The large, black, pesky flies that show up in bed rooms and on window sills from late fall through early spring have been very abundant this year, possibly because of the wetter than normal weather we had last summer. These flies are known as cluster flies, a name that describes their habit of clustering in large numbers inside attics. Cluster flies do not reproduce indoors, and home owners bothered by these pests do not need to fear the flies are 'hatching' from a dead animal or other unpleasant material within the attic or walls.
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.