Cluster flies (Pollenia rudis) are often mistaken for common house flies, but they exhibit a distinct behavior that makes them a seasonal nuisance inside buildings. These insects are slightly larger and duller than house flies, recognized by the golden-yellow hairs on their thorax and their sluggish movement. Unlike typical flies that breed in decaying matter, cluster flies spend their lives outdoors, only seeking shelter inside structures to survive the winter. Effective, natural control focuses on disrupting their life cycle and repelling them during their outdoor staging phase before they attempt to enter the home.
Understanding the Cluster Fly Life Cycle
The cluster fly life cycle is unique because it depends entirely on a host organism outdoors. Adult females lay their eggs loosely in damp soil, typically near earthworm burrows. Once the larvae hatch, they actively seek out earthworms, burrowing into the worms to parasitize and feed on them.
The larval stage lasts for several weeks inside the earthworm before the fly pupates in the soil, eventually emerging as an adult. This parasitic relationship means that controlling the flies by targeting their larvae is nearly impossible, shifting the focus to managing the adult population. In temperate regions, the flies can complete three to four generations per year, with the final generation seeking a warm, dry place to overwinter as temperatures cool. They commonly congregate on the sun-exposed sides of buildings in late summer and early fall before attempting to enter.
Natural Landscape Management for Prevention
Long-term prevention begins by modifying the outdoor environment to make it less hospitable to earthworm hosts and fly staging behavior. Cluster flies prefer rich, moist soil, which supports higher earthworm populations. Improving lawn drainage and reducing excessive watering helps limit the density of their larval food source.
Reducing the attractiveness of the soil near the foundation is a primary goal. Focus on creating a dry zone immediately surrounding the structure, as eliminating earthworms directly is impractical. Removing dense ground cover, heavy mulches, and debris near the perimeter eliminates shady, moist areas where flies lay eggs and earthworms thrive.
Cluster flies stage on sunny, south- and west-facing walls in the late afternoon while searching for entry points. Strategic planting can obscure the foundation and make walls less appealing. Planting low-growing, dense vegetation further away may provide an alternative staging area, but avoid planting anything that creates deep shade directly against the house. Maximizing direct sunlight exposure on the walls is important; flies are drawn to warmth but may be deterred by overly exposed, sun-baked surfaces when seeking a hiding spot.
Sealing the building exterior is the most effective preventative measure. Inspecting and sealing all cracks around windows, eaves, utility pipes, and siding with high-quality caulk should be completed by mid-August. Placing fine mesh screening over vents and openings provides a final layer of defense against entry.
Non-Toxic Active Outdoor Control Methods
Once cluster flies begin staging on exterior walls in late summer, active, non-toxic outdoor control methods are necessary to reduce the population before they overwinter inside. Physical removal is the most immediate and effective non-chemical solution. Large aggregations of flies can be removed using a powerful outdoor vacuum cleaner with a long hose attachment.
Vacuuming flies directly off sunny walls in the late afternoon is highly effective, as they are sluggish and concentrated in staging areas. After collection, the vacuum contents must be immediately sealed in a bag and disposed of far from the house to prevent escape and return. Another physical removal method is spraying the flies with a strong stream of water from a garden hose, which knocks them down and discourages them from landing.
Natural repellents can deter flies from landing on exterior walls. Essential oils like peppermint, clove, lemongrass, citronella, and eucalyptus oil are unappealing due to their strong aromas. A homemade spray can be created by mixing 10 to 15 drops of these essential oils per ounce of water with a small amount of dish soap to act as an emulsifier.
Apply these repellent sprays directly to exterior surfaces where flies congregate, particularly around window frames, door casings, and under the eaves. The most strategic time for application is the late afternoon or early evening, just before the flies cluster for the night. Application must be repeated frequently, perhaps daily, as essential oils dissipate quickly when exposed to sunlight and weather.
Decoy trapping can redirect flies away from the home. Cluster flies are attracted to light, making ultraviolet (UV) light traps a viable option for outdoor population reduction. Placing these non-toxic light traps far away from the house, such as near a shed or property line, lures adult flies away from the primary structure. The flies are attracted to the UV light and are then trapped on a sticky surface or killed by an electric grid, preventing them from seeking entry.