General

Cluster flies are not considered a pest of public health significance, but when present are a real nuisance one.  Their common name refer to their habit of forming clusters when they “hibernate”.

Secondly, cluster flies are not known to cause structural damage to property or furnishings etc.  And rest assured that they are not associated with dead animal bodies or faeces.  And also there are no maggots associated with cluster flies when found on domestic housing.

The term ‘cluster fly (flies)’ actually refers to a series of species of flies- these are then known collectively as ‘cluster flies’.  The four main species are:

• The Autumn Fly (Musca autumalis) – a small fly of about the same size and colour as the housefly, but with a yellow abdomen.

• The Common Cluster Fly (Pollenia rudis) – a larger fly of a dull, dark, dusty brown/grey colour with a “tessellated” abdomen.

• The Green Cluster Fly (Dasyhora caynella) – about the same size as a housefly, shiny green/blue in colour.

• The Yellow Swarming Fly (Thaumatomya notata) a smaller species of hibernating fly, yellowish body/black markings, sometimes confused with the fruit fly.

Biology and Habitat

Cluster flies are very common.  They become a nuisance by the fact that the adults hibernate during the winter months in the roof spaces of houses and/or farm buildings.   Once the slightly warmer months appear the adults lay their eggs outside in the soil and/or animal dung.  As the soil temperature increases in late spring, the eggs will hatch.  The larval stage is parasitic, entering earthworms and feeding upon them until emerging as an adult fly.  There may be two to four generations of flies in a single year.

The common cluster fly prefers a warm wet summer, whereas the autumn fly prefers dryer, hotter conditions.

Cluster flies migrate from the outside conditions into the lofts of house and/or farm buildings during the winter months.  The cool weather causes them to search for more protection from the elements and they may be seen in lrage numbers.  Obvious signs of an infestation include large quantities of lethargic/dead flies around windows, in roof spaces, lofts etc.  Once in the roof spaces the flies will stay in hibernation until the spring with several thousand flies clustered together.

Control options

Cluster flies emit a clustering pheromone that encourages them to hibernate together and for subsequent generation to follow to the same property.

For small infestations around windows and behind curtains, they can be removed with the use of a vacuum cleaner.  For large infestation in loft spaces, treatment is best carried out after the first frosts of winter, this usually ensures that all of the hibernating flies are in the treatment area.  The treatment best designed at this time is to treat with an insecticide space spray, will quickly knock down any flies present.

We do find that houses do tend to suffer year-on-year.

IMPORTANT NOTE: There is a possibility of bats living in the same areas of buildings used by cluster flies, and great care must be taken to survey the area for signs of bat presence.  Bats are a protected species in the UK