A cluster fly (Pollenia rudis) is a common pest that frequently invades homes as the weather cools. They are considered a nuisance because they appear indoors in large numbers, particularly during the fall and winter months. Unlike common house flies, cluster flies are not associated with unsanitary conditions or decaying matter, nor do they transmit human diseases. Their presence is primarily an issue of unwanted indoor aggregation, as they seek sheltered places to survive the cold.
Physical Characteristics and Unique Life Cycle
Cluster flies are larger than the average house fly, measuring about one-third of an inch in length. Their bodies are dark gray to brownish-black and lack the metallic sheen common to blow flies. A key identifying feature is the presence of short, yellowish-golden hairs covering the thorax, giving the fly a fuzzy appearance. When at rest, cluster flies hold their wings completely overlapped over their abdomen, a posture distinct from the parallel-wing stance of a house fly.
They move slowly and appear sluggish, making them easy to catch. This species’ life cycle is unique among common indoor flies because it is not tied to garbage or food waste. The adult female lays her eggs in cracks in the soil, usually close to earthworm burrows.
Upon hatching, the larvae seek out and burrow into earthworms (typically Eisenia or Allolobophora species) to complete their development. The larva lives as an internal parasite, feeding on the worm for two to three weeks until it pupates in the soil. The adult flies that emerge spend the warmer seasons feeding on flower nectar and fruit juices outdoors.
Seasonal Behavior and Home Entry
The appearance of cluster flies inside a home is linked to their need to find a safe location for overwintering (diapause). As summer ends and daylight hours shorten, adult flies cease breeding and begin searching for sheltered sites. Buildings, especially the tallest structures in an area, become prime targets.
The flies are drawn to the sunny, warm exterior surfaces, often congregating on south- and west-facing walls in late August and September. They release an aggregation pheromone that attracts other cluster flies, leading to the “clustering” behavior that gives them their name. From these gathering points, they crawl into any available opening to access a void space where they can hibernate.
They exploit structural flaws, squeezing through gaps around window and door frames, utility line penetrations, and openings beneath the fascia boards or roof eaves. Once inside, they move into secluded areas such as attics, wall voids, and false ceilings to remain inactive for the winter. If the home’s heating system or a sunny day warms the voids, the flies can become prematurely active.
When awakened, they instinctively move toward light, often entering the living space through small cracks around light fixtures, electrical outlets, or window trim. This emergence is why homeowners frequently find the flies buzzing slowly around windows during the colder months.
Control Methods and Exclusion Strategies
Managing cluster flies is most effective when focusing on prevention before they enter the structure. The most successful long-term strategy is mechanical exclusion, which involves sealing all potential entry points during the late summer or early fall. Homeowners should use silicone caulk to seal cracks around window frames, door frames, and where utility lines enter the building.
Attic and soffit vents should be tightly screened with fine mesh hardware cloth. Damaged or ill-fitting window and door screens must also be repaired or replaced before the flies begin their migration. This proactive sealing process prevents the flies from settling into the wall voids for the winter.
If flies are already present inside the living space, the most practical removal method is using a vacuum cleaner. Since the flies are slow-moving and often cluster, a shop vacuum or a standard vacuum with a hose attachment can easily remove large numbers. Insecticide sprays or foggers are not recommended for indoor use because they cannot penetrate the hidden spaces where the majority of the flies are overwintering.
For comprehensive prevention, professional pest control services can apply a residual insecticide to the home’s exterior surfaces. This treatment is applied around windows, doors, eaves, and vents in late August or early September, creating a chemical barrier that kills flies seeking entry. This exterior treatment is a temporary solution that must be reapplied annually but significantly reduces the number of flies entering the home.