Unfortunately, this annoyance will persist through the spring. Cluster flies are named for their habit of clustering in large numbers on outside walls in September and October and inside attics during the winter. Their abundance varies from year to year in relation to the weather. Cluster flies do not reproduce indoors. They cannot feed or hatch within the attic or walls.
Cluster flies develop during the summer as parasites inside the bodies of earthworms.
There are three generations of flies produced each summer, and the final generation of the season migrates to houses and other buildings starting in mid to late September. Reports suggest that houses located on an exposed hill top or high ground are most attractive to these migrating flies. The flies cluster on the warm sides of buildings in late summer during the day. When the sun goes down and the temperatures cool, these flies crawl indoors through cracks under the eaves and around windows or through gaps in the siding. Once inside and secured in a protected location, they remain in hibernation until warmed by heat from the furnace or the sun.