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Cluster Flies

During the fall, many flies may enter homes. They usually are unwelcome visitors, especially when they plan to stay for the duration of the winter. One common type of fly is called a cluster fly.

Cluster flies are similar in appearance to houseflies, but they are larger, nonmetallic and dark gray in color. They have golden hairs on their bodies. Cluster flies enter homes and structures one at a time and may begin to collect throughout the house in swarms. In large numbers, cluster flies may seem like a swarm of bees.

Once inside, these flies occupy many places in the home, such as nooks, dark corners, beneath curtains, and in the angles of walls. Also, they will rest under the edges of closets, in hats, and behind pictures or furniture. Cluster flies usually are found in the upper portions of the house, especially the attic. During sunny days many of them may be in clusters on window sills trying to find their way outdoors.

The life cycle of cluster flies is short, spanning only a few months. They lay single eggs in cracks in soil, during the summer. The eggs hatch within three days and the emerging maggots (larvae) are parasitic on earthworms. The maggot stage lasts 13 to 22 days and the pupal stage lasts 11 to 14 days. Outdoors, adult cluster flies primarily are found around fruit and flowers.

Cluster flies are nuisance pests. They are not destructive and do not harm humans or damage homes. Problems are most noticeable during the springtime when the flies begin to emerge in large numbers to exit the structure.

There are no practical means of controlling cluster flies. The best way to prevent them from entering the home is to use exclusion methods such as window screens, screening around attic vents, and weather stripping around doors and windows.  If cluster flies do gain entry into a home, use a vacuum to physically remove them. Be sure to discard the vacuum bag in a sealed container and place in an outside receptacle. If cluster fly numbers are so large that insecticide applications are necessary, only spray where flies may enter the home, and where the flies are harboring.

Prepared by Michael Vickers, Graduate Assistant, Patricia A. Zungoli, Extension Entomologist/Professor, and Eric P. Benson, Extension Entomologist/Associate Professor, Department of Entomology, Soils, and Plant Sciences, Clemson University. EIIS/HS-33 (New 05/2001).

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