The cluster fly is a pest in homes, schools, and commercial buildings in Europe, Canada and the United States. Cluster flies derive their common name from their habit of entering a home or building in the fall and gathering together in clusters, usually in the attic or upper regions of buildings. The behavior of the flies is a good identifying feature. The flies are strongly attracted to light and will blunder about lights and windows, colliding with any object in their path. They often drop to the floor on their backs and spin noisily until exhausted. The annoyance of cluster flies usually begins mid-August and may continue until April.

Description:

Cluster flies closely resemble house flies but are larger and slower in their movements. They are dark gray with checkered black and silvery-black abdomens. A newly emerged fly has many golden hairs on its thorax which may be lost as the fly ages. The stripes on the thorax are not as prominent as on the house fly, and the wings of cluster flies overlap over the abdomen when at rest. The wings of houseflies do not overlap over the abdomen when at rest. Cluster flies, when crushed, may have an odor similar to buckwheat honey and, when gathered together, they may emit a 'sickly sweetish' odor.

Biology:

Adult female flies that have survived the winter deposit eggs in cracks and crevices in the soil. The larvae, upon hatching, parasitize the earthworm Allolobophora rosea. This earthworm is red and about one inch in length when contracted (these earthworms are not the large earthworms, Lumbricus terrestris, commonly seen in the soil). Control of earthworms to control the cluster fly is not recommended or effective, since flies may have originated from up to a mile away from the site of infestation. The total developmental period of the fly, from egg to adult, varies from 27 to 39 days, depending upon temperature and other environmental factors.

Cluster flies can be found in fields throughout the summer, and movement towards shelter appears to be initiated by a sudden drop in temperature. From this point on, the behavior of the flies is characteristic--in the afternoon they settle on the upper parts of walls and on roofs, facing south or southwest, sunning themselves. As the sun sets, they crawl into any crevice on the exterior of buildings, but usually near the roof. For a few days, they come out during the day and return to the warmth of the building at night. Eventually they continue to move into the interior of the building and remain there to spend the winter. As warm spells occur throughout the winter, flies may break their dormancy and begin to move about inside the infested building. Flies that survive the winter reverse the behavior exhibited during the fall, emerge, and begin the next generation.

Control:

It is not fully known why flies 'choose' a particular house to infest. There is some evidence that the shape and construction (openings) of the roof are involved in promoting infestations. Also, once flies have entered a structure, the sweetish odor they emit may attract other flies. Homes with large shade trees are seldom attacked, presumably due to the cooler exterior temperature of the house. Control measures include caulking any openings to the interior (windows, doors, vents, etc.), screening (fine mesh) vents, including air conditioner openings, and taking care not to crush flies in the home, as this may attract more flies. Dead flies should be swept or vacuumed up. Because flies hibernate in inaccessible areas between walls, use of insecticides inside buildings is generally unsuccessful. Fly populations, like any other insect species, are susceptible to large increases and decreases in numbers, and the problem may simply correct itself over time.

By Steven R. Alm, URI Entomologist, 1999