How to Get Rid of House Flies, Cluster Flies, and Blow Flies

The Most Common Fly Control Mistake

Most homeowners dealing with a fly infestation focus on the adult flies they can see — spraying, swatting, and setting traps. These measures reduce adult counts temporarily but do nothing about the source that is producing new adults continuously. A house fly can complete its life cycle from egg to adult in as few as 7 days in warm conditions, which means a single breeding source can sustain an apparently endless infestation indefinitely. Identifying the fly species tells you where to look for the source — and finding the source is what actually ends the infestation.

Identifying Your Fly Species

The physical characteristics and behavior of the flies you're seeing narrow down the source before you even begin looking. Species identification should be the first step in any fly control investigation.

• House Fly (Musca domestica) — Medium-sized, gray with four dark stripes on the thorax. Active throughout the day. Found near garbage, animal waste, and decaying organic matter. The most common filth fly in homes and food facilities. If you have large numbers of house flies, the source is outdoor — garbage management and entry exclusion are the primary interventions.
• Cluster Fly (Pollenia rudis) — Larger than a house fly, slightly golden-haired thorax, sluggish movement. Found in large numbers on upper-floor windows in fall and spring. Overwinters in wall voids and attics. Not associated with sanitation problems — it is an overwintering insect, not a filth fly. The source is the wall void, not something decaying.
• Blow Fly (Calliphoridae family) — Metallic blue or green, slightly larger than a house fly, loud buzzing flight. Breeds in carrion, meat, and animal carcasses. Blow flies emerging from inside a structure — particularly from a specific wall or ceiling area — almost always indicate a dead animal in the wall void, attic, or crawl space.
• Phorid Fly (Megaselia species) — Very small, hump-backed, runs in short erratic bursts rather than flying continuously. Found at floor level in kitchens, restrooms, and mechanical rooms. Indicates decaying organic matter in a plumbing system, grease trap, or crawl space with moisture. A plumbing investigation is often needed alongside pest treatment.

House Fly Control: It Starts Outside

House flies breed outdoors and enter structures — they do not establish breeding populations indoors under normal circumstances. Effective house fly management targets the outdoor source and the structural entry points, not just the flies already inside.

• Identify and Manage the Outdoor Source — Garbage containers without tight-fitting lids, uncleaned dumpster areas, pet waste accumulation, compost piles, and decaying organic material near the structure are the primary house fly breeding sources. Eliminating or managing these reduces fly pressure at the source.
• Entry Point Exclusion — House flies enter through open doors, torn window screens, and gaps around entry points. Air curtains (fly fans) over commercial entry doors are standard in food service — the equivalent for residential is tight-fitting door sweeps, intact screens, and self-closing mechanisms on frequently used exterior doors.
• Exterior Perimeter Treatment — Residual perimeter treatment applied to exterior walls, window frames, and entry areas around the structure reduces the fly population approaching entry points. Outdoor fly light traps positioned away from the structure (not at entry points, where they attract flies toward the building) can reduce outdoor population density.
• Interior Treatment for Active Flies — Interior fly light traps capture adult flies in the immediate area. Residual treatment to resting surfaces — interior window frames, ceiling areas, and wall surfaces where flies rest — provides additional knockdown. Interior treatment alone without managing the outdoor source produces only temporary reductions.

Cluster Fly Control: Stopping the Fall Invasion

Cluster flies are not a sanitation problem and do not respond to the same control approaches as house flies. They are outdoor-developing insects that overwinter as adults in wall voids and attics. The control strategy is identical to other overwintering invaders: perimeter treatment in late summer before aggregation, combined with structural exclusion of entry gaps.

• Treatment Timing Is Critical — Cluster flies begin seeking overwintering sites in late August through September as nighttime temperatures drop. A perimeter treatment applied to exterior walls, eaves, and soffit areas during this window contacts flies before they enter the structure.
• Entry Point Identification — Cluster flies enter through the same gaps repeatedly year after year — roofline gaps where siding meets soffit, gaps around windows in older construction, and openings around eave vents. Identifying these points and sealing them after the overwintering population exits in spring breaks the cycle.
• Spring Emergence Management — Cluster flies found inside in February–May are the overwintering population emerging. Vacuuming is the practical removal method. Do not apply interior insecticide to the wall void for cluster flies — dead insects in the void attract dermestid beetles that cause additional problems.
• Attic Fogging for Large Populations — For structures with very large established cluster fly populations in the attic, a professional insecticide fogging of the attic space can reduce the overwintering population. This is typically done in October after entry and before spring emergence.

Blow Flies Inside: Finding the Source

Blow flies emerging from inside a structure require a different investigation than any other fly problem. The source is almost certainly a carcass — a rodent, bird, or squirrel that died in the wall void, attic, or crawl space. Blow fly maggots develop in the carcass, pupate in surrounding materials, and adult flies emerge from the wall or ceiling surface days to weeks after the animal died.

• Locate the General Area — Blow flies emerging consistently from a specific wall section or ceiling area help triangulate the carcass location. Note where adult flies concentrate — they tend to cluster near the emergence point.
• Odor as a Locator — A decomposing carcass produces a distinctive odor that intensifies with temperature. The strongest odor point on the interior wall surface is often directly adjacent to the carcass location.
• Access and Removal — If the carcass can be accessed through the attic or crawl space, removal eliminates the source and ends the blow fly emergence within days. Wall void access requires cutting drywall — weigh the cost against waiting for natural desiccation, which can take 1–4 weeks depending on the animal size and temperature.
• Secondary Pest Management — Rodent carcasses in wall voids attract dermestid beetles as the carcass desiccates. If rodents are the source, a rodent exclusion program should follow blow fly treatment to prevent recurrence.