Problems with cluster flies begin in late August as they move to winter quarters to over-winter. They are seeking warm sites with protective cracks for shelter, crawling back as far as they can get. It is important to consider treatment before this happens. They have been known to squeeze around the edges of windows that are weather-proofed. As the number of flies attracted to the building increases, large clusters of flies huddle inside wall voids, attics, and false ceilings.
Most infestations occur in the upper regions of a buildings, such as the attics of homes.
In multi-story buildings, they can be found in the upper two or three floors, and almost always of the south and west sides of the buildings. If you have unseasonably warm weather in the late fall or winter, they may emerge thinking it is spring, going for the warmer air outside. They fly very slowly when they just wake up. They are strongly attracted to light, so they are usually found around windows. At night they are attracted to lamps.