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Cluster Flies Over-Winter in the House

By Whitney Cranshaw, Specialist in Entomology, Colorado State University Cooperative Extension

During warm winter and spring days you may sometimes see flies about the living room and kitchen. Don't worry; it isn't because you forgot to take the garbage out.

Most flies survive winter in the adult stage, and many may migrate to buildings for shelter. Most notorious of these is the cluster fly, a moderate-sized dull black species that commonly winters in homes.  During this time of year, they are in a sort of suspended state of development known as diapause and neither feed nor reproduce.   During warm periods, however, they do come out from their hiding sites behind walls and fly about slowly and erratically.

Cluster flies don't develop on garbage but instead are parasites of earthworms.   They either will leave on their own or die out out without reproducing.   Control is difficult because it requires a major effort to prevent their entering the home.

This usually occurs in late summer, during which time the flies migrate to sun-exposed sides of buildings and infiltrate through any available crack or opening.  Before their annual indoor migration, thoroughly seal and caulk all such potential entries.

Once the flies have found their way behind the walls, the only option is to destroy them as you see them or wait for them to die or depart.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010