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Michigan State University Extension
Home Maintenance And Repair - 01500591

Cluster Flies

Cluster flies are large, black, bumbling flies that can invade homes in fall and become a nuisance  throughout the winter and following spring. They look like house flies; however, they are slightly larger. Also, they do not have conspicuous stripes on the upper surface of the thorax between the wings. They can be identified by the presence of golden hairs underneath the base of the wings.
Cluster Fly (Vis. 1) Cluster flies can become a nuisance because they enter buildings in fall in search of a place to spend the winter. Often large numbers will enter through cracks around loose fitting screens, in siding and along eaves. They search out dark places such as wall voids and attics and can congregate in large masses. Once in the house, if they become warm they are attracted to light. Because of this, they may move into living areas of the house through window casings or other wall openings.

Cluster fly control starts with measures aimed at keeping the flies out of dwellings. When they are seeking shelter, the flies have the habit of congregating on the sunny sides of buildings. They gradually move upward until they find a point of entry. Thus, all cracks in siding and along eaves should be sealed to prevent flies from gaining entry to the house.

In the house, some control measures may be used. If flies have congregated in accessible spaces such as attics, and wall voids a ready to use pyrethroid is recommended.

For a complete listing of suggested control options for all home, yard and garden insect pests, check with your local Extension Service, found under local government in the phone book.

Read and follow instructions on the pesticide label.


This information comes from Michigan State University Extension bulletin E-955, Cluster Flies.

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