Cluster flies can be found in fields throughout the summer, and movement towards shelter appears to be initiated by a sudden drop in temperature. From this point on, the behavior of the flies is characteristic–in the afternoon they settle on the upper parts of walls and on roofs, facing south or southwest, sunning themselves. As the sun sets, they crawl into any crevice on the exterior of buildings, but usually near the roof. For a few days, they come out during the day and return to the warmth of the building at night.
Cluster flies are nuisance seasonal pests. When cold weather proceeds, they search for protected areas for over-wintering. They gather on roofs of buildings, wall corners, cracks, crevices, weather stropping on windows, under clothing in closets, under curtains, behind pictures and furniture etc. They also may leave their hiding areas and appear on sunny warm days in winters. Cluster flies do not cause damage or bite. When crushed, they leave a greasy mark on the surface.