The flies that suddenly appear in your house beginning in late fall and ending in the spring are not houseflies but Cluster flies. They are slightly larger than houseflies and live most of their lives outdoors. As days grow shorter and the weather turns cold, they seek protection in the walls of buildings and will try to hibernate. The reason they appear indoors during this period is that they have been awakened out of hibernation by the combination of indoor heat and the heat of the sun on the outside of the building. Thinking it’s spring, they are seeking the sunlight at the windows and attempting to get back outside. Since adult flies live on flower nector, they will die after a few days of flying around in the house.
Preventing cluster flies from getting into the walls is very difficult and once they are in the walls they are almost impossible to kill because the insulation protects them from sprays and bombs. Most people resort to vaccuming up the flies once they die. The only effective solution for managing them is to capture them once they come out of the walls.
Carter Notch Managment, LLC offers a variety of fly traps that have been tested and proven to be effective in capturing cluster flies and other flying insects including lady bugs.
For more information on Cluster flies and the best use of traps to capture them see the attached page. http://catch-flies.com/Overview.aspx