Quick facts
- Species
- Pollenia species, often called cluster flies or attic flies
- Size
- About 8 to 10 mm, slightly larger than a house fly
- Appearance
- Dark grey with golden hairs on the thorax; they fold their wings flat over the back
- When
- They gather in autumn and reappear on warm days in late winter and spring
- Where
- Lofts, roof spaces, unused upstairs rooms and window frames, especially on south and west facing walls
How to identify cluster flies
Cluster flies look like large, slow house flies. They have a dull grey body, a covering of short golden hairs on the thorax and wings that overlap neatly when at rest. They are sluggish, buzz loudly against windows and often seem half asleep, which is exactly what they are: they are hibernating.
Why they choose your home
Cluster flies breed outdoors. Their larvae are parasites of earthworms in grassland, so homes near parks, playing fields, golf courses and open land see the most. In autumn the adult flies look for sheltered, dry places to overwinter, and the warm, sunlit upper walls of houses act as a beacon. The same houses tend to be chosen year after year. That is why we see cluster flies more often in houses on the edges of South London, such as around Chislehurst, Orpington, Eltham and Shooters Hill, and in parts of Kent, than in central flats.
Signs of cluster flies
- Clusters of flies in loft corners, around roof lights and in window frames
- Flies appearing indoors on sunny days in winter and early spring
- Dead flies collecting on window sills and loft floors
- A sweet, sickly smell where large numbers have died
Are cluster flies harmful?
Cluster flies do not breed indoors, do not feed on your food and are not linked to dirt or disease the way house flies are. The problem is the sheer number of them, the noise and the mess of dead flies.
Professional cluster fly treatment
Treatment usually involves a fogging or residual spray treatment in the loft or roof space, timed for when the flies have gathered, followed by clearing the dead flies. In occupied rooms we use targeted treatments around window frames and entry points. Because new flies arrive each autumn, some homes choose a yearly treatment.
Prevention
- Seal gaps around window frames, soffits, fascias and roof tiles before late August
- Fit fine insect mesh over loft vents and air bricks where it does not block ventilation
- Vacuum up clusters as soon as they appear so they do not attract more
Cluster fly control across Southwark and London
We deal with cluster fly problems in homes and businesses across South London. We cover South London, Central London (the City, the West End and Westminster), East London out to Stratford and Newham, and nearby parts of Kent, from our base at South Dock Marina in SE16. A standard visit is £90, and you can book online or call 020 8050 2022.
- Southwark
- Bermondsey
- Peckham
- Camberwell
- Rotherhithe
- Lambeth
- Brixton
- Lewisham
- Greenwich
- Bromley
- Bexley
- Kent
- Central London
- City of London
- Westminster
- East London
- Tower Hamlets
- Hackney
- Newham
- All areas
Cluster fly control: questions and answers
Why do I get flies in my loft every autumn?
Cluster flies return to the same buildings each year to hibernate, drawn by warm, sunlit walls. Proofing and a timely treatment reduce the numbers.
Are cluster flies a sign my house is dirty?
No. They breed in grassland and only use your home to shelter in winter. They do not breed indoors.
When is the best time to treat cluster flies?
Early autumn, once the flies have started to gather, or on mild days in late winter when they become active again.
Can I use fly spray?
Household sprays kill the flies you can see but not those hidden in cavities and roof spaces. A professional loft treatment reaches far more of them.
Related pages
This page is general information, last reviewed on 19 October 2026. It is not a substitute for an on-site inspection.