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.