The large, black, pesky flies that show up in bed rooms and on window sills from late fall through early spring are a common household pest. These flies are known as cluster flies, a name that describes their habit of clustering in large numbers inside attics. Their abundance varies from year to year, possibly in relation to the amount of rainfall through the summer.
Cluster flies develop 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 during mid to late September. Casual observation of client reports suggests 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 into the building 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.