Cluster Flies Cluster flies are about the same size, or slightly larger than houseflies (about 8mm, or 1/3 of an inch long), and are dark gray to black. The adult flies are non-biting and rarely cause any damage, except for spotting of walls and windows from defecation and regurgitation. During late summer or early fall, as adults they seek outdoor locations to over winter, often hollow trees, out buildings or sheds. They become a nuisance when they attempt to enter warm buildings to over winter; most cluster flies over wintering in heated buildings eventually die. They tend to cluster on the exterior walls of buildings, hence their name.
Cluster Fly Identification Cluster flies are distinguished from other flies by the presence of golden yellow hairs on the front, top of the thorax. Another noticeable feature of the Cluster Fly is the way its wings are held flat over its back while resting; the wing tips overlap when held in this position.

Cluster Fly Life Cycle The life cycle starts as an egg in the soil, usually in a well-drained silt-loam with a grass cover. During the summer, eggs hatch in about 3 days and become larvae which survive by becoming parasites on several species of earthworms. After about 2 weeks, the larvae leave the parasitized worm and pupate in loose surface soil. In about another 2 weeks, they emerge as adults. There may be 2 or 3 generations per year, depending on the weather. Wet summer weather is conducive to increased earthworm and cluster fly populations.

Cluster Fly Management
Since little can be done to prevent the development of cluster fly populations, action must focus on minimizing the impact of the flies as they seek sites to overwinter.

Cluster flies crawl through small openings and are difficult to keep out of houses. Once inside, they may cluster in attics, closets, wall voids or in false ceilings. Most flies overwintering indoors eventually die.

Cluster flies can be prevented from entering buildings by diligently blocking all entry points into the building. Screens should be installed on windows, doors, vents and even on air conditioners. Caulk all cracks and crevices carefully - around windows, doors, and vents; and along foundations, soffits and so on. This must be done very thoroughly to be effective. Repair any cracks or openings in the house siding or roof.

Cluster flies are sluggish, especially when overwintering, and may be easily swatted or simply vacuumed. If vacuumed, seal and properly dispose of the bag.

Exterior walls where cluster flies congregate in the fall - usually south-facing walls - may be sprayed with any one of a variety of registered pesticides. If the spraying is done as the flies begin to accumulate, a short-residual pesticide may be used. If the goal of the spraying is to prevent the flies from accumulating on the wall in the first place, a longer-residual product should be used.

Spraying the soil to kill cluster fly larvae and their host earthworms is not recommended. Earthworms play an important role in soil ecology.

Inside, a “no-pest” strip may be used, or an insecticide dust may be used in attics or wall voids. If large numbers of dead flies result, it is recommended to vacuum them to avoid an infestation of larder beetles, which will feed on the carcasses. Household sprays may also be used.