The sudden, sharp sensation of a bite, often followed by intense itching, can quickly ruin time spent outdoors. When this happens, the culprit is usually one of several species known as biting flies, which are distinct from the common house fly. Unlike non-biting flies that only land to feed on surface materials, these insects possess specialized mouthparts for drawing blood. Understanding why these flies target humans is the first step toward effective prevention, along with learning about the specific types of flies, the biological cues that draw them to us, and effective control strategies.
Identifying the Common Biting Flies
Biting flies vary significantly in size and behavior, and identifying the species attacking you can inform your defense strategy. The Stable fly, sometimes called the “biting house fly,” closely resembles a common house fly but features a stiff, piercing proboscis. These flies deliver a painful bite, often targeting the lower legs and ankles, and require several blood meals to reproduce.
Horse flies and Deer flies belong to the same family and are much larger. They inflict a particularly painful wound because they use scissor-like mouthparts to slice the skin open before consuming the pooling blood. Horse flies can be up to an inch long, while the slightly smaller Deer fly is recognizable by the dark, patterned bands across its wings.
Black flies, or “buffalo gnats,” are small, dark, and possess a distinct humpbacked appearance. They swarm aggressively, often around the head and face, creating a small, itchy, bleeding puncture wound, sometimes accompanied by localized swelling. The Biting Midge, commonly known as a “no-see-um,” is small enough to pass through standard window screening. Despite their size, their bite inflicts a burning sensation and can lead to intensely itchy welts.
Sensory Cues That Attract Biting Flies
The primary reason flies seek out humans is the biological necessity of obtaining a blood meal, which provides the protein female flies need to develop their eggs. These insects locate warm-blooded targets by detecting a combination of sensory cues. One of the most significant attractants is the carbon dioxide (CO2) exhaled from human breath.
Flies use specialized sensory organs to track this CO2 plume over long distances, effectively zeroing in on the source. As the fly approaches, other cues guide the final attack. The body heat radiating from the skin signals a potential blood source, as warm-blooded animals maintain a temperature gradient distinct from the surrounding air.
Chemical compounds released in human sweat and skin odor further narrow the target search. Substances like lactic acid and carboxylic acids, which are byproducts of metabolism, are highly attractive to many biting species. The fly relies on a combination of these signals—CO2, heat, and odor—to confirm a host is present.
Immediate Personal Protection Strategies
When flies are already present, immediate action focuses on creating a barrier and masking your attractive biological signature. Wearing protective clothing is an effective defense, as most biting flies cannot penetrate fabric. Wear long-sleeved shirts, long pants, and a wide-brimmed hat, ensuring shirts are tucked in to prevent flies from reaching bare skin.
The color of your clothing also influences attraction, as many biting flies are drawn to dark colors. Opting for light-colored fabrics, such as khaki, tan, or white, makes you a less appealing target. For high infestation areas, a permethrin-based insecticide can be applied to clothing and outdoor gear, which repels and kills flies upon contact, providing a long-lasting chemical shield.
Topical repellents applied directly to exposed skin disrupt the fly’s ability to sense chemical cues. Products containing DEET (N,N-Diethyl-meta-toluamide) or Picaridin are recommended for broad-spectrum protection against various biting insects. If you are indoors or on a porch, physical barriers such as electric fans can be used, as the strong air current easily disorients and prevents small flies like midges from landing.
Eliminating Breeding Habitats
Long-term management requires eliminating the materials and locations where flies lay their eggs and develop. Stable flies breed in moist, decaying organic matter, including wet straw, hay bales, grass clippings, and animal manure. Regularly cleaning up and drying out these materials, especially in livestock areas, is the most direct way to break their life cycle.
Proper composting is a significant control measure; piles should be turned frequently to generate heat, which kills fly larvae and eggs. For species that favor aquatic environments, such as Deer flies and Biting Midges, source reduction involves removing standing water and drying out muddy areas. This includes clearing clogged roof gutters and leveling ground depressions where water collects.
Black flies are unique because they require clean, fast-running water, like rivers and streams, for their larvae to develop. Control in these situations is complex and often involves community-wide efforts to treat the water source. For the homeowner, structural exclusion is a final defense, meaning sealing small cracks and ensuring all windows and doors have screens. Fine mesh screens are necessary to block tiny pests like no-see-ums.