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Cluster flies derive their common name from their habit of entering a home or building in the fall and gathering together in clusters, usually in the attic or upper regions of buildings. The behavior of the flies is a good identifying feature. Cluster flies closely resemble house flies but are larger and slower in their movements.

Pest Facts

Color: Dark grey – Brown
Flies Controlled by Expert Pest
Legs: eight
Shape: Oval
Size: 1/4 to 3/8 inch
Antennae: Yes
Flying : Yes
Region : All 50 states



The flies are strongly attracted to light and will blunder about lights and windows, colliding with any object in their path. They often drop to the floor on their backs and spin noisily until exhausted. The annoyance of cluster flies usually begins mid-August and may continue until April. There are three to four generations of cluster flies per year. In the late fall, between late August to early September (depending on the temperature and region), the adults seek protected areas usually in the sunny sides of buildings for over-wintering.


Cluster flies can be found in fields throughout the summer, and movement towards shelter appears to be initiated by a sudden drop in temperature. From this point on, the behavior of the flies is characteristic–in the afternoon they settle on the upper parts of walls and on roofs, facing south or southwest, sunning themselves. As the sun sets, they crawl into any crevice on the exterior of buildings, but usually near the roof. For a few days, they come out during the day and return to the warmth of the building at night.


Cluster flies are nuisance seasonal pests. When cold weather proceeds, they search for protected areas for over-wintering. They gather on roofs of buildings, wall corners, cracks, crevices, weather stropping on windows, under clothing in closets, under curtains, behind pictures and furniture etc. They also may leave their hiding areas and appear on sunny warm days in winters. Cluster flies do not cause damage or bite. When crushed, they leave a greasy mark on the surface.


In the summer and before late fall or early winter, seal or caulk crack, crevices, voids and all possible entry openings in and around your building. As well as screen windows and doors, ventilator and louvers.

When the pest is becoming a nuisance, an inspection and proper treatment of the structure is recommended.

For more information got to  Pest World

Category: flies
Tagged As: flies

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Winter has come! Now is the time when our desire for indoor warmth increases. Rodents, squirrels and raccoons also like to sit by the fire and warm their hands! Call us today if you hear scurrying or scratching in your home. 



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