Black flies, often called buffalo gnats or biting gnats, are a seasonal menace found near the running water where they breed. The female black fly requires a blood meal to produce eggs, and their bite is painful due to their unique feeding method. Unlike a mosquito that pierces the skin, the black fly uses scissor-like mouthparts to cut a small hole, allowing them to lap up the pooling blood. This results in an irritating wound and a localized allergic reaction during their active season.
Personal Protection Strategies
Defense against black flies focuses on chemical repellents and physical barriers applied directly to the body and clothing. Picaridin is an effective synthetic compound for topical application; a 20% concentration offers reliable, non-greasy protection for several hours against biting flies. Picaridin does not damage synthetic fabrics or plastics, unlike DEET.
DEET remains a gold standard, with formulations in the 25% to 50% range recommended for sustained protection. Because DEET can degrade certain synthetic materials, it should be applied carefully to exposed skin only. For those preferring a plant-derived alternative, Oil of Lemon Eucalyptus (OLE), specifically the refined active ingredient p-menthane-3,8-diol (PMD), provides protection for a few hours.
Physical barriers offer a straightforward method to prevent bites, as black flies cannot penetrate woven material. Wearing long-sleeved shirts and long pants is recommended, especially when choosing light colors like white, gray, or khaki. Black flies are visually attracted to dark hues, which they may mistake for a host animal. Tucking the shirt into the pants and the pant legs into socks eliminates small entry points. Permethrin is an insecticide that can be applied to clothing, hats, and gear for added protection, but it should never be applied directly to the skin.
Modifying Your Outdoor Environment
Creating a localized “no-fly zone” around outdoor sitting areas uses environmental conditions against the black fly. A simple electric fan is a powerful tool because black flies are weak fliers, and the slightest breeze makes navigation difficult. A fan’s airflow creates a physical obstacle, establishing a turbulent pocket of air that prevents the insects from landing.
This airflow disrupts the black fly’s ability to locate a host by scattering the carbon dioxide plume exhaled through breath. Dispersing this primary chemical attractant makes it harder for the flies to target you. Positioning a fan to create a steady current across a patio or deck can significantly reduce the number of flies in the immediate vicinity.
It is beneficial to adjust the timing of outdoor activities to avoid the flies’ peak swarming periods. Black flies are active only during the daytime, and their biting activity often peaks in two main windows. These periods typically occur in the late morning (9:00 AM to 11:00 AM) and again in the late afternoon (4:00 PM to 7:00 PM, or until sunset). Scheduling work or recreation for midday, when the flies are less active, or in the early morning or late evening can provide relief.
Identifying and Addressing Breeding Sources
The most effective long-term deterrence involves addressing the source of the infestation: the water where the larvae develop. Black flies breed exclusively in clean, fast-moving water, such as streams, rivers, and roadside ditches with a steady flow. The adult female lays eggs on vegetation or rocks in the current, and the larvae then attach themselves to submerged surfaces to filter-feed.
Controlling the larval stage is efficient because the larvae are concentrated in predictable locations before they emerge as biting adults. The biological larvicide Bacillus thuringiensis israelensis (Bti) is the primary material used in control programs. Bti is a naturally occurring soil bacterium that produces a protein crystal toxic only to the larvae of black flies and mosquitoes.
When black fly larvae ingest the Bti spores while feeding, the alkaline conditions in their digestive system dissolve the crystal, destroying the gut lining and killing the insect. This method is desirable because Bti is specific and non-toxic to fish, birds, mammals, and other non-target organisms. However, individuals should not attempt to treat moving water on their own property. Applying any pesticide to surface waters is often regulated by state or local environmental agencies and typically requires a special permit. Source reduction is most often handled by municipal or county vector control specialists who have the expertise and authorization to apply the larvicide safely and effectively.