A little larger than a house fly, the Cluster Fly will enter a house or structure in the fall one at a time and then congregate together in one place; generally in nooks and dark corners and cluster together. Coloring of the Cluster Fly is a nonmetallic, dark gray color. The Cluster Fly lives outdoors during warm seasons, where they can locate flowers and fruit easily to feed on. As with other types of overwintering insects, cooler weather outsides is when inside appearances of Cluster Fly populations become apparent, and when they become a pest issue.
In warm weather, Cluster Flies are known to also swarm windows on the warm sunny days and are extremely annoying to homeowners and business owners. Although they do not cause any extreme damage, as with the presence of any fly, it is undesired and can create issue with occupants.