Spotted Wing Drosophila is a common pest that affects strawberries, cherries and gooseberries in Scotland. Here is what it looks like, how to spot the damage, and which controls have been tested.
What does it look like?
Adults are small, pale brown flies, roughly 2-3mm long. Males carry a single dark spot near the tip of each wing; females lack the spot but have a serrated, saw-like egg-laying organ instead. Larvae are small white maggots up to 4mm long, found inside the fruit rather than on its surface, and can't be told apart from ordinary fruit fly maggots by eye.
What causes it
Most vinegar flies (the wider fruit-fly family this species belongs to) can only get into fruit that's already split, bruised or overripe. This species is different: the female's serrated egg-laying organ lets her cut directly into the skin of firm, ripening fruit, so damage can start on fruit that would otherwise still be perfectly sound.
Is this pest active now?
Watch from around April, when overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → adults become active again, right through to November. In Scotland specifically, the real build-up in numbers comes later and lighter than that full window suggests, often after strawberries, cherries, gooseberries and blueberries here have already been picked.
🌱 Plants at risk
Spotted Wing Drosophila also affects these crops:
Symptoms
Fruit collapses or develops small puncture marks, often with a tiny hole visible at the centre of each, on skin that otherwise still looks sound. Cutting into affected fruit usually finds white larvae feeding inside, and juice can seep from the puncture sites as the fruit breaks down.
Life cycle
Several generations are produced in a single season, each building on the last through summer before the population dies back for winter. Adults spend the colder months sheltering in nearby hedges, shrubberies and woods rather than dying off completely.
Why this matters in Scotland
First recorded in Scotland in 2014, with over a decade of trap monitoring since.
Numbers have been rising here since 2021, even though trap catches still run lower than England's and peak later, in November rather than through the broader UK's summer build-up. That upward trend, not the current gap with England, is why this earns a moderate rating rather than a low one: the population base to build from is already established and growing.
Controls: what actually works
All the methods below are backed by real evidence, not just repeated folklore.
Crop hygiene: harvest promptly and bin fallen or damaged fruit
Pick ripe fruit without delay, and clear fallen, surplus or damaged fruit rather than leaving it on the plant or ground. Bury it or seal it in a bag rather than adding it to the compost heap, since compost doesn't stop the fly breeding in it.
Fine mesh: net or sleeve fruit before it ripens
Standard fruit-cage netting has gaps wide enough for this fly to pass through. Mesh under 0.98mm is needed instead, distinctly finer than the netting used against most other fruit pests, or ready-made sleeves fitted over individual trussesThe flowering, then fruiting, stem cluster on a tomato plant; each truss ripens its fruit together. Full definition → or branches.
Monitoring: set traps to catch adults early
Vinegar or red-wine-baited traps, homemade or bought, catch adults before they've had a chance to lay eggs. Check traps regularly from spring through to November, matching how late in the year this species stays active.
Myths about spotted wing Drosophila
5 common claims about spotted wing Drosophila, checked: 2 busted1 disputed0 untested2 hold up
“Spotted wing drosophila, like other vinegar and fruit flies, can only lay eggs in fruit that's already split, bruised or overripe, so firm ripening fruit is safe from it.”
This is the one thing that sets spotted wing drosophila apart from the ordinary vinegar flies gardeners are used to. Females carry a serrated, saw-like egg-laying organ that can cut directly into the skin of firm, ripening fruit, rather than needing an existing split or soft spot to get in. That means fruit that still looks completely sound on the outside can already have eggs or larvae inside it.
What to do: Treat ripening fruit as at risk before it shows any damage: get traps and fine mesh in place early in the season rather than waiting for visible puncture marks or soft patches as a warning sign.
1 source
- RHS, Spotted wing drosophilaUnlike most native vinegar flies, which feed on decomposing fruit, spotted wing drosophila females can cut into and lay eggs in firm, ripening, unblemished fruit.
“Standard fruit cage or brassica netting is fine for keeping spotted wing drosophila off soft fruit.”
The mesh gaps on most fruit cage and brassicaThe cabbage family: cabbage, kale, broccoli, cauliflower, Brussels sprouts, kohlrabi, swede, turnip and more, sharing the same pests, diseases and rotation rules. Full definition → netting are too wide for this fly. Effective exclusion needs mesh under about 0.98mm, distinctly finer than the netting sold for keeping birds or larger insects off soft fruit, or individual sleeves fitted over trusses and branches before the fruit ripens.
What to do: Check the mesh size before buying or reusing netting for soft fruit at risk from this pest; if it's not specified as sub-1mm insect mesh, it almost certainly won't stop adults getting through to lay eggs.
1 source
- RHS, Spotted wing drosophilaVery fine mesh netting, less than 0.98mm, is needed to physically exclude adult spotted wing drosophila from ripening fruit.
“Putting out a handful of vinegar or red wine traps around the fruit patch is enough on its own to stop spotted wing drosophila damage.”
RHS material on this pest is not fully consistent on what trapping alone achieves. Its main pest page frames trapping as primarily a monitoring tool, catching adults so gardeners know when to act, working alongside mesh and hygiene rather than replacing them. Its separate grow-your-own feature goes further, describing traps placed along a garden's boundary as actually intercepting incoming flies and drawing existing ones away from the fruit itself, which implies a real population effect and not just detection.
Our read Traps clearly matter for timing, since they show when adults are active and eggs are about to be laid, but there's no suggestion that trapping by itself prevents damage without netting and prompt fruit clearance alongside it. Treat trapping as one part of a combined approach rather than a standalone fix.
What to do: Use traps to time when to start netting and tightening up fruit hygiene, not as a substitute for either. Run all three together through the April to November activity window.
2 sources
- RHS, Spotted wing drosophilaTrapping and monitoring is the best way to prevent damage to fruit, used alongside physical barriers once flies are detected.
- RHS, Grow Your Own feature on spotted wing drosophilaTrapping along the garden boundary from April to November will intercept flies invading from nearby hedges and woods, and attract flies already present away from fruit bushes.
“Soaking suspect fruit in a strong salt water solution for a few minutes will make any drosophila larvae inside float out, without needing to cut the fruit open.”
This is a genuine detection method, developed and documented by university fruit research programmes rather than a home remedy. Fruit is submerged in a saturated salt solution in a light-coloured, shallow container; the salt irritates any larvae inside, which crawl out and float to the surface within about fifteen minutes, where their pale, tapered bodies, up to 3 to 4mm long, are visible against the pan.
What to do: If damage is suspected but larvae aren't visible by eye, dissolve about a quarter cup of salt per litre of warm water, submerge the fruit in a pale shallow dish, weight it down, and check after fifteen minutes for larvae that have floated free.
1 source
- Rutgers University, Philip E. Marucci Center for Blueberry and Cranberry Research: Salt Float Method for SWD DetectionSubmerging fruit in a salt water solution in a pale shallow pan causes spotted wing drosophila larvae to emerge and float to the surface within about fifteen minutes, confirming infestation.
“Among garden soft fruit, cherries are more at risk from spotted wing drosophila than strawberries and other berries.”
Cherries are identified as the crop most vulnerable to this pest among the soft fruit gardeners commonly grow, ahead of strawberries, gooseberries, blueberries and grapes, all of which are also confirmed hosts. Thin skin and a short ripening window before harvest both favour the fly getting eggs into the fruit before it's picked.
What to do: Prioritise cherries for the finest mesh and the earliest netting if growing several soft fruit types and only able to protect some of them straight away.
In Scotland: Scottish trap monitoring shows spotted wing drosophila numbers building later and lighter than in England, often after cherries and other early soft fruit here have already been picked, which buys extra time to get netting and traps in place before the main risk period.
2 sources
- RHS, Grow Your Own feature on spotted wing drosophilaRHS experience suggests cherries are the most vulnerable crop to spotted wing drosophila among soft fruit.
- James Hutton Institute, Development, monitoring and control strategies for spotted wing drosophila in ScotlandScottish trap catches of spotted wing drosophila run lower than in England and peak later in the year, typically in November rather than through summer.
Frequently asked questions
What should I do first if I find spotted wing drosophila in my garden?
Start with crop hygiene. After that, fine mesh. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
Will spotted wing drosophila kill my plants?
They rarely kill established plants. The damage is usually cosmetic on outer leaves, and natural predators often bring numbers under control without you doing anything.
When are spotted wing drosophila most active in Scotland?
Check the seasonal activity section above. Scottish timings can run a few weeks behind the UK-generic figures.
Sources
3 sources, recorded with what each was used for
- RHS1: identification, symptoms, the intact-fruit egg-laying mechanism, life cycle and overwintering, fine mesh sizing, trap monitoring into November, absence of a specific amateur pesticide recommendation
- RHS1: cherries as the most susceptible crop, prompt harvesting and disposal of fallen or damaged fruit, spring reactivation timing, trapping window from April to November
- James Hutton Institute2: Scottish entomology monitoring: first detection year in Scotland, the rising trend since 2021, and the later, smaller seasonal peak compared with England