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 are difficult to control in homes because they hibernate within inaccessible places.
Because they hide inside walls or under insulation, they are usually not vulnerable to treatment until they appear within the living space 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.