Remember seeing a big, slow moving fly in the window on a sunny winter day in January and thinking, "Where the heck did that come from?" Well even though they look like a common house fly, these are indeed Cluster Flies, and they can be a real nuisance to homes, hotels, and many office buildings, especially if there is a wooded area near by.
Cluster Flies breed and spend most of their time in the woods, migrating to buildings in the fall, looking for a warm place for the winter. You can often see them on the sunny side of a building on cool days in the fall. They are attracted to the siding as the sun warms it up. Then they will climb the wall looking for an entry. Most times they find them at the roof eaves or under windows. They then hide in attics and crawl spaces and filter down in to the building over the winter months, so the top floor is usually the hardest hit.
Cluster flies are usually larger than a common house fly. They fly very slow and are so lethargic; one can almost touch them before they will fly slowly away. Lady Bugs and "stink bugs" (a / k / a Western Conifer Seed Bugs) will all enter the same way, and you may see them wintering in your facility as well. These pests do not pose any real health threat. They are more of a nuisance, although your hotel and restaurant guests will complain about them, and nobody in the hospitality industry wants to leave a negative impression with their guests.
In very light infestations, it's just a matter of vacuuming them up. Squashing them can leave an unpleasant musty odor (that's why they call them stink bugs) and / or leave a stain on your wall, window, or drapes.
In the case of Cluster Flies, especially if the infestation is mainly on the top floor, incandescent light traps with glue boards placed in attics and crawl spaces can often catch them before they filter down in to your building. Ask your pest management professional about these.
An even better approach is to have your pest management company treat the outside of your building in the late summer or early fall every year, especially if your problem involves Lady Bugs or Stink Bugs. This way they march through the material and die before they can become a problem on the inside.
If your home or business has a log construction, your problem is complicated because, with no attic or crawl space, these pests come directly in to your living space.
As with any pest problem, one should consult a professional. Home remedies can often exacerbate a problem by driving pests deeper in to the walls, only to have them reappear at a later date and in greater numbers. This makes a professionals job more difficult, and then, more expensive for the consumer. Many over the counter pesticides can be many times more toxic than what a professional would use, making the remedy much more of a health hazard than the original problem.