Growing Green

By Robin Trott, Extension Educator

Spotted wing drosophila (SWD, Drosophila suzukii) is an invasive fruit fly that has been present in Minnesota since 2012. It is a pest of home garden fruits,  especially raspberry, blackberry, blueberry, grape, aronia, strawberry and possibly tomato and apple.

SWD females use a serrated ovipositor to lay eggs in fresh, ripe fruit. They do not require damaged, cracked or rotting fruit to lay eggs. Eggs are laid in the flesh of soft-skinned fruit just prior to harvest and as sugars begins to accumulate and fruits start to change color.

The eggs hatch into tiny, white larvae that feed inside, causing the fruit to deteriorate. Larvae grow to 1/4-inch before dropping from the fruit to pupate and become adults. SWD can develop from egg to adult in 10 days to a month in cooler weather. Adults live for about two weeks, and each female can lay over 300 eggs, with successive generations attacking fruits through harvest.

Egg-laying punctures and larvae cause rapid decay in infested fruits. Though not harmful to eat, larvae make the fruit unappealing and unsuitable for storage. SWD adults are 1/8 inch long, yellowish-brown with dark abdominal bands and red eyes. Males have a dark wing spot; females don’t but have a serrated ovipositor. SWD overwinters in tree rows and wooded areas, becoming noticeable by mid-July and remaining active until frost.

Management

• Sanitation: Harvest frequently and cleanly. Remove and destroy fallen, damaged and overripe fruit.

• Refrigeration: Chill fruit immediately after harvest to slow larvae development. Infested fruits are safe to eat.

• Exclusion netting: Cover small plantings with fine netting (openings less than 1 mm) before flies arrive. Seal completely to prevent entry.

• Monitoring: Place traps when fruit starts to color. Check and replenish traps regularly. Construct traps from clear plastic cups with holes, filled with apple cider vinegar and soap, and a yellow sticky trap. Hang in shaded areas, check every other day, and replace vinegar weekly or after heavy rain.

Trapping SWD helps determine when to use insecticides, which control adult flies before egg-laying. Multiple applications may be needed throughout the harvest season. Follow label directions, especially regarding the pre-harvest interval.

Effective insecticides for home gardeners include spinosad and malathion.

For more information, visit https://extension.umn.edu/yard-and-garden-insects/spotted-wing-drosophila.

Until next time, happy gardening!