The female lays eggs in the soil near the burrows of earthworms. The tiny maggots that hatch from the eggs seek out earthworms to feed upon. The maggots of cluster flies have never been reared on any other food but living earthworms. When full grown, the parasitic maggot leaves the body of the host and enters the soil. There are about four generations a year.
Cluster flies are parasitic on certain earthworms.
Complete control of cluster flies is almost impossible since the overwintering adults are concealed in walls, attics, etc. 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.