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Cluster Flies

One of the most commonly encountered flies in structures, the Cluster Fly’s name is derived from their habit of forming compact clusters of overwintering individuals. They will vary in size slightly but are generally about 3/8” – ½” long and have a dull gray to black appearance.  This fly is NOT associated with filth or the presence of a dead animal. 

Females lay their eggs in cracks in the soil. The eggs hatch in about 3 days and the larvae will parasitize earthworms. In late summer to early fall, adults seek locations to overwinter. This is when they enter structures and is also the optimum time to perform exterior treatments to reduce their numbers. They will often be found around the inside of windows and doors on a warm day in the winter and spring in their attempt to leave the overwintering site.

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