Spotted wing drosophila
Drosophila suzukii
also called SWD, Cherry vinegar fly
A vinegar fly that lays in sound ripening fruit rather than spoiled, so berries collapse into maggots on the bush. Pick early, pick clean, and chill the harvest at once.
How to know it
A vinegar fly 3 mm long with red eyes; the male carries one dark spot near the tip of each wing, which is what names it. The female has no spot but a saw-edged ovipositor she uses to cut into firm fruit — the reason this fly matters and the ordinary vinegar fly does not.
What you'll see
- A soft, sunken dimple on an otherwise perfect berry, with a tiny breathing scar at the center.
- Raspberries and blackberries that collapse into mush a day after picking.
- White maggots 3 mm long in the flesh when a berry is broken open, or floated out in salt water.
- Fruit souring and leaking on the bush, with vinegar flies gathering on it.
- A late crop that is far worse hit than the early one from the same plant.
Where it comes from
Adults overwinter in sheltered leaf litter and hedge bottoms and become active in spring, but numbers stay low until midsummer and then rise very steeply — a generation takes as little as ten days in warm weather. Females lay in fruit at the first blush of color, and the maggots are through the flesh in three or four days.
Warm, humid summers and a long soft-fruit season are what build the population. Everblooming raspberries and late blackberries take the worst of it because they ripen at the peak; June strawberries often escape entirely.
Dealing with it
- Pick every two days and take the whole crop, including the over-ripe and the fallen — leaving them is feeding the next generation.
- Chill fruit to fridge temperature within an hour of picking; cold stops the eggs already in it from developing.
- Bag the culls in the sun for a week or freeze them, and never compost infested fruit in an open heap.
- Cover a small planting with very fine exclusion mesh (under 1 mm) from first color, sealed at the ground.
- Hang cider-vinegar traps to tell you when the flight starts; they monitor well and control poorly.
Keeping it out
- Grow early cultivars that finish before the population peaks, and think hard before adding a late one.
- Prune canes and bushes open so the fruiting zone dries and lights quickly.
- Clear wild brambles, elderberry and honeysuckle nearby, which carry the fly between crops.
- Keep the ground under the bushes clean of dropped fruit all season.
- Harvest strawberries at full color rather than leaving them a day for sweetness.
What it goes for
Raspberries, blackberries, blueberries, strawberries, cherries, grapes, plums and peaches.
Who hunts it for you
- Parasitic waspsBraconidae & Trichogramma Tiny, stingless wasps that lay eggs in or on caterpillars, aphids, and moth eggs — the white cocoons riding a hornworm are their calling card, so leave that caterpillar be. Umbel flowers with shallow nectar keep the adults fuelled. Grow to recruit it: Cilantro,Dill,Fennel,Lovage,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com