"Winter Fly Infestation – Cluster Flies"
Cluster flies are named for their habit of over wintering in large clusters within attics or upper wall voids of homes and buildings. Most often they are mistaken for house flies because they look similar to the common house fly. Cluster flies are black and a little larger than the house fly. They range from 3/8 inch to 1/2 inch long and have short, yellow hairs on the thorax. Their wings overlap when at rest.
Cluster flies are unique because they are parasites of earthworms. In the summer they lay eggs in cracks in the soil, and the larval or maggot stages of these fly attack and eat earthworms. The development time from egg to adult is about 27 to 39 days. There are two to three generations of flies produced each year and each time the female returns to lay eggs in the soil. In August or September, the final generation of flies spend the winter in a protected location, such as your home, and start the life cycle over again the following spring.
Adult cluster flies can be found over wintering in 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.
Cracks around windows, doors, siding, utility pipes, behind chimneys, and underneath the fascia and other openings should be sealed with good quality silicone or silicone-latex caulk. Damaged screens on doors and windows should be repaired or replaced.
Typically, flies will emerge from cracks under or behind baseboards, around window and door trim, and around exhaust fans or lights in ceilings. Seal these openings with caulk or other suitable materials to prevent the insects from crawling out.
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.