To rid flies and keep this problem under control, inspection is the first and foremost step.
There are four main species of fly collectively known as cluster flies.
Cluster flies are very common indeed. The adult cluster fly hibernates during the winter season in roof spaces. The flies lay eggs in the earth and also in animal dung. As the temperature climbs in late spring, the eggs will hatch. The larval stage is parasitic, entering earthworms and feeding on them until the larva emerges as an adult cluster fly. There may be two to four generations of flies in one year.
Cluster flies migrate from the outside conditions into the lofts of house and farm buildings during the winter season. Obvious signs of an infestation include large quantities of lethargic/dead flies around windows. Once in roof spaces the flies will stay in hibernation until sapring.
Small infestations behind curtains and around windows can be simply removed with a vacuum cleaner.
For a larger problem in areas such as loft spaces, treatment is best carried out by a qualified pest control technician after the first winter frosts. This usually ensures that all of the hibernating flies are in the treatment area, resulting in a more cost effective solution. We usually treats this problem with an insecticide space spray, will quickly eradicate any cluster flies present.