Cluster flies, scientifically known as Pollenia rudis, are a common sight in homes, particularly as seasons change. These insects are larger than typical house flies, often measuring around 8 to 10 millimeters in length, and are characterized by a dark gray body with short, golden hairs on their thorax. Unlike the fast-moving common house fly, cluster flies exhibit a more sluggish and clumsy flight pattern. They are frequently observed gathering in significant numbers indoors, especially in the fall and winter, seeking shelter from the colder weather.
Understanding Cluster Fly Behavior
Cluster flies exhibit a unique life cycle that explains their presence inside homes. During their larval stage, these flies are parasitic, developing within earthworms found in soil. Adult cluster flies, however, feed on plant sap and flower nectar outdoors. As outdoor temperatures begin to drop in late summer or early autumn, adult cluster flies seek sheltered, warm places to overwinter, with residential buildings being a preferred choice.
These flies typically enter structures through small openings, such as cracks around windows, doors, or vents, and will then congregate in secluded areas like attics, wall voids, and seldom-used rooms. Their name derives from this habit of forming clusters while dormant. Unlike common house flies, cluster flies are not attracted to indoor food waste or unsanitary conditions. Their slow movement and tendency to rest in quiet, undisturbed spaces make them primarily a nuisance.
Assessing Harm to Humans and Property
Cluster flies are generally considered nuisance pests and do not pose a significant health threat to humans or pets. They do not bite or sting, nor are they known to transmit diseases. Cluster flies do not breed indoors or contaminate food sources, as they do not feed on human food or waste while overwintering.
Regarding property, cluster flies do not cause structural damage. They do not feed on wood, fabrics, or stored products within a home. The only potential impact on property is minor staining from their droppings, which may appear as tiny black spots in areas where large numbers of flies congregate, such as windowsills or walls. These stains are typically easy to clean and do not indicate destructive activity. While large numbers of dead flies might attract secondary pests like carpet beetles, the flies themselves do not directly harm household materials.
Managing Cluster Fly Presence
Preventing cluster flies from entering a home is the most effective management strategy. This involves sealing potential entry points in late summer or early fall before they seek overwintering sites. Inspect and seal cracks and gaps around window and door frames, utility line entry points, and unscreened vents or soffits. Ensuring screens on windows and doors are intact also helps prevent their entry.
For flies that have already entered, physical removal methods are often recommended. Vacuuming is an effective way to remove active or dead flies, especially when dealing with large numbers. Fly swatters and sticky traps can also be used to manage individual flies. Indoor insecticides are generally not recommended for overwintering cluster flies, as they often hide in inaccessible wall voids and attics, and dead flies can attract other scavenging pests. For severe or recurring infestations, consulting a professional pest control service may be beneficial.