Why Do Flies Swarm on the Outside of Windows?

Homeowners are often dismayed to find flies swarming outside their windows. Cluster flies and face flies are commonly seen flying in sunny weather around home and office windows. These flies appear in large numbers and often become nuisance pests if they wander indoors.

Cluster flies and face flies are two types of insects often found swarming around windows outdoors. When they wander indoors, they can annoy homeowners with their loud buzzing and numbers. Also known as attic flies, cluster flies typically measure 5/16-inch in length and are gray in color. Cluster flies overlap their wings while at rest, unlike house flies that do not bring their wings together. Face flies are quite similar in appearance to house flies, but they are very dark in color and much larger.

In late August, cluster flies move indoors to hibernate in protected areas. These flies often hide in attics, basements, underneath eaves and in vinyl siding. While indoors, cluster flies do not reproduce but move outdoors in the spring to breed. According to Ohio State University, cluster flies are seen swarming in warm, sunny weather on the outside of windows in preparation for spring reproduction. Face flies are usually seen outside homes in rural areas, where horses and cows are present. Like cluster flies, these flies swarm together in large numbers, becoming a nuisance if they come inside.

Preventing cluster flies and face flies from coming indoors is key to controlling infestations. Seal all cracks and crevices that may allow flies to enter your home. Install small-meshed window screens to keep flies outside where they belong. Cluster flies and face flies typically swarm on the south side of buildings so be sure to seal holes and cracks around vents, siding and underneath eaves to keep them out of your home.

Spraying the outside of your home with pyrethroid-based insecticides can help control cluster flies and face flies. Aerosol sprays of pyrethrin, resmethrin or propoxur can help control light swarms, both outdoors and inside. Outdoor bug zappers can help control large populations of flies during the peak swarm months of March and April.