Cluster Flies

Cluster Flies

15th September 2010

As Autumn approaches we are coming into Cluster Fly Season (weather dependent but approximately October to April).

Location: Numerous locations, but usually in lofts and roof spaces. South-facing windows and walls often favoured.

Detection: Visual sightings of large numbers of adults.

There are three main species of fly collectively known as cluster flies. They are Musca autumalis, a small fly of about the same size and colour as the house fly, Pollenia rudis, a larger fly dull brown/grey in colour, and lastly Dasyhora caynella which is about house fly size and shiny green/blue in colour.


Cluster flies, particularly Pollenia rudis are very common occurring throughout Great Britain. The adult flies during the summer months, lay their eggs in the earth or in animal dung. Permanent areas of grassland are favourite sites e.g. permanent grazed pasture or parkland as solid cultivation such as ploughing disrupts the flies breeding cycle. The larval stage is parasitic, entering earthworms and feeding upon them until emerging as an adult fly.

Cluster flies migrate from outdoors into the lofts of houses and farm buildings during the autumn months causing a nuisance to householders. Obvious signs of an infestation is a roof space containing a large quantity of lethargic flies and a quantity of dead flies. Unless the loft is used for storage purposes or is accessed regularly the flies will hibernate successfully leaving in early spring dependent on the weather.

Reasons for control:

Cluster flies are not damaging to property when they enter buildings in the autumn purely to hibernate over the cold winter months. As mentioned above they feed on decaying vegetation and earthworms. Cluster flies are not to be associated with dead animal bodies, faeces and the resulting maggots. Fibre glass insulation material in lofts can look unsightly if covered with the bodies of flies that have not hibernated successfully although its insulation properties will not be altered and no damage to the roof structure will occur.

If the loft is home to a hibernating colony of flies, first floor lighting diffusers or sunken spot light lenses can become full of dead or dying flies. This often occurs in houses where the loft area has been converted into a living room. In most buildings built after 1950, the external roof covering has an under felt layer. Cluster flies can successfully hibernate undetected between these two layers. Often cluster flies can be seen basking on sunny grassland or resting on south facing walls of buildings. Cluster flies, like birds, return to roof spaces previously occupied so re-infestation is almost inevitable.


Unfortunately the only option available is to try and control the number of flies by either:-

Insecticidal space spray by us or another pest control company (N.B. if bats are present in your loft/attic this will not be possible). If you are not in our area please see the BPCA’s web site to locate a pest control company near you search BPCA member. Number of visits required will depend on the time of year the treatment is carried out and also on weather conditions (mild autumn) as to when flies come in to hibernate.

The use of fly control units also works well (N.B. it is not recommended that electric grid machines are used in loft areas/roof voids, however if you wish to use this type of machine it is important that these are used with an extra deep cluster tray and checked on a regular basis). Please see our online eshop for fly killer units which are suitable for controlling cluster flies.

Below photo shows a glueboard covered in cluster flies:-

N.B. If bats are present in the roof space, then it is not recommended that any sort of fly control is used.

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