Looking like overgrown house flies, cluster flies are often mistaken for their slightly smaller cousins. Dull gray with black markings, cluster flies have golden hairs on their thorax which can give them a slightly golden sheen. As summer ends, adult cluster flies increase in number as the final brood of the summer reaches maturity. Toward the end of September, cluster flies begin to search for a warm place to spend the winter. Like stink bugs, box elder bugs and Asian lady beetles, cluster flies can become a serious nuisance problem in the fall, NJ pest control companies warn. Like other overwintering insects, cluster flies begin to enter New Jersey homes in early fall to overwinter in wall voids, storage rooms, basements and attics.

These flies spend the winter in a semi-dormant state, but may leave their winter hiding places to sun themselves on south and west facing walls and windows on warm winter days. In multi-storied apartment buildings, condominiums and office buildings, cluster flies, which tend to migrate upwards, may congregate on upper floors where they can become a considerable nuisance problem during the winter.

Unlike common house flies, cluster flies do not breed in garbage, animal waste or other types of filth. Cluster flies are earthworm parasites. Adult flies lay their eggs in the soil. Once hatched, the maggots seek out and feed on earthworms until they reach maturity. Three to four generations typically develop each season. Cluster flies do not feed or breed during the winter months. Like other overwintering New Jersey insects, cluster flies leave their winter resting places in the spring and return to the outdoors.

Controlling cluster flies is best done in late summer well before they begin to seek winter shelter. A New Jersey pest control professional can treat the exterior of your home to discourage cluster flies and other overwintering insects from invading your home.