These flies may be found in large numbers in houses, usually lofts, causing annoyance to the occupants. They enter during the autumn to hibernate and leave in early spring, sometimes appearing on warm winter days. It is at these times when they become a nuisance and treatment is necessary.

There are 3 types of cluster fly: cluster fly, parasitic on earthworms, autumn fly, and green cluster flies, both breed in cow dung. All 3 species lead solitary lives except when they gather in the autumn to hibernate.Before moving into their hibernation place they will collect on the outside of buildings, especially on sunny walls before finding their way inside to form clusters of many thousands in such sites as roof spaces, under tiles, window frames and in window sash boxes. They are not a health hazard in the same way that House Flies are and are only coming into the building to hibernate, not to breed or feed.