The presence of biting black flies, also known as buffalo gnats, can quickly make a yard unusable during the late spring and early summer months. These small, dark pests are distinct from common house flies or mosquitoes. Unlike mosquitoes, which favor stagnant water, black flies depend entirely on moving water for their development, meaning different strategies are needed for control. Effective management involves eliminating the source of reproduction and implementing measures to repel or manage the adult flies already present.
Confirming the Pest
Black flies are small, ranging from 1 to 5 millimeters in length, and have a dark, black or gray, color. A distinguishing physical feature is their noticeably arched or “humped” back. Only the female black flies bite, as they require a blood meal to produce eggs, using blade-like mouthparts to slash the skin rather than piercing it like a mosquito. These biting adults appear in swarms, flying around the head, ears, and eyes, and are most active during the daytime, particularly on humid, cloudy days.
Targeting Breeding Sources
The most effective long-term solution involves addressing the black fly’s requirement for an aquatic habitat. Black flies exclusively lay their eggs and develop their larvae in flowing, oxygenated water, such as small streams, creeks, drainage ditches, or irrigation runoff. The larvae attach themselves to submerged objects like rocks or vegetation, where they filter feed on organic matter in the current. Simply treating standing water will have no effect on black fly populations.
Homeowners must locate any source of moving water, even a small trickle, on or near their property and determine if it can be eliminated or treated. For water sources that cannot be removed, the application of a biological larvicide containing Bacillus thuringiensis israelensis (Bti) is the industry standard for control. Bti is a naturally occurring soil bacterium that is highly specific and only affects the larvae of black flies, mosquitoes, and a few other fly species when they ingest it. It is considered environmentally sound because it does not harm fish, birds, mammals, or beneficial insects.
Applying Bti to moving water requires precise calculation based on the stream’s flow rate, size, and temperature to ensure the correct concentration reaches the larvae. This specialized application is often managed by local vector control districts or certified professionals, especially for larger water bodies, to maximize effectiveness and comply with regulations. Since larvae continue to emerge throughout the season, Bti treatments must be repeated every two weeks to continuously interrupt the life cycle and prevent the development of biting adults.
Quick Solutions for Adult Flies
While source control is working, immediate relief from adult flies is necessary for enjoying the yard. Personal repellents applied directly to the skin provide a temporary barrier against biting flies. Highly effective options include those containing DEET or Picaridin, which repel black flies for several hours. Picaridin is often favored because it is less likely to damage plastics or synthetic fabrics compared to DEET. Repellents based on Oil of Lemon Eucalyptus (OLE) also offer protection, though it may not last as long as synthetic options.
Using yard-wide insecticide foggers or sprays is discouraged for black fly control. The flies can fly in from breeding sites several miles away, making the relief extremely brief. These broad-spectrum treatments also pose a risk to beneficial insect populations, such as pollinators, with little long-term impact on the problem.
Long-Term Yard Prevention
Modifying the outdoor environment can make the yard less attractive to adult black flies. A highly effective physical tactic is the use of electric fans on patios and decks. Black flies are weak fliers, and the air movement from a fan creates an air current they cannot easily push through, preventing them from landing and biting. The fan’s breeze also helps disperse the carbon dioxide that humans exhale, which is a primary attractant for biting insects.
Physical barriers are another reliable prevention method, especially for outdoor seating areas. Installing fine-mesh screening on porches, gazebos, or windows can block the small flies from entering the space. Unlike mosquitoes, black flies are not attracted to light, so switching to yellow insect-resistant bulbs is not a specific deterrent for this pest.