Cluster flies, also known as attic flies, are household pests. They appear on the sunny side of the structure in heaviest concentrations in late fall and early winter, as they seek warm locations in which to live during cold months. Although cluster flies are observed buzzing and congregating at windows, screens may prove ineffective in preventing their entrance. Cluster flies are capable of crawling through small openings in the walls of a structure. They hibernate in secluded parts of houses like attics and wall voids. On sunny winter days, the wall voids become warm and the cluster flies try to move toward light. Very often they find themselves in the inhabited parts of the house and the move to the windows. They cluster around the windows and they leave stains on walls and curtains if crushed.
Adults measure 8 to 10 mm length and have light and dark grey-checkered abdomens. The thorax of an adult cluster fly is covered in short golden hairs and the wings overlap when at rest. Cluster flies are slightly larger and darker than the common housefly and move more sluggishly.
The cluster fly life cycle begins when a female lays her eggs in the soil in late summer or early fall. These eggs hatch within a few days, after which larvae seek to enter the body cavities of earthworms. Cluster fly larvae feed on earthworm hosts for several days, at which time they molt and pupate in the soil. Cluster flies’ development time from egg to adult is about 27 to 39 days.
Cluster flies are commonly found in quiet, undisturbed parts of your home, such as attics and wall voids. They require warm places to hibernate over winter. Cluster flies do not bite humans or animals. They also aren’t attracted to garbage. Their mouthparts, like many fly species are like a trunk and used for sucking. You may see a large group of cluster flies around a window, as they are attracted to the light on sunny winter days.