Spotted Wing Drosophila on Blueberries — How to Manage This Pest

Spotted wing drosophila (Drosophila suzukii, often abbreviated as SWD) is the pest that has most changed the difficulty of growing soft fruit over the past fifteen years. Unlike conventional fruit flies that attack only overripe or damaged fruit, SWD can lay eggs inside firm, ripening blueberries before there is any external sign of damage. The first indication that your crop is infested is often a ripe or near-ripe berry that collapses suddenly into mush, or a small sunken dent on an otherwise normal-looking berry. By then, the larvae are already inside.

Identifying spotted wing drosophila

The adult fly is small — about two millimetres long — and looks similar to other small fruit flies. The distinguishing feature is a dark spot near the tip of each wing in males. The females are identified by their serrated, saw-like ovipositor, which allows them to cut through the skin of firm fruit — a capability that distinguishes them from the common fruit fly, which can only enter through breaks in the skin. Infested berries initially appear normal but develop a small, slightly sunken puncture mark at the egg site, then soften and collapse rapidly as the larvae feed inside.

Why blueberries are particularly vulnerable

SWD populations build through summer and peak in late summer and early autumn, which coincides with the ripening window for mid- and late-season blueberry varieties. Early-season varieties that ripen in June and July often escape the worst pressure. Late-season varieties ripening in August and September are most heavily exposed. Any ripe or over-ripe fruit left on the bush or on the ground is a further breeding site that feeds the next generation of flies.

Physical exclusion: the most effective control

The single most effective control is excluding the flies from the ripening fruit using fine-mesh netting. The mesh aperture must be one millimetre or smaller — larger mesh allows flies through. Standard bird netting does not exclude SWD. A properly fitted fruit cage with fine mesh net installed before the berries begin to colour prevents SWD from reaching the fruit entirely. Ensure there are no gaps around the edges; SWD will find them.

Harvesting and hygiene

Harvest frequently — every two to three days at the height of the season — and never leave ripe or overripe berries on the bush. Collect and dispose of all fallen fruit immediately: freeze it, seal it in bags for the general waste, or submerge it in water to drown any larvae present. Do not compost infested fruit. The less ripe and rotting fruit is available around and below the bush, the lower the local SWD population pressure will be.