Cluster fly females lay their eggs singly in cracks in the soil; the larvae emerge in three days and begin to seek out their food source: earthworms. After burrowing into the earthworm, basically becoming a parasite in the earthworm, the larvae feed for about three weeks before pupating. The adult Cluster Fly emerges from its pupa after 12 to 14 days. The adult flies (of which there 3 to 4 generations per year) feed on flowers.
Adult cluster flies will begin to hibernate when the days get shorter in mid-August and move to a protected area for the winter. When Cluster Flies come out of hibernation, they will cluster together on warm sides of buildings during the day. When the sun goes down and the temperature begins to cool, they will crawl into buildings through cracks, under eaves, gaps in siding, cracks in windows, etc. They will group together in attics, empty or unused rooms, wall voids, basements, tree holes, and other darkened areas. They are attracted to light, light colored siding, and other structures on lawns, especially areas that are inhabited by earthworms.