Cluster flies are common insects that often become household nuisances, particularly during specific times of the year. These flies are larger and slower-moving than typical house flies, and they do not bite. This article explores the distinct factors that draw these insects to homes and other structures.
Understanding Cluster Fly Behavior
Cluster flies exhibit a unique life cycle that influences their attraction to human dwellings. During warmer months, these insects primarily live outdoors, and their larvae are parasitic on earthworms. Adult female cluster flies lay their eggs in the soil near earthworm burrows, and upon hatching, the larvae burrow into and feed on the worms for several weeks before pupating in the soil. This outdoor existence means they are not typically attracted to decaying matter or food sources inside homes, which differentiates them from common house flies.
As temperatures begin to drop, cluster flies shift their behavior from outdoor reproduction to seeking shelter. Their tendency to “cluster” or aggregate is a key aspect of their survival strategy. They gather in large groups to overwinter, seeking protected, undisturbed spots, often within buildings.
Key Attractants for Cluster Flies
The primary factors attracting cluster flies to structures are related to their need for overwintering shelter. Buildings provide quiet, undisturbed, and protected spaces that mimic their natural hibernation sites, such as under tree bark or in hollow trees. Attics, wall voids, and unused rooms are particularly favored locations due to their seclusion. These areas offer the ideal conditions for the flies to enter a dormant state and survive the winter months.
Warmth emanating from structures is another significant attractant, especially as outdoor temperatures decline. Cluster flies are sensitive to heat sources and will gravitate towards the warmer sides of buildings, particularly south and west-facing walls that receive ample sunlight. Light also plays a role in their movement, as cluster flies exhibit phototactic behavior, meaning they are drawn to light. This attraction can guide them towards entry points like windows and light-filled rooms.
Structural gaps and openings are not attractions in themselves but serve as crucial entry points. Cracks, crevices, gaps around windows and doors, and openings in siding provide the means for flies to access sheltered spaces within a building. Once inside, they then locate quiet, warm areas for clustering.
Seasonal Patterns of Attraction
Cluster flies are most noticeably attracted to structures during specific seasonal transitions. Their movement from outdoor environments to indoor spaces primarily occurs in late summer and early fall, typically from mid-August through early October. This timing coincides with dropping outdoor temperatures, signaling their need to find overwintering sites. Their attraction to homes intensifies as they seek shelter for hibernation.
As winter progresses, cluster flies remain largely dormant within their chosen overwintering spots. However, warm, sunny days in late winter or early spring can interrupt their dormancy. During these periods, they may become active and emerge into living areas, often drawn to light from windows as they attempt to exit the building and return outdoors to resume their life cycle. This emergence is typically the same flies stirring from hibernation.