During the summer, Cluster Flies congregate in fields. They eventually move towards shelter when their is a significant drop in temperature. Once the temperature has dropped, they adopt a consistent cycle of behavior: during afternoon hours, they warm themselves in the sun on the roofs and upper areas of walls that face south or southwest, which is followed by crawling into cracks and crevices around the building once the sun sets. This cycle repeats daily.
Cluster Flies function as seasonal nuisance pests. They inhabit protected areas once winter comes around. They will congregate on wall corners, cracks, crevices, roofs, under curtains, under clothing, etc. Cluster Flies may leave these hiding places during warmer days in winter months. They do not damage anything directly, but if smashed, they will leave a greasy mark.