The sudden appearance of numerous flies covering the exterior of a home is a common experience. This phenomenon indicates that the structure or its immediate surroundings are providing a strong attractant or shelter. Addressing the issue effectively begins with accurately identifying the species involved and tracing the specific source that draws them to your property.
Common Flies That Congregate Outdoors
One frequent culprit is the cluster fly (Pollenia rudis), which appears dull gray and moves slowly. These insects do not breed indoors or near garbage; their larvae are parasitic on earthworms. Their presence on the sunny sides of a building in late summer and fall signifies they are seeking sheltered locations to overwinter. This behavior is driven by seasonal temperature changes. They are strongly attracted to light-colored surfaces and warmth, using small cracks and crevices to gain entry into wall voids or attics. A large congregation suggests a nearby population of earthworms and suitable structural entry points.
Blow flies, or bottle flies, are easily recognized by their striking metallic coloration, usually shades of blue, green, or bronze. They are fast fliers and highly sensitive to odors associated with decomposition. Their congregation strongly indicates nearby decaying organic material.
These flies are drawn to dead animals, such as rodents or birds, that may have died within a wall void, attic, or crawl space. They lay eggs on this material, and the resulting larvae feed on the carrion, completing their life cycle quickly. The emerging adults can create a localized swarm on the house exterior.
The common house fly is a smaller, grayish insect distinguished by four dark longitudinal stripes on its thorax. Unlike the seasonal cluster fly, house flies are a year-round nuisance, drawn intensely to moisture and fermenting organic waste.
A swarm of house flies suggests an accessible food source or breeding site is close by, typically involving high moisture and decomposing food residue. They frequently breed in manure, spilled animal feed, or wet garbage, completing their life cycle from egg to adult in as little as ten days.
Locating the Source of Attraction
Blow Flies and Decomposition
The first diagnostic step involves searching for sites of decomposition, the primary attractant for metallic blow flies. A deceased animal, such as a squirrel or mouse, may have expired within a chimney flue, behind a fascia board, or under a deck. Locating and safely removing this material is the only way to eliminate the resulting adult swarm.
House Flies and Sanitation
For house flies, attention must turn to sanitation and waste management. Open or poorly sealed garbage containers, particularly those holding food scraps, provide an ideal breeding substrate. Pet waste not routinely collected from the yard can also serve as a significant source of attraction.
Outdoor areas where food or sugary liquids are consumed should be scrutinized for residue. Even small amounts of spilled liquids or grease near a patio can support a fly population through fermentation. Compost piles must be managed correctly, ensuring they are turned often and covered to prevent access.
Moisture Sources
Moisture accumulation around the foundation or in landscaping can also support fly development. Clogged rain gutters or leaky outdoor faucets create persistently wet areas, which encourage conditions ideal for larval development. Eliminating standing water is crucial for breaking the reproductive cycle.
Cluster Flies and Structure
When cluster flies are the problem, the source is the structure itself. They seek protected, internal harborage for the winter, and the warm, sun-exposed side of the house acts as a beacon. Focus must be directed toward finding and sealing the minuscule gaps in siding, window frames, and utility penetrations that allow entry.
Immediate Actions for Fly Reduction
Immediate sanitation changes are necessary to disrupt the fly life cycle once the source is identified. This includes securely disposing of all pet feces and ensuring all outdoor trash cans have tightly fitting lids. Cleaning the exterior of garbage cans with a detergent removes residual organic film that attracts flies.
Addressing structural issues is essential, especially for cluster flies seeking overwintering shelter. Inspecting and sealing cracks or crevices larger than an eighth of an inch around utility lines, window frames, and door jambs prevents entry into wall voids. Replacing or repairing damaged window and door screens limits access to the home’s interior.
Effective water management involves clearing debris from rain gutters to ensure proper drainage away from the foundation. Repairing leaky hoses or sprinkler heads eliminates moisture-rich breeding sites. Reducing excess moisture in mulch beds near the perimeter makes the environment less hospitable for fly reproduction.