Pest Control
How to stop cluster flies taking over your loft
Dozens of slow, sleepy flies around the loft window. Cluster flies come back to the same buildings every autumn, and the timing of what you do matters.
Opening the loft hatch to find dozens or even hundreds of slow-moving flies is a common autumn and winter surprise. They are almost always cluster flies. They are harmless, they do not breed indoors, and they tend to come back to the same buildings every year.
The short version
- Cluster flies spend the summer outdoors and come into lofts and roof spaces in late summer and autumn to overwinter.
- They do not bite, breed indoors or spread disease, but large numbers are messy and unpleasant.
- Timing matters: the best results come from proofing and treatment before or as they arrive, not in spring.
What are cluster flies?
Cluster flies are a group of flies in the genus Pollenia. They look like a house fly but are slightly larger and slower, with a dark grey body and fine golden hairs on the thorax. At rest their wings overlap neatly over the back.
Their larvae are parasites of earthworms, so the flies breed in grassland and fields, not in your home. They are not attracted to food or rubbish, which is why you find them in the loft rather than the kitchen.
Why lofts are so attractive to cluster flies
From late August into October, as the weather cools, the adult flies look for somewhere sheltered to spend the winter. They gather on sunny, south and west-facing walls, then crawl in through gaps at the eaves, under tiles, around windows and through vents. A loft is dark, dry and undisturbed, which suits them well.
Buildings near open grassland are most affected. The same buildings tend to be chosen year after year, so if you have had them once you are likely to have them again.
Through winter they are mostly dormant. A sunny day warms the roof and wakes some of them, and they head for the light, which is when they appear at loft windows, roof lights and sometimes in the rooms below.
Signs of cluster flies in your loft
- Clusters of flies in corners, around windows, on roof timbers or tucked into the roofing felt.
- Slow, sluggish movement. They are easy to catch compared with a house fly.
- A low buzzing from the loft on mild days.
- Small dark spots on beams, sills and stored items, and piles of dead flies near windows.
The odd fly in a loft is normal. Dozens or hundreds together in autumn and winter is the pattern that points to cluster flies.
Are cluster flies harmful?
No. They do not bite, sting or spread disease, and they do not lay eggs indoors. The problems they cause are practical:
- Mess from droppings and dead flies, which can mark surfaces and stored belongings.
- A sickly smell when large numbers are crushed or die together.
- Other pests, because a heap of dead flies is food for larder beetles and carpet beetles.
- Coming back every year if nothing is done.
How to deal with cluster flies
- Vacuum them up rather than squashing them, which stains surfaces. Empty the vacuum outside.
- Clear the dead flies from the loft in spring so they do not feed other insects.
- Use a fly screen or light trap at the loft hatch or a loft window to catch those that wake on warm days.
- Seal what you can around windows, eaves and service entries. Fit fine mesh over vents, but never block loft ventilation, which protects the roof timbers from condensation.
Proofing a whole roof against an insect this size is rarely possible, so treatment is usually needed where numbers are large. It works best at the right time: a residual treatment on the outside walls in late summer, before the flies move in, or a treatment of the loft space itself once they have settled in for the winter.
Preventing cluster flies from coming back
Because the same buildings are chosen each year, prevention is about the calendar. Check the loft in early autumn, deal with gaps before September, and if you have had a heavy infestation, book treatment for late summer next year rather than waiting until they are inside.
When to call us
If you have a loft full of flies, or they keep spilling into bedrooms on sunny winter days, we can confirm what they are, treat the loft safely and advise on timing so next autumn is better. See our fly control page for how we work.
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