Spray recommended residual insecticides, such as Bifen IT, D-Fense SC or Cyper WP. Apply these insecticides outdoors to window frames, door
frames, soffits and eaves as well as any other areas that are vulnerable
to entry. Spray beneath the eaves and around the windows and other areas where flies are likely to seek entry.
Application of to the surrounding soil with D-Fense SC or Bifen IT may also help minimize Cluster
Fly populations and later entry.
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, an aerosol contact pyrethrin aerosol can be used as a space spray to get some immediate relief. Recommended pyrethrin aerosols are CB-80, PT 565, or V-One pyrethrum
Identifying the Cluster Fly
The cluster fly averages 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 of Cluster Flies
The cluster fly is a parasite
of earthworms and breeds outdoors in lawns and fields during the spring
and summer. You can find cluster flies almost everywhere in the United
States and Canada, except for the Southern states bordering the Gulf of
Female Cluster Flies lay their eggs in cracks in
the soil, which 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, the cluster flies begin to enter structures in
large numbers. Problems with cluster flies begin in late August as they
move to winter quarters to over-winter. The cluster fly is seeking warm
sites with protective cracks for shelter, crawling back as far as they
can get. It is important to consider treatment before this
Cluster flies have been known to squeeze around the edges of windows
that are weather-proofed. As the number of cluster 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 buildings, such as the attics of homes. In multi-story buildings,
the cluster flies 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, the
cluster fly may emerge thinking it is spring, going for the warmer air
outside. Cluster flies fly very 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.
Cluster Fly Inspection
Check around windows for live
or dead flies. If you can find the voids in which they are over-wintering,
you can treat the voids with a dust or aerosol, but that is not an easy
task. In most cases, the voids can't be located.
To locate the voids, start with
an inspection of cracks and crevices on the southern and western exterior
walls. Usually the only accessible voids are the attics, crawls paces and