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.
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.
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.
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.
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
Steven R. Alm, URI Entomologist, 1999