Adult cluster flies gather in clusters in warm buildings to overwinter.  They are considered a household nuisance. The name is derived from tight “clusters” made by hibernating individuals in wall voids or attics, throughout Canada, most of the United States and Europe.  On the first warm days of spring you may notice cluster flies gather outdoors, buzzing on lawns. They may also congregate in huge numbers on sunny walls in the fall.
The adult cluster fly looks like a very large house fly. The difference is that at rest, the fly overlaps its wing tips over the abdomen, like a pair of scissors, while the house fly does not. The cluster fly is dark gray to almost black with a checkered grayish abdomen. There are numerous short crinkly golden hairs on the sides of the thorax. In old specimens, these can be rubbed off. The fly is a sluggish flyer, buzzing loudly while flying aimlessly in concentric circles in buildings. Although this pest is less hazardous than other infestations, you don’t want cluster flies in your home or building.
Control is generally achieved by applying a pesticide to the exterior walls of the structure in an effort to discourage the insects from entering. This application should be made before any insects enter the structure, generally mid to late August depending upon the weather in any given year. The best way to identify and remove pests is by hiring a professional who can deal with the problem quickly and effectively.