Cluster flies are found throughout the UK. Their common name refers to their habit of forming clusters when hibernating. 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. 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
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. 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 with a range of insecticides and fly strips.