- Size
- 8–10 mm
- Look
- Grey, with golden hairs on the thorax
- Behaviour
- Sluggish, gather in large groups
- Peak season
- Autumn, and warm days in winter and spring
They don't breed indoors and have nothing to do with hygiene. Their larvae live in soil, feeding on earthworms. The frustrating part is that they return to the same buildings year after year.
Signs of cluster flies
- Large numbers of slow, buzzing flies at upstairs or loft windows
- Clusters of flies in loft corners, around window frames or in unused rooms
- Buzzing in the loft on sunny autumn and winter days
- Piles of dead flies on windowsills and in the loft
How to be sure
House flies are smaller, faster and found in kitchens in summer. Cluster flies are sluggish, gather in upper floors and appear in autumn. If you hear buzzing in the loft in summer instead, consider a wasp nest.
What you can do yourself
- Vacuum up live and dead flies. Clearing dead flies also removes food for carpet beetles.
- In the loft, an electric fly killer or insect light trap can reduce numbers over the winter.
- Insecticide smoke generators designed for lofts can help in a closed, unoccupied loft. Follow the label carefully and keep pets out.
- Seal gaps around windows, eaves and roof vents where you can, in late summer before they arrive.
Call a professional if
- You get very large numbers every year
- You want the outside of the house treated in late summer, before they move in
Keeping them out
- Seal gaps around window frames and fascias in summer
- Fit fine mesh over roof vents where it won't block ventilation
- Clear dead flies from the loft each spring
Kit that helps
- Insect light trap or electric fly killer for the loft
- Loft insecticide smoke generator
- Vacuum
Product types only. We'll only ever point you to things we'd use ourselves.
Questions people ask
Are cluster flies dirty?
They don't breed in homes and aren't associated with rubbish or food. They're a nuisance rather than a hygiene risk.
Why do cluster flies come back every year?
They're thought to be drawn to the same buildings by scent and position, especially tall, pale or south-facing walls.
Will they go away on their own?
They leave in spring, but numbers often rise each year unless you reduce them.