If you live in a northern climate, you may be familiar with cluster flies. Not the typical house fly or filth fly, the cluster fly is often found in rural areas on the warm side (south and west) of a building during fall afternoons. The worst part is, these flies crawl up the side of the building and look for the smallest crack to gain access to warmer environs inside. Their plan is not to reproduce, but to hibernate in the attic.
Which would all be well and good until a warmish day when the congregate at the highest window in the home, banging against the glass trying to get out. It’s not unheard of for hundreds of flies to gather.
The best treatment is prevention, sealing the possible entry points, but in older homes, and perhaps even newer homes, this can be a challenge. Caulk is a good start, tight construction is another.
Sprays work marginally, as do strips. Fly paper will catch some. I finally resorted to leaving the windows and screens open in the top windows and the flies escape — this of course let other pests in and out, but a few was better than the hundreds.
The solution I’m currently using this the Powder Trap. I ordered mine via catalog and the day they arrived, the temperature dropped so I cant say for sure if it is the weather or the trap — but my fly troubles dropped to non existent.