Cluster Flies gather into a group of individuals, or clusters, to hibernate in the winter. They gather in attics, upper rooms and window frames. The young are parasites of earthworms. Eggs are laid on the soil and newly hatched larvae enter earthworms where they feed. Because the young develop in worms, Cluster Flies are common in areas where there are large grassy areas and in farmlands.
In the fall Cluster Flies congregate on the warm sides of houses and buildings. When the sun goes down and it cools the flies enter the structure through cracks and crevices. They then gather together and hibernate in clusters. In the spring the flies break their dormancy and want to fly to grassy areas but are often trapped inside the house.
Screen windows and doors. Caulk outside and inside around windows and doors, and fill any cracks and holes on outside walls to prevent entry. Check that the attic hatch fits snugly, if not use weather stripping to keep flies out of the living areas. If cluster flies are a problem every year, professional applications of pyrethroid insecticides can be performed in the late summer to prevent cluster flies from overwintering indoors.
Once cluster flies have entered a building control can be very difficult. Pesticide can be injected into cracks or sprayed on surfaces. Sticky fly strips in attic spaces and around windows will reduce the numbers of flies inside. Lights installed in attic areas can attract flies to them and away from living areas of buildings. In heavy infestations the use of electric fly traps may be desirable.