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Cluster Flies

Cluster flies, while not harmful, are annoying and troublesome to eliminate because there is not always a clear solution as to how to best reduce their presence. Typically cluster flies are seen on the south and west sides of a building. They hang out by windows, door frames, and gaps in the side of the structure. Initially the number of flies is small but, if left unchecked over a period of time, they can become a major nuisance. They enter structures in early fall to seek shelter from cooling temperatures. When the weather warms, the cluster flies emerge from their hiding places and either exit the building or enter interior areas. Once stimulated, cluster flies are attracted to light.

The female flies mate in the spring and lay their eggs in soil crevices. The eggs hatch in three days and the larvae burrow into the bodies of earthworms where they develop. Development (egg to adult) requires 27 to 39 days. There are usually four generations per year.
There is no effective control of the larval stage of these flies because they develop in earthworms. Control tactics for cluster flies should be initiated before they enter buildings in large numbers. The most effective long-term control in structures attractive to over-wintering adults is to seal entry points in the walls and roof of the structure. 

It is important to treat cluster flies before their population gets out-of-hand, as they may attract other household pests (Larder Beetle) which feed on dead cluster fly carcasses.

Elimination of cluster flies requires the expertise of a professional applicator.