The trouble with cluster flies

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 often 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.

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. 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.

Prevention and Control

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 the areas or voids used are difficult, if not impossible, to locate.

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.

Although proofing is seldom 100% successful, sealing around window and door frames and other obvious entry points can also assist in controlling their presence.

Cluster flies do not breed indoors and once they are inside a property, control can be relatively easily achieved by a professional pest controller.

Use of EFKs

We have found that using a ultra violet electronic fly killer that has been specifically adapted for cluster fly control to be a very effective method of controlling these flies.

Use of insecticides and pesticides

Insecticides can be purchased from garden centres/shops, hardware/ironmongers shops, DIY stores, etc.

Insecticides and pesticides are potentially dangerous. Store them away from children and pets. Always read the label and follow the manufacturer's instructions.

If you need more information or advice, please contact Castle Pest Control Services on 01760 7755574.

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 that it is an offence even to disturb bats. In case of doubt, Natural England must be notified before any insecticidal or other treatments are applied.