Cluster Fly

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Cluster flies are commonly found in quiet, undisturbed parts of your home, such as attics and wall voids. They require warm places to hibernate over winter. You may see a large group of cluster flies around a window, as they are attracted to the light on sunny winter days.

Fast Facts

Color Dark grey–olive thorax clothed with crinkled golden–brown hairs.
Size 6–10mm in length.
Appearance A cluster fly looks a lot like a standard housefly, but is slightly bigger. They have large reddish eyes and a thorax that is covered with short hairs. Their wings overlap when at rest and are sluggish in flight.

Diet

Cluster fly larvae feed on earthworm hosts for several days, at which time they molt and pupate in the soil. They prefer not to eat garbage and other waste products.

Habitat

Cluster flies are named for their tendency to “cluster” together when they overwinter in the adult stage. They live outdoors in the warmer months of the year and go unnoticed. Cluster flies begin to be noticed in the fall when they look for a secure place to spend the winter. As fall approaches and temperatures begin to fall, the cluster flies begin to enter homes and buildings in large numbers.

The west and south facing buildings are exposed to more sunlight, so the cluster flies are attracted more to such buildings due to the warmth they can get there. They enter these buildings through small openings or cracks and crevices near window or door frames, open or unscreened windows/vents.

Once inside, they gather together in an isolated, safe place such as attics or false ceilings and begin to hibernate. When the flies end up inside your home during the cold winter months, it’s because either your house has gotten warm enough that they emerge confused and looking for a way out, or the temperature outside has warmed enough to accomplish the same thing.  They do not lay eggs or breed indoors; they are only looking for shelter from winter weather.

Life Cycle

The adults leave their protected over-wintering sites in the spring and lay eggs on the soil, which contains earthworms. The hatching larvae enter earthworms to feed for several days, develop to their last larval stage, pupate and emerge as adults to begin the cycle anew. Development time from egg to adult is about 27 to 39 days. Adults live for 12-18 days. With the onset of autumn, adults will seek protected over-wintering sites in such locations as under the bark of dead and dying trees or within the cracks, crevices and voids in sheds, garages, barns, houses and other man-made structures. There may be three to four generations per season.

Damage

Fortunately, cluster flies are not as unsanitary as house flies and other species of blow flies. Joining other over-wintering pests, the Asian lady beetle and boxelder bug, cluster flies are mainly considered a nuisance pest. Occasionally, the flies may leave small dark-colored spots of excrement on windows and walls, but they will not damage your home. In addition to the ‘clustering’ on the sunny exterior of buildings in the fall, the flies will gather in large numbers at windows within the home on warm winter days. The flies are typically sluggish in flight and can be easily swatted or captured.

Prevention & Control

Cluster Fly Prevention

The best way to stop a cluster fly infestation is to prevent them from getting in. Like with other pests, making sure that all cracks and spaces are sealed in your home’s siding, near where the roof meets walls, roofing tiles and screens that lead into attics or provide access to spaces between walls and siding. It is beneficial to get your home treated before winter arrives by a trained pest control specialist.

Cluster Fly Control

A Batzner Service Specialist will perform a thorough inspection to look for signs of infestation and inspect for potential openings or entry points. An exterior treatment is performed in late summer to prevent cluster flies from entering. If cluster flies have already entered the building, a winter or spring interior treatment can be done to eliminate them inside.

If a cluster fly infestation occurs, count on a professional pest management service to take care of the problem knowledgeably and successfully.