The presence of slow-moving, buzzing flies indoors during colder months can be unsettling, often leading people to assume they have a standard house fly infestation. While the common house fly is drawn to food and decaying matter, the cluster fly (Pollenia rudis) is a distinctly different pest. These flies are slightly larger and darker than their common counterparts, identified by yellowish-golden hairs on their thorax and the way their wings overlap across the abdomen when at rest. Managing their appearance indoors requires understanding the cluster fly’s specialized life cycle.
Do Cluster Flies Reproduce Indoors
The question of whether cluster flies lay eggs inside houses can be answered with a definitive no. Despite the sight of dozens or hundreds of these insects clustered in attics or around windows, they are not breeding within the structure. Cluster flies are strictly nuisance pests that use buildings for shelter, not for reproduction or feeding. They do not deposit eggs on food, garbage, or animal waste indoors, meaning they pose no health risk like the common house fly. The adult flies found inside are in a non-reproductive, semi-dormant state, requiring an outdoor host to complete their life cycle.
The Unique Cluster Fly Life Cycle
The cluster fly’s entire reproductive cycle is tied to the outdoor environment and a specific biological host. Adult female flies lay eggs in cracks and crevices in the soil, usually near earthworm burrows, during warmer months. Once the larvae, or maggots, hatch after about three days, they actively seek out and bore into earthworms. The fly larvae are obligate parasites, meaning they must develop inside the earthworm, feeding on its tissues for up to three weeks. After this parasitic phase, the mature larvae exit the earthworm, which often does not survive, and pupate in the surrounding soil before emerging as adult flies.
Why Cluster Flies Invade Homes
The sudden appearance of cluster flies is a direct result of their overwintering behavior, not their breeding cycle. As late summer transitions into fall and outdoor temperatures drop, adult flies cease reproduction and seek protected sites for hibernation. They are attracted to the sun-warmed sides of buildings, particularly those facing south and west, where they congregate in large numbers. This behavior leads to the nickname “attic flies,” as they gravitate toward the highest and warmest points of a structure.
The flies gain access to the interior by exploiting the smallest gaps in the building’s exterior envelope. They squeeze through cracks around window frames, door casings, loose-fitting siding, and utility pipe penetrations. Once inside, they migrate into secluded, undisturbed spaces like attics, wall voids, and seldom-used closets to enter dormancy. Their presence becomes noticeable when indoor heating or a warm winter day causes them to awaken prematurely and emerge into living spaces.
Preventing and Removing Cluster Flies
Since cluster flies do not reproduce indoors, control efforts must focus on preventing their entry and removing those already present. Exclusion is the primary strategy, involving sealing all potential entry points on the exterior of the home before late summer. This means applying quality silicone caulk to seal cracks around window and door frames, utility lines, and eaves. Damaged window screens or unscreened attic vents should also be repaired or covered with fine mesh to block access to voids.
For existing infestations, active removal is the primary solution, as insecticides are often ineffective in reaching flies hidden deep within wall voids. The most practical method for dealing with active flies in a room is using a vacuum cleaner to suck them up, as they are sluggish and easy to catch. Placing light traps in attics or other non-living spaces where the flies cluster can help reduce the population by attracting and capturing the awakened insects.