Every autumn, people across the UK climb into the loft, or open a spare bedroom that hasn't been used for a while, and find hundreds of dull, slow flies crowded around the window. More turn up on warm days through winter, buzzing against the glass or spinning on their backs on the sill. Those are almost always cluster flies.
The good news is that cluster flies don't bite, don't breed indoors and aren't linked to spreading disease. The frustrating part is that they tend to come back to the same houses year after year, and no spray on its own stops that. This guide explains what they are, why your home in particular attracts them, and what actually reduces the numbers.
Quick answer: what are cluster flies and how do you get rid of them?
Cluster flies are slightly larger, darker relatives of the house fly that spend summer outdoors and hibernate in lofts and upper rooms from autumn to spring. They are a nuisance rather than a health risk. Vacuum up the flies you can reach, seal gaps around windows and the roofline before late summer, and use professional treatment for large loft clusters.
If you're in Tonbridge, Tunbridge Wells or elsewhere in West Kent and not sure whether you're dealing with cluster flies or something else in the loft, a pest control survey will tell you what's there, where it's getting in and whether treatment is worth it.
Cluster flies or house flies? How to tell the difference
At a glance the two look similar, which is why most people assume they have a house fly problem. The differences show up when you look closely at a fly sitting still on the window.
| Feature | Cluster fly | House fly |
|---|---|---|
| Size | About 7 to 8mm, a little bigger than a house fly | About 6 to 7mm |
| Colour | Dark grey to olive, with a chequered grey pattern on the abdomen | Grey, with four dark stripes along the thorax |
| Hairs | Short, golden or yellowish hairs on the thorax | No golden hairs |
| Wings at rest | Folded over each other across the back, like closed scissors | Held apart in a V |
| Behaviour | Slow, sluggish, often found dazed on sills or spinning on their backs | Quick and active, hard to swat |
| Where you see them | Lofts, upper rooms, windows in rooms that are rarely used, autumn to spring | Kitchens, bins, food areas, mostly in warm months |
| Breeds indoors? | No. Larvae develop in soil, inside earthworms | Yes, in food waste, rubbish and other decaying matter |
The golden hairs are the giveaway, so check a few flies rather than one. Crushed cluster flies also have a sweetish smell, and a large mass of them gives off a noticeable odour in an enclosed loft.
If the flies are active in the kitchen in summer, hanging around bins or food, you are probably dealing with house flies or blowflies instead, and the cause is a breeding site nearby rather than a loft that's being used for winter shelter.
Why cluster flies end up in your loft
Their life cycle starts with earthworms
Cluster flies (Pollenia species, with Pollenia rudis the most common in Britain) spend the warmer months outdoors. Females lay eggs in damp soil, often near the openings of earthworm burrows. The eggs hatch in about a week, and the larvae work their way into earthworms and feed on them before pupating in the soil. In the UK there are usually two generations a year, and more in a hot summer.
That's why cluster flies are more common in rural and semi-rural areas, and around homes that back onto fields, lawns, paddocks or grazing land. Lots of healthy soil means lots of earthworms, which means lots of flies.
Autumn sends them looking for shelter
From late summer, as the days shorten and nights cool, the last generation of adults starts looking for somewhere dry and sheltered to spend the winter. They gather on the warm, sunny walls of buildings first, usually the south and west facing sides, then crawl into any gap they can find: around window frames, under fascias and soffits, through gaps in tiles and roof felt, around vents and where pipes and cables go through the wall.
Once inside, they move upwards into the loft, wall cavities and the upper corners of rooms. They are not breeding or feeding there. They're simply hibernating, living off fat stored in their bodies before winter.
Why the same houses get them every year
Many homeowners notice that their house gets cluster flies every autumn while the neighbours seem fine. Several things make a property more attractive:
- It stands out in the landscape, for example on a hill or at the edge of open countryside
- It has large south or west facing walls or roof slopes that catch the afternoon sun
- It is surrounded by grassland, lawns or farmland with plenty of earthworms
- It has a roof space with lots of small gaps, such as an older tiled roof or unsealed eaves
The British Pest Control Association notes that cluster flies may return to the same location year after year. Flies hibernating in the loft this winter are a good sign that more will try again next autumn.
Signs you have cluster flies
You don't usually find a "nest". Cluster flies don't build one. What you find instead is:
- Large numbers of sluggish flies at the windows of the loft, landing or rooms that aren't heated much
- Dead flies collecting on windowsills, along skirting boards and on the loft floor
- Flies appearing indoors on sunny days in winter or early spring, often seeming to come from nowhere, around light fittings, ceiling roses or window frames
- Dense clumps of flies in the corners of the loft, around the hatch, or between rafters
- A sweet, musty smell in the roof space when numbers are high
The "appearing from nowhere" part catches people out. On a mild winter day the flies wake up, move towards the warmth and light of the house below, and find their way out through gaps around loft hatches, downlighters and window reveals.
Are cluster flies harmful?
Cluster flies don't bite or sting, they don't damage furniture or the structure of the house, and they don't breed indoors. Unlike house flies, they don't feed on food waste and aren't associated with spreading disease.
They do cause some problems, though:
- Sheer nuisance. Hundreds of flies buzzing at the windows of a bedroom or holiday let is unpleasant, and it's worse when guests or tenants are involved.
- Smell. A large mass of cluster flies gives off a noticeable odour in an enclosed space.
- Dead flies attract other pests. Piles of dead flies left in a loft are food for other insects. Carpet beetle larvae and larder beetles feed on dead insects, so a loft full of dead flies can lead to a second problem.
If you find something else in the loft while you're up there, such as papery wasp comb or droppings, it's worth reading our guide on wasp nests in lofts and wall cavities or on noises in the loft, because those need a different response.
How to get rid of cluster flies
There's no single fix that clears cluster flies for good, and anyone promising one is overselling. What works is combining removal now with prevention before next autumn.
What you can do yourself
- Vacuum them up. Cluster flies are slow and make little effort to escape, so a vacuum with a crevice tool is the quickest, cleanest way to clear the windows and loft corners. Empty the vacuum into a sealed bag and put it in the outside bin straight away so the dead flies don't feed other insects.
- Clear up dead flies regularly. Sweep the loft floor, windowsills and skirting boards every few weeks through winter rather than letting them build up.
- Keep unused rooms closed off. Shut doors to spare rooms and the loft hatch so flies waking on warm days don't spread into the rest of the house.
- Seal the indoor routes. Fill gaps around loft hatches, ceiling roses, downlighters, window reveals and where pipes come through ceilings. This doesn't stop flies getting into the roof, but it keeps them out of the living space.
- Open the window on a mild day. When flies gather at a window, opening it lets some leave. It won't solve the problem, but it can help in a spare room.
Fly papers and sticky traps aren't much help indoors. Cluster flies don't fly around much once they've settled to hibernate, so they rarely end up in them. There's also little evidence that the "natural" remedies you'll find online, such as essential oils, keep them away.
Stopping them getting in next year
Prevention is mostly about timing. Gaps need to be sealed before the flies start looking for shelter in late summer, not once they're already inside.
- Check and seal gaps around window frames, door frames and where pipes, cables and vents pass through external walls
- Look at the roofline: fascias, soffits, gaps under the eaves and around the edges of the roof
- Fit fine mesh over roof and soffit vents where it won't block ventilation the roof needs
- Repair damaged or missing pointing, tiles and flashing
Be realistic about what this achieves. The BPCA's own guidance says that preventing cluster flies in some properties is mostly impossible, because you can't remove the fields and earthworms they breed in, and a roof can never be made completely airtight. Sealing the obvious gaps does reduce the numbers and, more importantly, keeps them out of the rooms you live in. If you want a hand finding and sealing those gaps, our pest proofing service covers exactly this kind of work.
When professional treatment makes sense
For a few dozen flies at a window, a vacuum is all you need. Professional treatment is worth considering when:
- There are large clusters in the loft, in the thousands, every winter
- Flies keep coming into bedrooms or living areas despite sealing the indoor gaps
- It's a rental, holiday let or business where guests and tenants can't be expected to put up with it
- You're not sure the flies are cluster flies, or you've found other signs of pests in the roof
A professional treatment for cluster flies in a loft usually involves a space treatment, such as a fog or smoke, carried out by a trained technician when the flies are clustered. You'll be asked to keep out of the treated area for a few hours afterwards. It kills the flies present at the time, so it's best done when they've gathered rather than in summer, and it doesn't stop new flies arriving next autumn. That's why we always pair it with advice on proofing.
Check for bats before any loft treatment
This is the one step that should never be skipped. Lofts are also used by bats, and in the UK all bat species and their roosts are protected by law, whether or not bats are present at the time. A responsible pest controller will check the roof space for bats and bat droppings before using any treatment there, and won't treat if a roost is found.
If you think you have bats as well as flies, contact the Bat Conservation Trust for advice before doing anything in the loft.
What not to do
- Don't set off a shop-bought smoke or fogger in the loft without checking for bats first. It's a legal risk as well as a wildlife one.
- Don't leave piles of dead flies in the loft. Clear them up so they don't attract beetles.
- Don't seal the outside of the roof in midwinter and expect it to work. The flies already inside are trapped in with you. Seal in summer, before they arrive.
- Don't block roof or soffit vents completely. Lofts need ventilation to prevent condensation and damp. Use fine mesh, not filler.
- Don't spray insecticide around windows and bedding indoors. It isn't needed for a hibernating fly that doesn't feed or breed, and it means more chemicals in the rooms you live in.
Frequently asked questions
Why do I suddenly have cluster flies in my house?
Usually because the weather has changed. Flies that crawled into the loft or wall cavities in autumn wake up on warm, sunny days and head towards the warmth and light of the rooms below. They have often been there for weeks before you notice them.
Do cluster flies bite?
No. Cluster flies don't bite or sting people or pets, and they aren't known to spread disease. They are a nuisance pest.
Do cluster flies lay eggs in the house?
No. Their larvae develop in soil, feeding inside earthworms, so they can't breed in a loft or living space. The flies you see indoors are adults sheltering for the winter.
How long do cluster flies stay?
They usually arrive from late summer to autumn and stay until spring, when they leave to lay eggs outdoors. Through winter they appear on and off whenever the weather turns mild.
Will cluster flies go away on their own?
The live ones will leave in spring, but the problem tends to return. Cluster flies often come back to the same building every autumn, so without sealing gaps you can expect them again next year.
What smell do cluster flies hate?
There isn't good evidence that any smell keeps cluster flies away. Sealing gaps before late summer and vacuuming up the flies that do get in is far more reliable than scents or essential oils.
Can you get rid of cluster flies permanently?
Not completely in most rural or edge-of-town homes, because the fields and soil they breed in are outside your control. You can reduce the numbers a lot by proofing the roofline and windows and keep them out of your living space by sealing indoor gaps.
Need help with cluster flies?
If you've got flies massing in the loft every winter, or you're not sure whether it's cluster flies or something else in the roof, and you are in Tonbridge, Tunbridge Wells, Sevenoaks or another area we cover across West Kent and selected nearby parts of Surrey and Sussex, speak to Protego Pest Control.
We are a local pest control team in Tonbridge, BPCA Level 2 trained, part of the Prompt CPD Scheme and fully insured. We are available for contact 24/7 and will give you straightforward advice based on what you have found. Find out more about our pest control surveys.
Contact Protego Pest Control, call 07897 314478 or email info@protegopestcontrol.co.uk.
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