Blackflies, small, dark, humpbacked biting flies from the family Simuliidae, are a well-known nuisance. Measuring only 1 to 5 millimeters, these insects are often called buffalo gnats due to their humped appearance. They are notorious for bothering humans, pets, and livestock, with their populations peaking in late spring and early summer. Female flies are responsible for biting, as they require a blood meal for their eggs to develop.
Blackfly Lifecycle and Habitat
The lifecycle of a blackfly is tied to clean, fast-flowing rivers and streams. Female flies deposit 150 to 500 eggs on submerged surfaces like rocks, logs, and vegetation. These eggs hatch into larvae that attach to these underwater objects, using specialized mouthparts to capture organic particles from the current.
After several larval stages, the insect develops into a pupa anchored to the submerged substrate. When development is complete, the adult fly emerges, floating to the water’s surface in an air bubble before flying away. Adult flies then inhabit wooded and shaded locations near the moving water where they developed.
The Blackfly Bite
A blackfly obtains a blood meal differently than a mosquito. Instead of piercing the skin, a female blackfly uses sharp, blade-like mouthparts to cut the skin. This creates a small wound, allowing the fly to lap up the blood that pools at the surface. The fly’s saliva contains an anticoagulant to prevent clotting while it feeds.
This feeding method results in an uncomfortable reaction. The initial bite may be painful, leaving a small spot of blood. Following the bite, many people experience significant swelling, intense itching, and a hard, raised welt. A large number of bites can cause “blackfly fever,” an immunological response with symptoms like fever, headache, and nausea.
Associated Health Risks
While blackfly bites are a painful nuisance, they can also pose direct health risks in some parts of the world. In specific regions, primarily sub-Saharan Africa and parts of Central and South America, certain species are vectors for the parasitic nematode Onchocerca volvulus. This parasite causes Onchocerciasis, or “River Blindness,” which can lead to severe skin disease and permanent vision loss.
This disease is not a health concern in North America or Europe, where blackflies are considered pests whose bites do not transmit disease to humans. However, their presence can have a significant economic impact on agriculture. Swarms of biting blackflies can cause stress to livestock, disrupt grazing, and in severe cases, lead to death from blood loss or allergic reactions.
Prevention and Control
Protecting yourself from blackfly bites involves physical barriers and repellents. Wearing light-colored clothing, including long sleeves, pants, and socks, can make it difficult for flies to land and bite. In heavily infested areas, a head net can shield the face, neck, and ears. Insect repellents containing DEET or Picaridin are effective when applied to exposed skin and clothing.
Controlling blackfly populations over a large area focuses on their larval stage and is managed by regional authorities, not individual homeowners. This involves the application of Bacillus thuringiensis israelensis (Bti) into the rivers and streams where larvae are present. Bti is a naturally occurring soil bacterium toxic only to the larvae of insects like blackflies and mosquitoes, but harmless to fish, birds, and other wildlife. This targeted approach reduces the number of adult flies that emerge.