Frances from Rocanville, Sk. writes
They seem to be coming from the floor joists and up the air vents.
What you are describing actually sounds more like a cluster fly. Cluster Flies usually live outside during the warm spring and summer months, feeding off of flowers or fruit. As the cooler fall weather approaches, cluster flies find their way into buildings through small cracks and crevices. Once inside, the flies hide in clusters in dark corners, wall voids, and virtually any tight space they can find-- beneath curtains, behind pictures, underneath furniture, inside door and window casings, and under baseboards. These flies start becoming active and emerging flying around in the house when the heat kicks on or when the weather warms up. Like any pest, the best way to get rid of them is to stop them from coming entering in the first place. Now that they are already indoors your options are a bit limited.
Use a vacuum hose to remove single flies or clusters of adult flies as they appear.
A contact pyrethrum spray like CB-80 can be kept on hand and sprayed lightly as needed for contact kill.
Inspect around windows for live or dead flies. Any void where cluster flies are found or suspected should be treated using a residual dust insecticide such as Delta Dust or Drione Dust. Apply using a hand duster to any enclosed void including underneath baseboards and behind electrical outlets and window and door frames. Some enclosed wall voids in critical areas may need to be drilled, dusted, and then resealed.
A residual spray insecticide such as Suspend SC or Demon WP may also be used to treat areas where flies are frequently seen after they emerge in the spring, such as attics, basements, storerooms, closets, and other areas.
Fly Light Traps
Installing insect or fly lights in attics and false ceilings is an effective approach to dealing with cluster flies when the overwintering site cannot be determined. When the cluster flies "awaken" in the spring, a fly light will attract and capture a large portion of the cluster fly population before they are able to inhabit living spaces.
Answer last updated on: 11/18/2012
