Main Navigation

  • Sales & Tech Helpline: 01332 340898
  • UK Delivery £6
  • Free Delivery Over £100

Cluster Fly Control

The Cluster Fly is often mistaken for the common house fly but it is more sluggish and often appears to fly very slowly. It can overwinter in vast numbers in lofts, cellars and dark voids. These flies are of no particular harm but can be alarming because of their great numbers. They are generally noticed on the first warm days of spring when they congregate on the inside of windows. Frequently found in window frames, particularly sash windows.
They can be easily controlled with many of the cluster fly products we stock.

15 products
  • If you can get access to the flies' hibernating voids, and are comfortable with using insecticidal fumers, then they are easy to control.
  • If the flies' hibernating area is extensive and you would rather not use insecticides, you may be better to use a non-toxic spray like Oa2ki.

Cluster Flies are not easy to kill without insecticides because of the difficulty of accessing all their hibernating places.

Electronic Fly Killers: These need to be plugged into a live mains socket as they emit an attractant light which draws the flies towards an electrified stainless steel grid which kills the flies and the dead bodies are collected in a removeable tray beneath. The unit should be located at the opposite end of the room to any existing source of light in order to attract the flies to the electronic lights rather than the window lights. Extra deep collection trays are available for cluster flies.

Insecticide Fumer: These fumers contain Permethrin and are lit using a match, they then give off a pall of smoke which gets into all cracks and crevices coating all surfaces with a very thin layer of permethrin insecticide. The flies - and other insects - are killed upon contact with the permethrin dust.

For safety, place each unit on an old saucer or tin lid before lighting.

Make quite sure there are no bats or bees in you loft as they will also be killed and that is illegal. Also, it may be a good plan to inform the Fire Brigade in case a helpful neighbour sees smoke coming from your loft and phones the Fire Brigade! Remember, if you need to use several fumers because of the size of your loft, light the furthest away first to enable you to get back out.

Do not use fumers if you are asthmatic.

Insecticidal Dusts: can be puffed onto surfaces used by the flies and into difficult to reach voids. The dust settles on surfaces and insects subsequently come into contact with the dust and so are killed.

Non-Chemical Dusts: these dusts do not contain chemical insecticides but work by rubbing off the protective waxes covering the insects body causing it to die from dehydration.

Insecticidal Sprays: are a means of dispensing the active ingredient onto surfaces via water rather than smoke or powder. The result is the same but without the active ingredient hanging around in the air as it does with powder, dust or smoke.

Use pesticides safely and always read and understand the label.

The Cluster Fly overwinters in lofts, cellars and voids within the structure of buildings.


  • slightly larger than the house fly
  • slower flying
  • non-metallic, dark grey in colour
  • dark grey abdomen with lighter patches
  • characteristic golden hairs on body
  • wings overlap at the tip when at rest


  • adult lays her eggs singly in damp,moist soil or leaf litter
  • emerging maggot penetrates and enters a living earthworm
  • feeds on worm for 13 - 22 days
  • emerges from worm at pupal stage and pupates after 11-14 days
  • up to four generations over the summer months
  • hibernates as an adult fly in lofts etc
  • crowds on the internal side of windows trying to get outside with the first warm days of spring

Cluster flies tend to overwinter in lofts, cavity walls, cellars and other enclosed dark voids. There they can accumulate in very large numbers and subsequently appear with the first warm sunny days of spring.

They are usually first noticed at the windows or in conservatories as quite large, sluggish, dark coloured flies. Most simply exit the building the way they came in and go unnoticed by us. In some cases many hundreds can find their way into rooms through ill-fitting window frames, especially so in the case of sash windows.

In most cases it is very difficult to prevent cluster flies entering your buildings as they can gain access through the tiniest of cracks. It might be worth looking for obvious entrance points but realistically these may be difficult to find and seal.

Not every year is bad for cluster flies so treating them when present is often the best option available.

Be aware that bats may be using your loft as a winter roost or a summer breeding site and that it is illegal to disturb bat roosts. Blocking access to cluster flies may also block access to bats and, given that the only time to proof against cluster flies is during the summer when they are living outside, blocking holes may also trap nursing bats and their young in your loft where they may then die.

Q. I have a Prevent Auto dispenser in my loft to control cluster flies, which I have found works very well. My question is, when should the dispenser be active, and when can I leave it switched off?


I would keep your ammo dry for as long as possible, the refills are quite expensive. The flies will not be active during this cold weather so no point in having it in use right now. Come the warmer days of Feb/March/April it would be worth re-activating for short spells. Choose still, calm days so the insecticide does not get sucked or blown out before it has time to work. The real time to use it is Autumn when the flies come into the buildings to hibernate. Hopefully by now anything in the house is long since dead.

Q. Hi. I have a cluster fly problem at my cottage in France (which is a holiday home). They appear in late September onwards, and congregate at windows trying to get out. They leave deposits/spots on my curtains - is there a spray that I can use on my curtains so that the flies avoid them (they leave spots on my windows too but at least I can simply wipe them clean). Thank you, Nicola


Hi Nicola,

This is a very common problem. If you are infrequently at the house then perhaps the best remedy is to install a Prevent Auto insecticide dispenser (available on our website) either in the house or preferably in the loft space where the flies will initially congregate. That unit will dispense a measured amount of insecticide at intervals you can select and will kill the flies before numbers can escalate. (Not to be used if bats or bees are resident in your loft)

Q. I have a house in France which has been empty over winter I've return to find the loft infested with tiny flies all stuck to the walls they are only 2 to 3 mm in size any idea what they do they pose any threat to building structure ? and how to cure them Thank you Ian Creuse


Hi Ian,

Very sorry, French flies are outside my ken. I doubt they will be a threat in any way but the simplest way to eliminate them is with an insecticidal smoke bomb or two, depending on the loft volume. Make sure you have no bats before using the smokers.

Q. The last few days have been unusually warm and I have large numbers of cluster flies at the sash windows in South facing rooms at the moment (inside and out). Are these flies on the way into the house looking to hibernate or do you think they were already in the house ( attic or window sash boxes) and the unusually warm weather has revived them? Is there a product for application to external walls and around window frames etc. to reduce the number of flies getting in? And finally - is there an attic "bomb" treatment which is safe for bats?


Andrew a smoke fumer is by far the best way of getting a quick knock down effect. This cannot be used where the bats are I'm afraid as they are a non target species and protected. It could however be used in the room where the cluster flies are gathering as it leaves no lasting effects, such as smoke residue. Protector C is an effective insecticidal spray safe for use in the home. This can be used as a space spray and also onto hard and porous surfaces. We sell a kit for cluster flies called Blitz Kits for cluster fly. It contains the above two products plus an Agrothrin powder which can be applied in any cracks or crevices where the insects may be entering.

Q. We've recently moved into the top floor of an old manor house converted into apartments - set in mature gardens of 6 acres with many surrounding trees. In the summer we had major problems with honey bees - then wax moths and now the latest is lady birds and cluster flies there are 19 single glazed sash windows in the apartment and I am currently having to collect about fifty flies each day from the windows - one month ago it was up to 200 a day! There is a very large attic but can't see much evidence of them there - I'm beginning to wonder about fitting secondary double glazing on some of the windows on the south end of the building where most of the infestation occurs - but this would be very expensive as it's a listed building. Any suggestions would be much appreciated. Management is responsible for coping with the infestation but they don't seem to know what to do other than pay for the occasional visit by a pest controller. Peter


The solution we usually recommend for cluster flies is the Blitz Kit for Cluster Flies because it contains everything you need. One Blitz Kit will be enough to treat a room up to 400m cubed. Evacuate the room closing all the windows and doors and making sure any pets are removed or fish tanks covered. Light the fumer and leave for a couple of hours after which time you should open the doors and some windows to clear any residual smoke. There will be no smoke or residue left. Treat all surfaces and soft furnishings with the spray and fill any cracks and crevices where the insects might be entering with the powder. although you say there is nothing in the loft it might be as well to treat there as well as this is normally the source for cluster flies. Another solution for the loft would be aPrevent Auto Dispenser which dispenses at intervals which you determine over a period of time. Each refill lasts about 30-60 days depending on the frequency

Q. When can I stop using smoke fumigators(to kill cluster flies) in my loft?


Cluster flies are a problem in loft spaces in the spring and autumn. I would suggest that as winter sets in fumigation is no longer necessary, but be vigilant as spring returns in case the cluster flies do too!!

Q. The last few days we have had 6-7 flies appear in the house, they are big, black and slow moving. we can not locate where they are coming from, do you have any idea's where we should start looking?


It's very difficult to say. I would suggest they are being attracted by something rotting. Check under floorboards, behind cupboards etc. in case any rodents or small animal has found its end there. Other than that I assume you have looked in all the obvious places.

Q. i've noticed the accumulation of about 5 flies now in between my bedroom window and the screen. Fat ones. Probably cluster flies? Not sure how they even found a way into there, it is well sealed! Should I just wait till they die? Or is there a good way to get rid of them now? And they can't lay any eggs in there can they?


Cluster flies will get into any tiny crevice so your windows aren't as well sealed as you think. They are very resilient and will over winter only to become active with a little warmth. They will not lay eggs there and they do not present a health hazard as regular flies do

Q. When is the best time to spray for cluster flies?


Cluster flies are active twice a year - as soon as it warms up in spring, and between September and December. So a good time to spray would be any time now, providing the weather ever warms up!

Q. how can I eliminate cluster flies from my attic?


The most effective way is to invest in a Prevent Auto dispenser. They are not the cheapest option but ideal for a loft where the timer can be set to spray permethrin at the interval you choose. Replacement sprays can be bought much more cheaply.

Q. Having a very annoying problem with these flies they seem to be coming from under the floor in the kitchen please could you give me a resolution to this problem I'm using fly spray at the moment . Please help. Thank you.


You most likely have a cellar under the kitchen. If you do you need a Prevent Auto dispenser. This takes an aerosol that you put into the place where the cluster flies are and it releases a timed spray of permethrin. This will get rid of your problem

Q. Hi, I have huge numbers of cluster flies in my loft, and like the sound of the Prevent Auto Dispenser – but the flies are very near our cold water tank (we are not on the mains, the tank is fed from a spring). Is the dispenser safe to use in this situation, as some may inevitably get into our water? Many thanks, Jonathan


Not quite sure how your water system works as water tanks should be covered to prevent anything falling in. The spray is safe to use in most environments but as it is not advised to spray on foodstuffs or utensils I would make sure the tank is covered before using

Q. We have flies in our loft. I have seen available methods are Prevent dispensers and smoke bombs. I believe the flies are cluster flies but assume the above method are successful for any fly? Also one concern i have is our children 9 months to 6yrs of age. If i used either of the above does it represent a risk. I am aware we should vacate for several hours but would I then open loft hatch and all up-stair windows, or best to leave loft hatch closed? I will have to cover the water tank, any tips for this to ensure no contamination? Sorry for the amount of questions just want to do it right and without endangering my family. Thank you so much.


The best thing is to stick with the Prevent Auto Dispenser. The spray used is of no danger at all to your family and it can be left to spray in the loft happily until it runs out (by which time it should have killed the flies. Surely the water tank in the loft does not supply drinking water but cover it as a precaution if you wish

Q. I have cluster flies in my garden cabin. Do I need to empty the cabin of furniture/soft furnishings before setting off cluster fly 'bomb' treatment...and how long will it be before it is ok for me (or my cat) to go back in after the treatment? Pam (Wrexham)


No not at all Pam. If you are planning on letting off a smoke bomb there should be no sparks. The cabin should be sealed for a few hours or overnight and then ventilated for a couple of hours after which you and the cat should be able to use the cabin again