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.