When possible, cover the louver to the attic with screen, and use a caulking compound to seal openings to the outside. Adults are sluggish enough in the winter to be picked up with a vacuum cleaner. Earthworms are important to soil, and cluster flies would simply come from surrounding areas. Chemical control should be concentrated in the home where the adults hibernate. Adults that buzz about the house can be killed with an aerosol spray Prevention of cluster fly infestations is very difficult, but certainly warrants a try.
Timing is the most important factor.
Applying a synthetic pyrethroid to the outside of the structure with an electric fogging machine will help treat all entry sites. Application should be made in the first two weeks of August. When using strips or spray, it may be more effective to concentrate the chemical on the sunny side of the house. These walls are warmer and generally attract more flies. When crushed, they leave a greasy spot on upholstery, carpets, and wood surfaces.