Indoors, they fly for long periods and rarely rest. The adult flies lay eggs in decaying organic matter, particularly manure, where the maggot-stage larvae feed. They are serious pests in office buildings and concentrate in the upper stories. Cluster flies are larger than house flies and during their indoor period they are semidormant and fly awkwardly. The habits of cluster flies are different from other common domestic flies.
Immature stages develop as a parasite of earthworms.
Eggs are laid in the soil and the maggots enter and feed within the earthworms. Cluster flies do not feed on garbage or animal manure. In late summer, cluster flies seek overwintering shelter. They fly to buildings in the afternoons and rest on sun-exposed areas. As the sun sets, the flies seek out cracks and other openings into the building and move to upper stories.