The cluster fly, scientifically known as Pollenia rudis, is a common pest that becomes a significant nuisance when it invades structures during the colder months. Unlike common house flies that seek out food waste, these insects are solely focused on finding a secure location to survive the winter. Understanding the specific factors that draw this pest to a property and into the home is the first step toward managing their presence. These flies are outdoor insects for most of the year, but their life cycle and seasonal behavior compel them to seek shelter inside human dwellings, usually starting in the early autumn.
Identifying Cluster Flies
Cluster flies are often mistaken for house flies, but they possess several distinct physical and behavioral characteristics. The adult cluster fly is slightly larger than a house fly, typically measuring about 7 to 8 millimeters in length, and it has a noticeably sluggish and slow flight pattern. Their bodies are non-metallic and dark gray, but the thorax is covered in fine, crinkly, yellowish-golden hairs that can give the fly a golden sheen, which is a key identifying feature. When resting, the cluster fly’s wings overlap over its abdomen, unlike the house fly, whose wings remain separated.
Cluster flies do not breed in garbage or decaying matter, meaning their presence is not an indicator of poor sanitation. Instead, they are often found in groups, or “clusters,” near windows, especially on warm, sunny days during the winter and early spring. When crushed, these flies may also emit a distinct odor, sometimes described as smelling like buckwheat honey.
The Primary External Attraction
The reason cluster flies are present in a general outdoor area is directly tied to the life cycle of their larval stage, which relies on a specific biological host. Female cluster flies lay their eggs in cracks in the soil, usually in lawns, fields, or other areas with turfgrass. Once hatched, the maggot-like larvae burrow into the soil and seek out earthworms, particularly certain species of introduced European earthworms. The larvae are parasites, meaning they enter the earthworm’s body cavity and feed on it, eventually killing the host.
The presence of a healthy, moist lawn or nearby field with an abundance of earthworms serves as the breeding ground, attracting adult flies to the vicinity. This process occurs throughout the spring and summer, resulting in multiple generations. Adult flies feed on plant nectar and fruit juices outdoors, but the primary attraction to the area is the viable earthworm population in the surrounding soil.
Why Cluster Flies Enter Homes
As the days shorten and temperatures drop in late summer and early autumn, cluster flies seek a sheltered, dry place to enter a state of dormancy (diapause) and overwinter. They are strongly drawn to warm, sun-exposed surfaces, making south and west-facing walls the most common gathering points. This warmth provides the necessary thermal cue for them to begin their migration indoors.
Flies aggregate on sunny exterior walls near the roofline, searching for entry. They squeeze through incredibly small structural gaps, such as cracks around window frames, loose siding, utility line penetrations, and openings under eaves or in attic vents. Once inside, they navigate into secluded, dark spaces like wall voids, false ceilings, and attics, where they gather in large groups. Successful flies leave behind an aggregation pheromone, which attracts subsequent waves of flies to the same location, often causing recurring infestations.
Preventing Cluster Fly Entry
Because cluster flies are attracted to the structure for overwintering, the most effective defense is physical exclusion carried out before the autumn migration begins. Sealing potential entry points on the exterior of the building is the primary focus.
Sealing efforts should target:
- Cracks and crevices around all window and door frames, using a high-quality silicone or acrylic latex caulk.
- Gaps where siding meets the foundation or roofline.
- Areas where utility lines, cables, and pipes enter the building envelope.
All attic vents, soffit openings, and exhaust fans must be equipped with fine mesh screening (1/16th of an inch or less) to prevent entry. Repairing or installing weatherstripping around doors and windows ensures these common entry points are sealed when closed. Prioritizing the sunniest, warmest sides of the home, typically the south and west facades, during this effort can significantly reduce the number of flies that successfully enter.