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How to get rid of Cluster Flies

Cluster flies belong to a family of flies called Pollenia and are medium sized, about 10mm in length. They are greyish olive in colour and adorned with golden hairs which makes them quite distinctive to the trained eye.

Closeup of a cluster fly

They get their name from their characteristic behaviour of congregating in groups of hundreds and even thousands in lofts, small spaces and window frames.  

Continue reading to get expert tips on how you can get rid of cluster flies with ease.

How do cluster flies enter my home?

Cluster flies like to enter window frames, especially casement and sash windows, where they leave significant deposits of fluid and faecal residues with an offensive sickly smell.

They also like to hibernate in the warmth and shelter of our lofts and the growing popularity of ceiling halogen lights means that large numbers of flies can collect around the light fittings and potentially create a fire risk.

Cluster flies entering through window frames

Signs of a cluster fly infestation

If you believe you may have a cluster fly infestation use these common signs to know for sure.

  • A sickly rich smell
  • Swarming flies if disturbed.
  • Large number of flies in windows.
  • Fly faeces on window frames.

Where are cluster flies usually found?

Cluster flies in windows are very, very common along with infestations in lofts.

Other common places you are likely to find the flies are the internal weight cavities or shafts of sash windows and the internal frame cavities of double glazed units.

How to get rid of cluster flies

We recommend anyone looking to get rid of cluster flies to use these 3 methods below for best results.


The key challenge when fumigating is timing. You need to ensure that any fumigation product is used when it will be most effective.

Cluster fly smoke bombs, available in 3 sizes are a good way to treat problem areas and are very effective at reaching inaccessible areas within your home. Simply, place the fumer on a slate or tray and light the touch paper.


An alternative way to kill cluster flies is by using the Prevent Auto Dispenser. This dispenser contains a pyrethrum based insecticidal spray which will activate at several intervals over a 24 hour period. This is effective at killing cluster flies as they enter to hibernate.

If you have a problem with cluster flies in your windows, Dethlac is the best spray to use. It is a lacquer based insecticide that dries into a hard layer and it remains active for a number of weeks.  Just spray into the window cavities and it will kill any flies that come into contact with it.


If you are looking to trap cluster flies we highly recommend using the Cluster Buster. This trap contains a quicksand made of exploded eggshells.

The natural behaviour of the cluster fly leads it to fall into the trap where it becomes stuck in the quicksand. The more flies that get trapped, the more pheromone the insects emit, so attracting yet more flies

Each trap will hold about 1000 flies and normally lasts about 2 years. For maximum effectiveness, place a unit on each window.

What causes cluster flies in the house?

Cluster flies are common in the loft and attic spaces of homes and business premises. The warmth generated here means that flies are naturally drawn into these cavities and voids, particularly during the months of October and November, where they congregate in groups or clusters.

Cluster fly infestation in the home

Can you prevent cluster flies?

It is generally said that prevention is better than cure, but in the case of the cluster fly, this adage is of little use.

Proofing contaminated areas is largely pointless. Your property requires ventilation to breath and without sufficient air flow, mould and rot would set in. 

What damage do cluster flies cause ?

None – unless they are present in significant numbers. Even though they don’t cause damage they can be of high annoyance due to the large numbers they travel in.

Do cluster flies pose any health risks?

Cluster flies have no recognized importance as carriers of disease, so are not a significant pathogenic risk. They should only be viewed as an unsightly nuisance and not as a sign of poor hygiene.  

What is important to think about  is that cluster flies will hibernate with other flies that ARE carriers of disease, such as blow flies found on faeces and rotting meat, the latter possibly attracted by the smell created by large cluster fly populations.


  1. Dear Samantha
    We seem to have an issue with cluster flies in our kitchen.
    Having read that they seek to hibernate in the house early autumn I’m at a loss to understand why they are coming in early July.
    We cleaned out the attic of thousands in January by vacuuming.
    It seems that when we clear them out of the kitchen through swatting glue board, fly paper and electric zappers another wave mysteriously appears.
    I’m not happy to use chemicals in the food preparation area , so I am frustrated and becoming obsessed ( as is my dog) in getting rid of them. The deposit they leave is everywhere ( and kitchen has been newly painted

    1. Hello Suzanne. I feel your pain! It is indeed odd why they are causing issues now, as they would normally happily be doing their thing outside by now. But insects don’t always follow the plan. It seems you have an issue with them both in the kitchen and the attic. The attic is relatively easy to deal with. The Prevent Auto Dispenser is ideal for up there. It is an insecticide but it can do no harm in the loft. I would definitely put one up there. It runs on battery and sprays intermittently until the can of insecticide runs out (we sell replacement cans). If you look in the loft and there are thousands of flies as you found in January, I would be prepared to let off a smoke fumer first, which will kill most of them in one hit, at least allowing you access without getting bombarded with flies. The prevent dispense is actually pretty safe to use elsewhere in the house, but if you don’t wish to use it I do understand. The kitchen is trickier and there is no magic solution. Regular fly sprays will knock them down instantly of course. Sticky papers and electric zappers will catch some, but are not terribly effective on cluster flies. If they are gathering on a particular window, which they tend to do, you might try a cluster buster. This is chemical free and attracts the flies to the slit in the top of the unit (a natural instinct). It traps the flies inside, and the more you trap, the more you attract because the pheromone they emit is in itself an attractant. You might find this slow to start, but for reasons just given, should work better with time. Sam

  2. For the past week I have had small blue/green flies in 3 bedroom windows in Abundance..after reading what you said I now think they may be cluster flies as we have had the loft door open due to a leaking roof..does this sound plausible to you..ive just been spraying them with fly killer but it sounds from what you wrote I may need to do more..I would be grateful for any help you can give me..many thanks

    1. Hello Carole, yes, it sounds totally plausible. It is exactly the right time of year for cluster flies to be inside, and they love lofts. The most effective way to get rid of them from there, if they’re bothering you, is a Prevent Auto Dispenser, that sprays on a timer. If you’re happy to shut your loft door again and leave them, they will quietly hide themselves away all winter and you won’t see them unless you go into the loft and disturb them. In the spring they’ll leave and be gone all summer. Cluster flies are quite harmless. They are clean and do not spread germs or disease. They don’t lay eggs inside either. They do that outside in the spring, in the soil, so you don’t need to worry. They can just be a bit alarming because there are always so many of them. For the flies in your bedrooms, they will succumb to regular fly spray as you can target them easily.

  3. Launa Woodruff

    Hi just been reading the above emails and wondered if I have a small problem too. I found and still find several flies in my bedroom every few days when weather is fine also found a bigger bunch ( about 9) in another bedroom which has no access to the attic and feel they are coming in via window frames . I have double glazing so am surprised but when I opened the window to get rid of then there were some in the frame! So just use spray.
    This is made worse because earlier I had some flying ants in bedroom coming in via opening in boiler /heater cupboard Floor, it went on for weeks ( 20/30 for couple of days then just 2/5 on warm days! ) no ants so not sure why these either! Has this Covid turned the world mad !

    1. Hello Launa
      It sounds like you too have experienced a few cluster flies. They are not solely inhabitants of lofts and very often congregate on and in the frames of sunny walls. It doesn’t sound as though you have a very big problem and in the numbers you describe, using spray is effective enough.

  4. Hello Samantha,
    We have suffered cluster flies since we’ve renovated this house and moved in 7 years ago, although I believe the previous owners had the problem too.

    It’s not too big of a problem in the main house, as there are only around 5-10 around the windows per day, in 3 of the bedrooms (sunny side) although there are a few on the north west side too sometimes.

    Our main problem is the loft, we have converted it into a guest bedroom with en-suite and velux windows. But at the moment it hasn’t got any furniture as it’s a no-go zone.

    The en-suite is yet to be finished, and there are therefore still lots of gaps-insulation-slate where they can come in through.

    The sides of the bedroom have been converted into storage, and it’s not fully sealed on ceiling as it’s only for containers with clothes/toys etc

    The Velux windows are full of them around the edges, they seem to love coming through the gaps.

    We have been given a spray and also have 2 smoke bombs-from last year-but I’m afraid it’ll only sort the problem this winter.

    I can’t use that bedroom until it’s sorted as hoovering around furniture/cushions etc would be extremely difficult. So I’m wondering if is there anything we can do to sort the problem once and for all so we can use the loft.
    We have thought of

    – seal the storage with plaster and cabinets making it as air tight as possible.
    – changing the velux windows to a different system that is tighter.
    Put silicone in all the cracks of the wood/around the spot lights etc.

    Or would we be wasting our time?

    We might need that room for a nanny if I go back to work, and as it is, I don’t think anybody would be happy to live in it.
    I Hoover over hundreds every week.

    Thank you so much for your time

    PS. What’s the best time in the day to spray them? My husband wants to do it in the morning when the kids go to school but I don’t want them to be out and about already sunbathing and missing our window of opportunity. Doing it in rain helps?

    1. Hi Bianca
      unfortunately there is no single one time remedy to cluster flies. If you live in an area that is prone to them you will potentially have an annual problem. As they have by now already taken up their hibernating places, you won’t be getting anymore than are already in your home. Smoke bombs are generally the most effective way of killing the majority that are present providing the fumes can reach them. You may need to do this more than once. Placing a Prevent Auto dispenser can also get the stragglers once the smoke bomb has done its job.

      Without doubt sealing the converted room would be the best permanent fix. It could also be worth trying to seal any gaps on the outside where they are entering, usually under the eaves. You mention the side storage areas, a smoke bomb in each of these would be a good idea. Dethlac or Protector C around the Velux edges may deter some as both are residual and remain active for a few weeks.

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