The cluster fly is slightly larger than the common house fly. There are
no distinct lines or stripes behind the head, and the abdomen has irregular
light and dark gray areas. Adult flies of the last generation of the year
become numerous during the latter days of September to mid-October.
As cold weather progresses, adults seek protected places to spend the winter. In many cases, this is within walls, attics, storage rooms, and basements of houses. Screens offer no protection since these flies prefer to crawl in through small openings elsewhere around the building. For this reason they are extremely difficult to keep out of houses.
Isolated houses in the country are especially prone to invasion, since they offer the only warm shelter for miles around. The female lays eggs in the soil near the burrows of earthworms. The tiny maggots that hatch from the eggs seek out earthworms to feed upon. The maggots of cluster flies have never been reared on any other food but living earthworms. When full grown, the parasitic maggot leaves the body of the host and enters the soil. There are about four generations a year.
Complete control of cluster flies is almost impossible since the overwintering adults are concealed in walls, attics, etc. When possible, cover the louver to the attic with screen, and use a caulking compound to seal openings to the outside. Adults are sluggish enough in the winter to be picked up with a vacuum cleaner. It is probably not practical to attempt to control the host of cluster flies -- earthworms in the ground around the house. Earthworms are important to soil, and cluster flies would simply come from surrounding areas. Chemical control should be concentrated in the home where the adults hibernate.