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Pest Control - Cluster Flies

What do they look like?

Cluster Fly

Where do they live?

Cluster flies are found throughout the UK and their common name refers to their habit of forming clusters when "hibernating" - in often large numbers - in buildings. Whilst there is a specific species of common cluster fly (Pollenia rudis), there are other species of cluster fly and swarming flies which have a similar hibernating nature and these may also be involved in forming mixed populations of flies inside suitable buildings.

The life cycle of the Cluster Fly is very much dependent on the prevailing weather conditions, and in this country, two generations a year are usual but in hot summers, up to four generations per year are possible.

Cluster flies are "field" flies and in summer and early autumn they are of no consequence. However, as the weather becomes cooler, they seek out shelter in nooks and crannies in houses and other buildings. As the weather becomes colder, they search for more protection from the elements and may be seen in large numbers, particularly in roof spaces, lofts, etc, sometimes with several thousand flies clustered together.

Curiously, it has been seen that a single house or one building in a row of similar buildings will be chosen year after year for this clustering phenomenon.

Pest Status

Large numbers of cluster flies hibernating together are capable of producing a sickly smell and, if their local environment becomes warmer for any reason, they can emerge to fly around, albeit rather lazily. They are attracted to light, and some will find their way into living areas, and the presence of large flies in winter, usually around windows, can cause concern to the building's occupants.

However, whilst cluster flies can be a source of nuisance on occasion, it is not considered that they pose any risk to human health and their presence should not be taken as evidence of poor hygiene.


As cluster flies do not breed indoors, the control of these flies outdoors is impractical. Control methods for cluster and swarming flies are often ineffective or incomplete. It is often impossible to keep flies from entering premises. Indeed, it is likely that in many premises used by cluster flies that the areas or voids used are difficult, if not impossible to locate.

Although proofing is seldom 100% successful, sealing around window and door frames and other obvious entry points can assist in controlling their presence. The best method of control is to prevent them entering the building in the first place, especially by blocking any access into cavity walls, e.g. replacing missing bricks, filling in other holes, etc.

Once flies are inside property, control can be relatively easily achieved with a range of insecticides and fly strips.

Use of Insecticides and Pesticides

Insecticides can be purchased from garden centres/shops, hardware/ironmongers shops, DIY stores, etc. Please note the following precautions:

Insecticides and pesticides are potentially dangerous, and always:

  • store them away from children and pets;
  • read the label on whichever product is being used and follow the manufacturer's instructions carefully;
  • take whatever precautious are advised/recommended;
  • ensure good ventilation;
  • avoid inhaling vapours and spray;
  • wash hands after use;
  • do not use near open food or on preparation surfaces.

Important Note:

There is a possibility of bats living in the same areas in buildings used by cluster flies, and great care must be taken to survey the area for signs of bat presence. Please note it is an offence just to disturb bats. In case of doubt, contact the Biodiversity Officer on 01656 643667 before any insecticidal or other treatments are applied.

Use of Repellents

Although not a proven method of control as such, the use of repellents such as citronella oil may be worth trying, particularly to prevent recurrence of cluster flies in future years. These repellents do not contain insecticides and may therefore be safer in some situations.

As in the case of insecticides, always follow the manufacturers' instructions.

Last Updated: 27/02/2017
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