Cluster fly

Desciption. Adult cluster flies are slightly larger than the common house fly, Musca domestica. They are dull-gray with black markings and have golden-yellow hairs on the thorax, which can give the appearance of a golden sheen. The hairs are more numerous on the under side of the thorax between and near the legs.
The immature stages, egg and larva, are seldom seen as the eggs are deposited on the soil and the larva or maggots burrow into earthworms on which they feed. The maggots are cream colored and are an elongated wedge shape. As members of the fly family Calliphoridae, the blow flies, the maggots resemble other blow flies commonly found on decaying carrion.
Life History. In Pennsylvania, there may be three to four, or more, generations of cluster flies per season. The adults leave their protected over-wintering sites in the spring and will lay eggs on the soil, which contains earthworms. The hatching larvae enter earthworms, develop to their last larval stage, pupate and emerge as adults to begin the cycle anew. With the onset of autumn, the current crop of adults will seek protected over-wintering sites in such locations as hedge rows; under the bark of dead and dying trees; in the crevices of south-facing cliffs and rock faces; and within the cracks, crevices and voids in sheds, garages, barns, houses and other man-made structures.
Damage. As a nuisance pest, Pollenia rudis joins the ranks of other over-wintering pests such as the multicolored Asian lady beetle, the western conifer seed bug and the boxelder bug. Cluster flies will not damage your home. Occasionally, the flies may leave small dark-colored spots of excrement on windows and walls, but they are not known to carry any diseases of medical importance to humans. In addition to the ‘clustering’ on the sunny exterior of buildings in the fall, the flies will gather in large numbers at windows within the home on warm winter days. The flies are typically sluggish in flight and can be easily swatted or captured.