Cluster flies can normally be found anywhere outdoors during the summer. Their migration towards shelter appears to be started by a sudden drop in temperature. The flies show characteristic behaviors; in the afternoon they settle on the upper parts of walls and on roofs, facing south or southwest, warming themselves. As the sun sets, they crawl into any opening on the exterior of any structure, but usually near the roof. For a few days, they come out during the day and return to the warmth of the structure at night.
Cluster flies are nuisance seasonal pests. When cold weather approaches, they look for protected areas for the winter. On the exterior, they gather on roofs of buildings, wall corners, cracks, crevices, or any other opening found. Once inside they may be found under clothing in closets, under curtains, behind pictures and furniture. They also may leave their hiding areas and appear on sunny warm days in winters. Cluster flies do not cause damage or bite. If crushed, they leave a greasy mark on the surface.