Often confused with the common house fly, cluster flies are roughly the same size. Some characteristics that differentiate the cluster fly: they fly somewhat more slowly than the house fly, they almost always fly toward windows on the warm side of a structure and their wings overlap almost completely, when at rest.
Cluster flies breed in the ground outside of buildings during the warm weather (late Spring into early Summer) using earthworms as a food source for the immature larva (maggots). The flies later pupate (go into the cocoon stage), then hatch as adult flies. In temperate areas, often in late August, these flies begins to migrate indoors finding any small cracks or crevices that permit entry into structures. These may include areas around window frames, door frames or eaves. Entry tends to be on the same, warm, sunny side (often the southern or western exposure) of the structure as the flies later emerge from.
During the Fall, Winter or Spring months, these flies may emerge, particularly on warm, sunny days. The flies appear at windows buzzing and "clustering" around those areas to the dismay of the occupants. This fly can become a problem in virtually any structure and they have, on occasion, been a problem in sensitive areas such as hospitals, where they are especially unwelcome.
Stop them on the Outside
One of the best ways to minimize entry of this pest is by applying a repellent, residual insecticide within a week or two of the time when these insects are known to begin migrating indoors. Within the US, Cooperative Extension is a good source to check with for timing information for your immediate region. These residual insecticides are applied outdoors to window frames, door frames, soffits and eaves as well as any other areas that are vulnerable to entry.
Some have suggested that application to the surrounding soil may also help minimize cluster fly populations and later entry, but others caution that since flies may come from other, adjacent areas, treating the soil is a futile attempt to control the problem.
Because some of the treatment areas are many feet off the ground, an application may require power equipment. Calling in a professional, commercial applicator to do the application for your may be best. Product used by commercial applicators (common name in parentheses) include Prelude or Dragnet (permethrin), Demon or Cynoff (cypermethrin) and Demand CS (lambda-cyhalothrin).
For existing infestations indoors, insecticides may be used selectively to control this pest. A total release fogger such as Pro Control Plus may be used in an attic or other room to control flies that may have migrated in.
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