"Why Am I Seeing So Many Black Flies?"
Chattanooga Times Free Press
Sunday, February 1, 2003

Cluster flies are large, black flies that come inside your home in winter and early spring. These flies are ¼" to 3/8" long, black or gray in color, with that are overlapping when resting. The name comes from their preference to live clustered together inside your attic. Experts believe the size of the clusters depend on the amount of rainfall throughout the summer months. Thus the huge quantities of winter flies you see may be due to the rainy Chattanooga area summer of 2003.

These pests luckily do not reproduce indoors and are simply wintering with you. They have an interesting lifecycle developing as parasites inside the bodies of earthworms. Three generations of flies are produced each summer and the last flies of the season enter your home during late fall. Houses located on hills or exposed elevated locations, particularly in light colored houses, typically experience more problems with cluster flies.

Cluster flies enjoy sunning themselves on the exterior surface of home's south or west side location during late summer days. When cold weather arrives, they enter you home through cracks in the foundation or siding and around windows and fireplaces. They remain in a hibernation state, enjoying both security and protection, until they feel the warmth of the sun or are warmed by your heating system.

Once warm, they come out of their dormancy state and begin moving about. They are sluggish, disoriented and crawl randomly. They enter the living space through electrical outlets or small openings around moldings, baseboards, and windows. They are seeking an escape route and you will see them moving toward doors and windows. They are not attracted to indoor food. You may find them on their backs on the floor or on windowsills, spinning and making noise until they become tired. You will want to remove the flies you see since they carry many organisms of infection on their bodies. Dead flies should also be removed since they attract other insects (as a food source).

Because the larvae of cluster flies require an earthworm host to survive, the more earthworms there are outside your home, the more likely you are to experience cluster flies inside your home. Consider adding a birdbath or feeder to attract birds to your yard to help reduce the earthworm populations.
Controlling the attic flies is best done in late summer and fall. Because they are hiding under insulation and deep in cracks in your attic space they are difficult to treat with spray or fogging machines. Flypaper, strips, and zappers don't work in attics for the same reason. Your pest management professional should treat your home's exterior in early fall with a residual pesticide. Spraying insecticides combined with sealing all interior and exterior cracks and installing screens behind attic vents is necessary for prevention and control. To kill the flies in your living space during the winter and early spring, your pest management professional will use a safe, insecticide aerosol sprayer along with integrated pest management techniques of flies attempting to make it into your home.