Tiny white maggots in just-picked berries may be the first noticeable sign of a spotted wing drosophila (SWD) infestation. This nefarious new menace to fall fruit crops throughout North America can cause severe losses to commercial growers and backyard gardeners.
“It will be a problem on any berry or soft fruit that ripens from mid-July on,” says Kathy Demchak, a Pennsylvania State University horticulturist.
Using a sawlike ovipositor, the female SWD lays eggs in ripening fruit—mainly fall raspberries, late-season blackberries, and day-neutral strawberries. It also attacks blueberries, summer blackberries and raspberries, peaches, nectarines, hardy kiwi, cranberries, cherries, and even grapes. Tissues surrounding egg-laying sites begin to discolor and decay within a few days. Larvae hatch from the eggs, growing up to 1⁄8 inch long as they burrow and feed inside the fruit.
Native to Southeast Asia, the SWD appeared in California in 2008. Within 3 years, it had arrived in Florida, Michigan, and parts of Canada, and it continues to spread.
About 1⁄10 inch long, the fly is light yellow or brown. Adult males usually have one obvious spot on the leading edge of each wing close to the tip, plus two black bands on each front leg. Females do not have spotted wings.
The fly moves freely on infested fruit. Cold weather slows its spread, but as temperatures return to the 70s, populations explode with eight to nine generations a year.
Cultural strategies offer the best control, Demchak says. Picking ripe fruit daily may be enough to fend off the fly. Refrigerate harvested fruit to slow development of any eggs present and kill some of the larvae that do hatch. Remove and destroy culls and overripe fruit; don’t compost them. Most compost piles aren’t hot enough to kill SWD eggs and larvae.
Finally, consider removing nearby wild hosts, including wild raspberries and blackberries, cherries, mulberries, dogwoods, viburnums, and even pokeweed. SWD thrives in dense foliage with high humidity and moderate temperatures.
Trapping usually does not control SWD populations, but it can help to identify and monitor the pest’s presence. Research is beginning on predatory insects as a potential control. Two insecticides approved for organic growers, Pyganic and Entrust (a spinosad), should be used only as a last resort. West Coast SWD populations are already resistant to natural pyrethrins.
Illustration: Jack Unruh