CLUSTER FLY
IDENTIFYING THE CLUSTER FLY
They average between 1/4 to 3/8 inch long. They
are dark gray, never metallic blue or green. When
crushed, they give off an odor like buckwheat honey.
Cluster flies closely resemble house flies, but
they are usually larger and have a yellowish sheen
on the thorax.
BIOLOGY AND HABITS
They are parasites of earthworms and breed outdoors
in lawns and fields during the spring and summer.
You can find them almost everywhere in the United
States. Females lay their eggs in cracks in the
soil and subsequently they hatch in three days.
The larvae use earthworms as a food source. The
larvae feed for about 22 days. After that, they
go into the pupae stage, which lasts 11-14 days
before emerging as adults. Adult flies feed on flowers.
There are about four generations hatched per summer.
When fall approaches, they begin to enter structures
in large numbers. Problems with cluster flies begin
in late August as they move to structures to over-winter.
They are seeking warm sites with protective cracks
for shelter, crawling back as far as they can get
(It is important to consider treatment before this
happens).
Cluster flies have been known to squeeze around
the edges of windows that are weather-proofed. As
the number of flies attracted to the building increases,
large clusters of flies huddle inside wall voids,
attics, and false ceilings. Most infestations occur
in the upper regions of a structure, such as the
attics of homes.
In multi-story buildings, they can be found in the
upper two or three floors, and almost always of
the south and west sides of the buildings.
If you have unseasonably warm weather in the late
fall or winter, they may emerge thinking it is spring,
going for the warmer air outside. They fly slowly
when they just wake up. They are strongly attracted
to light, so they are usually found around windows.
At night they are attracted to lamps and other artificial
light sources.
CONTROL MEASURES
If the voids in which they are over-wintering in
are located, treatment to the voids with a dust
or aerosol may be applied. however in most cases,
the voids can not be located.
Cluster fly problems can be partially prevented,
but after the fall little can be done. All of those
currently found within the home had infiltrated
walls months ago. A "mini-vac" is the
best control of the individual flies that are currently
present.
If this is not successful a pryethrum contact aerosol
may be used or an alternative method could be electronic
fly traps.
Late August and early September are the times for
treating the outside walls of the structure where
cluster flies are likely to land and rest before
entering the structures. April or early May are
the best times to treat for pre-emerging flies.
These residual insecticides are applied outdoors
to window frames, door frames, soffits and eaves
as well as any other areas that are vulnerable to
entry.
Along with the above treatments, thoroughly caulking
all exterior cracks and crevices on the sun-exposed
sides (south and west) of the structure is also
very important in controlling these flies.
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