There may be four or more generations of cluster flies per season. These insects are parasites of earthworms. The more abundant earthworms are, the more likely it is that cluster flies will abound and become a nuisance. Earthworms are most abundant around old farms and places where manure has been piled or stored. Siding without cracks, crevices or other protected areas are usually fly free, but white loose-fitting vinyl or aluminum siding will attract the creatures.
Indications are that the cluster fly is also attracted to light colored siding.
The flies may enter homes as early as the first part of August. On warm days in early winter, or when homeowners turn on indoor heat, the flies become active and move toward the warmth. Apparently this happens only after they are exposed to a period of colder temperatures. Thus, the flies can become a nuisance in the middle of the winter, as well as spring and fall, when warmth or light lures them from their hiding places into other rooms of the house. During the summer, cluster flies go unnoticed as they search for their host, the earthworm.