The annoyance of cluster flies usually begins mid-August and may continue until April.

Cluster Flies closely resemble houseflies but are larger and slower in movements. They are dark gray with checkered black and silvery-black abdomens. The wings of cluster flies overlap over the abdomen when at rest. The wings of houseflies do not overlap over the abdomen when at rest. Cluster flies, when crushed, may have an odour similar to buckwheat honey and, when gathered together, they may emit a “sickly sweetish” odour.

 Cluster flies can be found in fields throughout the summer, and movement towards shelter appears to be initiated by a sudden drop in temperature. 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. Eventually they continue to move into the interior of the building and remain there to spend the winter.