Even after adult flies leave in the spring the high protein associated with dead cluster flies in attics or wall voids provide a food source for carpet and larder beetles. This creates additional nuisance problems during the summer that can only be solved by controlling the cluster fly problem. The best solution is to prevent flies from entering a building before mid-September. Adult cluster flies congregate on the sunny sides of a building and crawl upward as the day warms during late summer and early fall. If they find a point of entry the fly will crawl inside.
Cluster flies collect in wall voids, under siding and overhangs and in attics.
Once inside flies can get access to the whole structure by working through the walls and are often attracted to the light from windows or light fixtures. All cracks in siding, along eaves and overhangs should be sealed to prevent entry. Check roof vents, ventilators and vent fans for screening that is small enough to exclude the flies. All openings around door and window frames, siding, and especially soffits need to be caulked or sealed to make the building fly-proof. If the flies are only a problem in one area, it usually signals what side of the building contains the main entry point.