Black flies (family Simuliidae) are small, biting gnats that cause a nuisance due to their swarming behavior and painful bites. They are found near running water, which is necessary for their life cycle. The female requires a blood meal to produce eggs, leading to intense localized swarms. Managing a black fly problem requires a multi-faceted approach, combining immediate personal protection with long-term population control.
Identification and Behavior
Black flies are tiny insects, typically between an eighth and a quarter of an inch long, characterized by a distinct humpbacked appearance that earns them the nickname “buffalo gnats”. They are usually dark gray or black, with short antennae and stout legs. Only the female black fly bites, as she requires a blood meal to mature her eggs.
The female’s feeding mechanism is unique; instead of piercing the skin like a mosquito, she uses specialized mouthparts to slice a small hole, allowing blood to pool for her to consume. She injects anticoagulants and a mild painkiller during the process, leading to the characteristic localized swelling and intense itching that results from an allergic reaction to the foreign proteins. Black flies are primarily daytime biters, with peak activity often occurring during the morning and early evening.
Black fly larvae develop exclusively in clean, flowing water, such as streams, rivers, and even small rivulets. The larvae attach to submerged objects like rocks and vegetation, filtering organic matter from the moving current. This dependency on running water means that black fly populations are often concentrated near these aquatic habitats, sometimes flying up to ten miles in search of a host.
Personal Protective Measures
Immediate relief from black fly bites involves creating barriers and using effective repellents to deter the insects from landing. Wearing appropriate clothing is the first line of defense, as black flies are attracted to dark colors. Opting for light-colored, loose-fitting clothing, long sleeves, and long pants can significantly reduce the number of bites.
Protecting the head and face is particularly important since black flies often swarm around the eyes, ears, and nose. A wide-brimmed hat and a fine-mesh head net offer a simple yet highly effective physical barrier against swarming adults. For chemical defense, traditional mosquito repellents may not be as effective against black flies.
Repellents containing DEET or Picaridin are recommended for application to exposed skin. Picaridin, a synthetic compound, is often more effective than DEET at repelling biting flies and is less likely to damage plastics. Clothing can be treated with Permethrin, an insecticide that binds to fabric and repels or kills insects on contact, but this product should never be applied directly to the skin.
Source Reduction and Prevention
Since black flies breed in flowing water, localized source reduction focuses on making the immediate area around a home less hospitable to adult flies seeking shelter. While homeowners cannot alter a river, they can manage the nearby landscape to reduce resting areas. Trimming dense shrubs, tall grasses, and other heavy vegetation near outdoor gathering spaces removes sheltered spots where adult flies rest before and after feeding.
A simple, non-chemical method for deterring adult black flies on porches or decks is the use of electric fans. Black flies are weak fliers, and the turbulent air created by a fan disrupts their flight path, making it difficult for them to land and bite. Placing an oscillating fan near seating areas can provide a localized zone of protection.
Other deterrents, such as essential oils like vanilla extract, cedarwood, or lavender oil, offer minor relief but are not primary solutions for heavy black fly pressure. Sticky traps are ineffective for controlling large populations. Homeowner efforts are best spent on personal protection and mechanical barriers rather than attempting small-scale source elimination.
Area-Wide Control Methods
The most effective strategy for reducing black fly populations involves targeting the larval stage in their aquatic breeding grounds. This large-scale control is typically conducted by municipal or county pest control programs, as treating running waterways requires specialized permits. The biological control agent of choice is Bacillus thuringiensis israelensis (Bti).
Bti is a naturally occurring soil bacterium highly specific to the larvae of black flies, mosquitoes, and fungus gnats. When larvae ingest the Bti spores while filter-feeding, the bacterium releases crystalline toxins that disrupt the digestive system. This causes them to stop feeding and die within hours. This larvicide is considered safe for humans, pets, and most non-target aquatic organisms, including fish.
Controlling adult black flies through traditional spraying is difficult and impractical due to their vast numbers and wide dispersal, making source control with Bti the preferred method. Community-wide programs monitor larval populations and apply Bti to streams and rivers when larvae are present. This aims to reduce the number of emerging adults before they become a nuisance.