Cluster flies (Pollenia rudis) closely resemble house flies, but they are usually larger and have yellowish hairs on the thorax. There may be four or more generations of cluster flies per season. They do not bite people or animals, and aren’t attracted to garbage. They are, however, parasites of earthworms, and are thus a good indication of an earthworm supply close by. The more abundant earthworms are, the more likely it is that cluster flies will abound. But when the flies manage to find their way indoors in search of warmer temperatures, they can quickly become a nuisance.
Additional Information: Cluster Flies