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If your loft window is crawling with slow, dozy flies every autumn, here is the short answer: for a small number of flies, a vacuum cleaner and a few sticky fly papers will do the job. For a loft that fills up year after year, an insecticidal smoke generator labelled for flying insects is the most effective product you can buy, as long as you have checked for bats first. For a large or recurring infestation in a big or old building, a professional residual treatment is the only thing that keeps working through the winter.
Below we go through every type of cluster fly killer and trap, what each one is genuinely good at, where it falls short, and which products we would pick. Our recommendations are based on product labels and on published guidance from the British Pest Control Association and local council pest services, not on marketing claims.
Quick Picks: The Best Cluster Fly Killer for Each Job
| Best for | Product type | Our pick |
|---|---|---|
| Clearing a loft full of flies | Insecticidal smoke generator | Compare loft smoke generators |
| Chemical-free control in a loft or spare room | Sticky fly papers | RAM 20-Pack Fly Paper |
| Flies gathering on window panes | Window fly traps | Compare window fly traps |
| Quick knockdown of flies you can see | Aerosol fly spray | Pest-Stop Wasp & Fly Killer Spray |
| Large or older houses with yearly problems | Spray multipack | 6 x 300ml Fly & Wasp Killer Spray |
| Picking off the odd fly without chemicals | Electric zapper racket | Electric Fly Zapper Racket |
Not sure you are dealing with cluster flies at all? Our guide to cluster flies in your loft explains how to identify them and why they keep coming back to the same house.
Insecticidal Smoke Generators
A smoke generator (often sold as a loft or garage “fumigator” or “smoke bomb”) releases a cloud of insecticide, usually permethrin, that drifts into the gaps between rafters, tiles and insulation where cluster flies hide. That is the key advantage: sprays and traps only catch the flies you can see, but most of an overwintering cluster is tucked out of sight. The BPCA notes that permethrin-based smoke formulations can be effective against cluster flies in the right situation.
Good for: enclosed lofts and roof voids with a heavy, recurring infestation.
Watch out for: fuse-lit generators carry a fire risk, you must leave the space closed for the time stated on the label, and you will need to cover smoke alarms while the product is working, then uncover them straight afterwards.
When you choose a product, look for one whose label specifically lists flying insects or cluster flies and lofts or roof voids, and check the volume it treats against the size of your loft. Compare loft smoke generators on Amazon.
Sticky Fly Papers and Window Traps
Cluster flies crawl towards light on mild days, which is why they pile up on windows. Sticky traps take advantage of that habit. They contain no poison and give off no vapour, so they are the safest choice for lofts with stored belongings, spare bedrooms and anywhere bats might be present.
The limitation is scale. Council guidance describes fly papers as offering limited control that won’t cope with a large infestation, so treat them as a top-up rather than the whole answer.
Our pick: RAM 20-Pack Pesticide-Free Fly Paper
Twenty double-sided papers is enough to hang several across the loft and still have spares for spring, when the flies wake up and head for the light again. They come with drawing pins for hanging from rafters. Read our notes on the RAM fly papers or check the price on Amazon.
For flies on glass, clear window fly traps that stick to the bottom of the pane catch them as they crawl, and are much less noticeable than hanging papers in a living space. Compare window fly traps.
Fly Killer Sprays
A pyrethroid aerosol kills exposed flies almost instantly, which makes it handy for the cluster you find on a window in October. What it can’t do is reach the flies hibernating in cracks and behind boards, so on its own a spray won’t clear a loft.
Our picks
- Pest-Stop Wasp & Fly Killer Spray (300ml): low odour and non-staining, so it is the better choice for living areas and conservatories. Check price on Amazon.
- Sanex Fly & Wasp Killer Spray (300ml): a straightforward, fast-acting aerosol for the odd cluster on a window. Check price on Amazon.
- 6 x 300ml Fly & Wasp Killer Spray: worth it if you have a large house, outbuildings or a holiday let that gets flies every year. Check price on Amazon.
Always cover food, remove pets and fish tanks, and leave the room to ventilate as the label directs.
Electric Fly Killers and Zapper Rackets
A plug-in UV fly killer can help in a loft or top-floor room, and the BPCA suggests switching one on only during the months when flies are active. Like any loft treatment, check for bats before installing one. Compare UV electric fly killers.
A battery zapper racket is the chemical-free way to deal with flies one at a time. It is satisfying, but it won’t make a dent in a loft cluster. Check the Electric Fly Zapper Racket on Amazon.
The Vacuum Cleaner Method
Don’t overlook the tool you already own. Vacuuming is simple, costs nothing and removes the flies along with the scent that attracts more to the same spot. Go up on a mild day when the flies are sluggish, work along the window frames and joists, and empty the bag or canister outside straight away. It only removes visible flies, so it suits a small problem or a quick tidy-up after a smoke treatment.
What Doesn’t Work
- Fruit fly traps: these use a fruity bait. Cluster flies aren’t looking for food when they come indoors, so they ignore them.
- Fly screens: council pest services report they are of little use, because cluster flies squeeze through tiny gaps around tiles, fascias and window frames.
- Treating the garden: the larvae develop inside earthworms in the soil, so outdoor control is neither practical nor necessary.
- Sealing every gap: filling obvious holes around frames helps a little, but complete proofing is rarely achievable.
When to Call a Professional
Call a pest control company if flies keep coming back every year despite smoke treatments, if you have bats or suspect them, if the loft is hard or unsafe to get into, or if the building is large, such as a farmhouse, church or block of flats. Professionals can use a low-pressure residual spray that keeps killing flies as they emerge for several months, or an ultra-low-volume mist that reaches high voids without scaffolding. Be aware that even professional treatment usually controls rather than eradicates cluster flies, so ask what follow-up is included.
Frequently Asked Questions
What is the best cluster fly killer for a loft?
For a loft with a heavy, recurring infestation, an insecticidal smoke generator labelled for flying insects in roof spaces is the most effective DIY option, because the smoke reaches flies hidden in gaps. Check for bats first, and use sticky papers if you can’t rule them out.
When is the best time to treat cluster flies?
Treat in autumn, once the flies have moved into the loft but while you can still reach them, and again on mild late-winter or spring days when they wake up and head for windows. Treating in summer does nothing, because the flies are out in fields and gardens.
Are cluster flies harmful?
No. They don’t breed indoors, bite or spread disease the way house flies can, and their presence isn’t a sign of poor hygiene. The problem is the sheer number, the mess of dead flies and the staining they can leave.
Why do cluster flies come back every year?
Houses that have had cluster flies tend to attract them again, possibly because of the scent left by previous clusters, or because the house is light-coloured and catches the autumn sun. Vacuuming up dead flies after each season and treating early can reduce the numbers.
Sources: BPCA PestWatch: cluster flies and Cotswold District Council cluster fly fact sheet.