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 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.
As the flies warm throughout the winter, and especially in the early spring, they come out of their cold temperature dormancy and begin sluggishly moving around. Their random crawling brings them into the house by way of electrical outlets, window pulley holes, and small openings around windows, moldings and base boards.
Cluster flies hibernate in inaccessible places, making them difficult to control. Hidden within walls or under insulation, they are protected from most treatments until they appear within the living spaces of the house. Preventing attic flies is a job for the summer and fall. As much as possible, seal cracks and openings around the outside of the house, especially under the eaves, as you would for energy conservation. Insecticides can be used on the outside of the house in mid-September if you have a persistent problem with attic flies. Remember the problem varies greatly from year to year and is worse than average this year and tends to be worse following a wet summer. The outdoor treatment with insecticides is difficult and potentially messy. I would not routinely advise this treatment for most home owners. Unfortunately, there is little that can be done for flies already inside the attic and walls. Insecticide sprays and fog treatments in the attic have little if any affect, as the flies are usually under insulation or deep in cracks and crevices. They do not fly around much in attics. Therefore, fly paper, fly strips and bug zappers are of no value. (A possible exception would be placement of a professional fly control electrocutor within suspended ceilings (warmed space) of commercial buildings by a pest control operator after a determination of likely fly routes of entry.)
Flies buzzing within a room can be dispatched with a fly swatter, or a vacuum.