They do no harm but can become a major nuisance in the home when they cluster in the tops of rooms and then when it warms up by swarming around the windows seeking a way outside.
Cluster flies are parasites of earthworms.
Spraying the entry points on the outside, once in September and again in October will greatly reduce the number of cluster flies, boxelder bugs and ladybugs entering into your house in the fall. Spraying talstar on the yard twice a year will reduce the numbers of cluster flies and other lawn pests. There are over 100,000 species of flies in the world. The common house fly is categorized as a filth fly because they breed in garbage, rotten food and manure. From egg to adult may only take 10 days.