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University
of Rhode Island GreenShare Factsheets
Cluster
Fly Pollenia
rudis F.
Diptera: Calliphoridae
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. 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.
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. 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
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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