Blackflies, often called buffalo gnats due to their distinct, humpbacked appearance, are a significant seasonal nuisance, particularly near moving water. These insects swarm during the late spring and early summer, making outdoor activities uncomfortable. Effective control requires a two-pronged strategy: immediate protection from adult flies and long-term prevention by targeting their aquatic breeding grounds.
Identifying the Pest and Its Threat
Blackflies are small, dark, gnat-like insects, typically 2 to 5 millimeters long, recognized by their slightly arched, hunched thoracic region. Only the female blackfly bites, requiring a blood meal to develop her eggs, and feeds primarily during the day. They locate hosts by sensing carbon dioxide and body heat.
Blackflies swarm in large numbers, especially near the running water sources where they breed, such as streams and rivers. Unlike mosquitoes, the female blackfly uses specialized mouthparts to cut a small wound to draw blood. This feeding method causes a small, painful puncture wound that results in localized swelling, bleeding, and intense itchiness lasting several days.
Immediate Personal Protection and Relief
Protecting yourself requires creating physical and chemical barriers. The most reliable chemical defense involves using topical insect repellents containing synthetic ingredients like DEET or Picaridin. Picaridin formulations, modeled after a substance found in pepper plants, are particularly effective against biting flies, often performing better than DEET.
When using DEET, concentrations between 25% and 50% offer sustained protection. Picaridin concentrations of approximately 20% are recommended, offering the benefit of being virtually odorless and not damaging plastics or synthetic fabrics. For a physical barrier, light-colored clothing is preferable, as blackflies are often drawn to dark colors. Wearing long sleeves and pants minimizes exposed skin. A fine-mesh head net provides immediate relief for the face and neck. Timing outdoor activities also helps, as adult blackflies are most active during daylight hours. Avoiding peak activity times, such as late morning and early afternoon, lessens the chance of encountering large swarms.
Controlling Blackflies in Outdoor Spaces
Reducing the adult blackfly population around decks, patios, and yards requires localized control measures. Electric fans are an effective tool, as the strong air currents easily disrupt the flight of these small insects, creating a fly-free zone. Placing multiple fans around a seating area establishes a barrier blackflies are reluctant to cross.
Commercial trapping systems also reduce the adult population near the home. Carbon dioxide (CO2) traps mimic the breath of a host, using dry ice or propane to release CO2 that attracts the blackflies. Once attracted, the flies are captured in a collection bag or on a sticky surface. Outdoor misting or fogging systems offer temporary relief by dispersing insecticides, but this method provides only short-term control and must be used carefully. Sticky traps can be placed near areas of high activity to capture swarming adults.
Eliminating Breeding Sources for Long-Term Prevention
The most sustainable strategy for long-term blackfly prevention is eliminating their larval habitat, as blackflies breed exclusively in moving water. Larvae attach themselves to submerged rocks and vegetation in flowing water, ranging from small creeks to large rivers. For large-scale control, many municipalities employ biological larvicides containing the naturally occurring soil bacterium Bacillus thuringiensis israelensis (Bti).
Blackfly larvae must ingest Bti, which releases a toxin that specifically disrupts their alkaline digestive system, leading to mortality. This approach is favored because Bti is highly specific, having negligible effects on humans, mammals, birds, and most other aquatic life, including fish and non-target insects. Homeowners cannot typically apply Bti to natural streams or rivers. However, they can focus on small, contained water features on their property. For decorative water features, altering the flow or removing submerged debris makes the habitat less suitable for blackfly larvae. Removing vegetative material or altering the water speed in small channels reduces the number of sites where larvae can attach and feed. Disrupting the larval stage is the only way to break the breeding cycle.