Fact Sheets - Cluster Flies
Pest Management Fact Sheet #5010
James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician
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Description & Biology
Cluster flies closely resemble house flies, but they are usually larger and have a yellowish hairs on the thorax. There may be four or more generations of cluster flies per season.
These insects are parasites of earthworms. The more abundant earthworms are, the more likely it is that cluster flies will abound and become a nuisance. Earthworms are most abundant around old farms and places where manure has been piled or stored. High earthworm populations are common in grassy areas, good soil and where moisture is adequate.
In the late summer, adults search for a protected overwintering sites such as attics, lofts, wall voids, loose bark, holes in trees, or other crevices and cavities. Siding without cracks, crevices or other protected areas are usually fly free, but white loose-fitting vinyl or aluminum siding will attract the flies. Indications are that the cluster fly is also attracted to light colored siding. The flies may enter homes as early as the second half of August.
On warm days in early winter, or when homeowners turn on indoor heat, the flies become active and move toward the warmth. Apparently this happens only after they are exposed to a period of colder temperatures. Thus, the flies can become a nuisance in the middle of the winter, as well as spring and fall, when warmth or light lures them from their hiding places into other rooms of the house. During the summer, cluster flies go unnoticed as they search for their host, the earthworm.
The best way to control cluster flies indoors is to “build them out”. Nailing wood over cracks or caulking them tightly helps reduce the annual buildup of the pest. Putting screening over attic soffit vents is another step you can take. You can also use the flies’ attraction to light to rid your attic of the creatures. Simply open the attic windows on sunny days. The use of a vacuum cleaner is a quick and effective means of reducing a cluster fly population in the home. Traps inside may be useful such as the “Cluster Buster.”
Aerosol sprays containing resmethrin or pyrethrins are available for use in homes. Insect strips or no-pest strips containing Vapona are also helpful. Never use no-pest strips where food is prepared or served, or in bedrooms where children, elderly or sick people spend much time. Use the strips in attics, window frames, spaces around louvers, under eaves and intersections of walls. Outside resting areas may be sprayed with permethrin by mid to late August. Look for these materials in the active ingredient list on product labels. Be aware that some spray formulations may stain siding.
On the positive side, cluster flies do not bite people or animals, aren’t attracted to garbage, and they are a good indication of an earthworm supply not too far away!
When Using Pesticides
ALWAYS FOLLOW LABEL DIRECTIONS!
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