The cluster fly (Pollenia rudis) is a common nuisance pest that often appears indoors during cooler months. These flies are primarily outdoor insects, completing their unique life cycle in fields and open areas. Unlike common house flies, they are not associated with poor sanitation or decaying matter. Interaction with humans begins only when adult flies seek sheltered, dry spaces to survive the cold season.
Recognizing the Cluster Fly
Adult cluster flies are slightly larger than the common house fly, measuring about 6 to 10 millimeters in length. They have a dull, dark gray body with short, crinkly golden-yellow hairs on their thorax. Unlike the house fly, their wings overlap scissor-like over the abdomen when resting.
When found indoors, these flies are noticeably sluggish, moving slowly and flying with a clumsy, erratic pattern. They often gather near windows, drawn to the light, or crawl on walls and ceilings. A buzzing sound originating from wall voids or attics indicates a large cluster is hibernating nearby.
The Complete Life Cycle Duration
The full life cycle, from egg to adult, is completed in approximately 27 to 50 days, depending on environmental factors like temperature and moisture. Adult females lay eggs singly in cracks in the soil, usually near earthworm burrows, during spring and summer. The eggs hatch in about three days into small, cream-colored larvae.
This larval stage is parasitic; the larva seeks out and burrows into a host earthworm to feed internally. The larva consumes the earthworm over 13 to 22 days before leaving the host to pupate in the soil. The pupal stage, where the larva transforms into an adult fly inside a tough, brown case, lasts for 11 to 14 days.
The adult fly emerges from the soil to reproduce, and multiple generations, often three to four, occur during the warm season. The adult stage is the longest part of their existence, particularly for the final generation. These adults live for several months as they enter hibernation, or overwintering, from late autumn until spring.
Seasonal Infestations and Management
The cluster fly becomes a pest at two distinct times, both tied to their need for overwintering shelter. The first occurs in late summer and early autumn as adults seek warm, protected spaces to hibernate. They enter buildings through small cracks, gaps, and openings around windows and eaves. They then congregate in large numbers, or “clusters,” in quiet areas like attics and wall voids, where they remain dormant.
The second time they become noticeable is during warm winter days or in early spring, when they are roused from hibernation. Mistaking the warmth for spring, they become active and move toward light sources. They find their way into living spaces through light fixtures, baseboards, and window frames.
The most effective management strategy is prevention. Sealing potential exterior entry points, such as gaps around utility lines, fascia boards, and window frames, is the primary preventative measure. This exclusion work should be completed in late summer or early fall before the flies seek shelter. Once the sluggish flies are inside, physical removal using a vacuum cleaner is the simplest way to manage them.