The cluster fly (Pollenia rudis) is a common indoor pest that frequently enters homes during the cooler months, often appearing sluggish and clumsy around windows. These insects are seeking a quiet, protected location to survive the winter, leading to the sudden appearance of large numbers inside a living space. This information helps to distinguish the cluster fly from other flying pests.
Do Cluster Flies Pose a Biting Threat?
Cluster flies pose no biting threat to humans or pets because they are incapable of piercing skin. The adult cluster fly possesses sponging mouthparts, designed exclusively for consuming liquids like flower nectar or fruit juices. These mouthparts function like a sponge, mechanically different from the sharp, stylet-like mouthparts used by biting insects.
The absence of mandibles and specialized piercing structures confirms that the cluster fly cannot bite. They are considered a nuisance pest rather than a medical risk. Cluster flies are not associated with the same level of disease transmission as house flies, which breed in unsanitary conditions.
The risk of disease transmission from cluster flies is low. Studies show that P. rudis can carry bacteria that may cause opportunistic infections, particularly in sensitive environments like hospitals. This transmission is mechanical, occurring on the outside of their bodies, not through biting. For the average homeowner, the primary issue is the sheer number of flies and their irritating presence.
Identifying the Overwintering Pest
Correctly identifying the cluster fly is important for effective control, as they are often mistaken for the common house fly. The cluster fly is slightly larger, measuring about 3/8 of an inch long, and appears dark-gray with a checkered abdomen. A distinguishing feature is the numerous short, crinkly golden or yellowish hairs on the thorax, which can give them a golden sheen.
When at rest, the cluster fly’s wings overlap neatly down the center of its back, resembling a pair of scissors. This trait sets it apart from the house fly, whose wings remain separated. Their behavior is also a strong identifier; they are notably sluggish flyers, often seen crawling slowly on walls or buzzing aimlessly around windows.
The cluster fly lifecycle confirms they do not breed indoors. The female lays eggs in the soil near earthworm burrows, and the larvae are parasitic, feeding exclusively on earthworms. The presence of flies indoors only signifies that the structure has been selected as an overwintering site, typically in protected voids like attics or wall spaces.
Practical Steps for Cluster Fly Removal
Control strategies for cluster flies focus primarily on exclusion and physical removal, as they do not breed inside the home. The most immediate and effective method for removing active, sluggish flies from living spaces is using a vacuum cleaner with a hose attachment. Remove the vacuum bag or empty the canister immediately after use and seal it outside to prevent the flies from escaping back into the house.
Sealing entry points is the best long-term preventative measure, ideally completed in late summer or early fall before the flies seek entry. Inspect the exterior for cracks and gaps around window and door frames, utility lines, and under the eaves. Use caulk or weatherstripping to seal these small openings, which the flies use to access wall voids and attics.
Ensure that all attic and gable vents, as well as any other openings like chimneys, are covered with fine-mesh screening. Blocking their access to these quiet, protected overwintering sites is the only way to prevent their appearance indoors the following year. Dead flies that accumulate in voids should be cleared when possible, as their carcasses can attract secondary pests like carpet beetles.