae directly damages the fruit, and the holes created during egg-laying allow the entry of disease-causing microorganisms.
SWD has not been shown to cause economic damage to intact wine grapes in B.C., and there are no established thresholds.
For table grapes, trap counts do not correlate with levels of damage, and producers currently need to rely on prophylactic sprays to susceptible table grape varieties.
For more information about chemical control, see the Pesticides page.
Parasitic wasps have been reared from SWD in B.C., and predators of other species of vinegar flies are likely to also attack this species.
Sanitary measures suggested for other fruit crops (i.e. removal of infested and unharvested fruit) might not be practical for commercially produced grapes.
Increased exposure of fruit early in the season (see Leafhoppers) that results in fruit with thicker skins and a heavier wax layer, combined with judicious use of fungicides, provides the best means to manage sour and bunch rots that are associated with increased numbers of vinegar flies.
Additional research would be helpful to determine the relative susceptibilities of table grape varieties and conditions that favour SWD infestations.
Visit the Ministry of Agriculture's information page on SWD.
