Regrettably cluster flies are a reoccurring nuisance. Cluster flies are one of the few insects where you can only treat the adults. Cluster flies lay their eggs on the soil. Earthworms come along and eat the eggs. The cluster flies then pupate from the earth worm and fly to the nearest structure. This is a cycle that is difficult to break. Cluster flies prefer the south side of a structure. In the early summer, once the flies have left the structure, sealing gaps on the exterior can significantly reduce the potential nesting sites. Entry points include gaps in window frames, doors frames, and any fixture on the exterior of the structure. A multi-prong approach of sealing and chemical applications is required to win the day over an extended period of time.