Cluster Flies

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Pest Management Fact Sheet #5010

James F. Dill, Pest Management Specialist
Clay A. Kirby, Insect Diagnostician

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Description & Biology

Cluster Fly
Cluster Fly

Cluster flies closely resemble houseflies, but they are usually larger and have 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, adult cluster flies search for protected overwintering sites such as attics, lofts, wall voids, loose bark, holes in trees, or other crevices and cavities. Based on casual observations, cluster flies seem to be attracted to light-colored buildings. If the siding of the building is tight, then the flies have less opportunity to make their way into the structure.

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’ve been exposed to a period of colder temperatures. 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 earthworm hosts.


The best way to control cluster flies indoors is to build them out. Nailing wood over cracks or caulking cracks tightly helps reduce the annual buildup of these pests. 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 also a quick and effective means of reducing a cluster fly population in the home. Traps such as the “Cluster Buster” may be effective when used indoors.

Aerosol sprays containing resmethrin or pyrethrins are available for use in homes. Insect strips or no-pest strips containing Vapona® are also helpful. Be sure to follow label directions and heed precautions. 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 this material on the active ingredient list on product labels. Be aware that some spray formulations may stain siding. Many people hire the service of a professional pest control company to apply a chemical barrier to the outside of their home in order to inhibit the entry of the cluster flies into the home.

On the positive side, cluster flies do not bite people or animals, are not attracted to garbage, and they are a good indication of an earthworm supply not too far away!

When Using Pesticides


Pest Management Unit
Cooperative Extension Diagnostic and Research Laboratory
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1.800.287.0279 (in Maine)

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