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How to get rid of Cluster Flies

Know your enemy

Before you learn how to get rid of cluster flies we need to learn more about them. Cluster flies belongs 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.

They are more sluggish in nature than other flies of the same size, such as blow flies, regularly flying into lights and windows, landing upside down and spinning on their backs.

They get their name from their characteristic behaviour of congregating in groups of hundreds and even thousands in lofts, crawl spaces and window frames.  The following information describes how to get rid of cluster flies.

cluster fly

The cluster fly begins life as an egg, laid by the female, during the warmer months of the year in the burrows of earthworms. Once the larvae hatch they wriggle along the burrow until they find the worm. They enter the body of the earthworm where they become parasites until mature, at which point they leave the worm to pupate. 

The size of a cluster fly population is very much dependent on climate and it has been asserted that the warmer the summer and autumn, the greater the overwintering population will be.

Being parasites of earthworms, cluster flies are most abundant in areas with lots of grassland, especially grazing land, and are also known as pasture or field flies.

They are also attracted to white and light painted surfaces, especially those that are south and west facing, where they can bask in the sun. This can give rise to swarms of flies in some areas, even causing homeowners to change the colour of their homes to more subdued tones.

Once they have mated and the outdoor temperature begins to fall in the autumn, the flies seek refuge inside buildings, under bark and in the hollows of trees etc.

Read more: Cluster Fly, House Fly what’s the difference?

What damage do cluster flies cause ?

None – unless they are present in significant numbers.

The problem facing most people who live in areas with plenty of open grassland is that their homes offer these little parasites the ideal environment to overwinter. 

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 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.

How to tell you have cluster flies

  • Cluster flies are common in the loft and attic space.
  • A sickly rich smell
  • Flies attracted to your light source.
  • Swarming flies if disturbed.
  • Large number of flies in windows.
  • Fly faeces on window frames.

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.

Because people rarely have cause to go into the loft, it is often not at all apparent that there is a problem. In fact, the first time you are likely to realize is when you go to collect the Christmas decorations.  

You will first notice a very sickly and rich smell and then a few flies attracted to the light source you are using. If you make the mistake of disturbing them though, it is not uncommon to find tens of thousands of flies swarming at you a little like in a horror movie.

As well as finding an infestation in your loft, cluster flies in windows are very, very common. You will find that these often rather plump flies are quite amazingly able to get into some of the tiniest gaps imaginable.

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. You are also likely to see a dark brown speckled covering of fly faeces where the windows close tight into the frame.

Before you go on the attack

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

spray cluster flies with fly spray

Proofing contaminated areas is largely pointless. Your property requires ventilation to breath and without sufficient air flow, mould and rot would set in.  It has already been said that cluster flies are adept at fitting themselves into the smallest gaps, and in most cases they will be able to find a hole large enough to fit through somewhere on your property.

It is believed that aggregation pheromones and other chemicals are responsible for the flies’ clustering behaviour. If you add to this the phenomenon shared with wasps that they tend to be selective over the properties they choose to infest.

It seems that once you have a problem with these flies they will keep coming back year after year and in greater numbers, even if adjoining or nearby properties of the same or similar age have no appeal at all. This is probably due to them leaving a pheromone trail that will continue to attract flies long after the original inhabitants have gone.

So the bottom line is that once these flies seek you out, you just have to deal with them, year on year. Before you have any treatment carried out it is important that you assess an affected space for bats as it is illegal to disturb or kill them.

How to get rid of cluster flies

Cluster Fly Control Kit

Cluster Fly Control Kit is a handy kit containing everything you need to effectively treat and control Cluster Flies in your home.

Cluster Fly Prevent Auto Dispenser

The Prevent Auto Dispenser is an excellent solution to cluster flies in the loft. It contains a natural pyrethrum based insecticide which it releases on a timed basis.

Insecto Cluster Fly Fogger

The Insecto Cluster Fly Fogger, formerly called the Flea and Fly Bomb, is an automatic release aerosol, containing 0.5% permethrin, that when let off in a room will reach cluster flies, bedbugs and other insects in difficult to reach places.

Insecto Cluster Fly Smoke Bomb

Insecto Cluster Fly Smoke Bombs are ready to use Insecticidal Smoke Generators providing highly effective control of flying and crawling insects in difficult to reach places, loft spaces or empty greenhouses.

Cluster Buster, Cluster Fly Trap

The Cluster Buster, Cluster Fly Trap incorporates a patented powder trap technology which traps cluster flies en masse in a bed of extremely low density powder known as Quicksand.

Over 1 billion micro-spheres in each trap stick to the feet and hairs, preventing the flies climbing or flying out of the trap. Completely non-toxic solution to cluster fly control.

Bayer Cluster Fly Spray

Bayer Cluster Fly Killer is specially formulated for the rapid control and quick knockdown of cluster flies.

This 400ml aerosol can be used for the rapid control of an extensive range of flying insects (see label for full details) and can be used in a wide range of areas including domestic and industrial premises.


With fumigation, the challenge is always to try and time it so that you kill as many flies as possible. Do it too early and you will need to re-treat a couple of weeks later to kill the final wave of overwintering flies.  The best time to fumigate is late October, early November, unless you need control more quickly.  Cluster Fly Smoke Bombs contain 13.25% permethrin and come in three sizes. To use, set the fumer on a slate or metal tray and light the touch paper (lighting the one furthest from the exit point first if you are using more than one)

One way to avoid having to find the right time is to invest in a Prevent Auto dispenser. This is a unit that houses a pyrethrum based insecticidal spray which you can set to spray at intervals over a 24 hour period. The idea is that the timed spray targets cluster flies as they come in to hibernate. To be most effective it should be used between August and December.

The flies can also be a problem in early spring when the weather starts to warm up and they become active in preparation for mating and egg laying, so it would be an idea to set up the unit again for then. Remember that they WILL NOT lay eggs in your loft. Be sure there are no bats roosting in the loft before attempting either of these eradication methods as bats are protected.

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.

The Cluster Buster is a brilliant product. It is a sleek plastic unit containing 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. It will hold about 1000 flies and normally lasts about 2 years. For maximum effectiveness, have a unit on each window.


The best way to get rid of cluster flies:

  1. Fumigation
  2. Sprays
  3. Traps

Remember that cluster flies are not associated with disease or dirt.  They are not harmful and do not lay eggs in your home. 

However, they can be a nuisance and alarming because of their numbers. For more information on how to get rid of cluster flies, get in touch today.

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