Black flies are small, dark, hump-backed insects that slash open skin and feed on the pooling blood. Found near rivers and streams across much of the world, they’re one of the most aggressive daytime biters you’ll encounter outdoors. At roughly 5 mm long (about the size of a sesame seed), they’re easy to underestimate, but their bites can swell to the size of a golf ball and, in some regions, transmit serious disease.
How to Identify Black Flies
Black flies, sometimes called buffalo gnats, belong to the family Simuliidae. Adults are small and dark with a distinctive hunched back, short wings, and stout bodies. They measure up to about 5 mm in length. Despite the name, not all species are jet black; many are dark gray or have orangish markings on the legs or abdomen. Their compact, rounded shape and the characteristic hump behind the head set them apart from mosquitoes, deer flies, and other biting insects.
Only females bite. They need a blood meal to develop eggs. Males feed on nectar and are harmless.
Where They Live and When They’re Active
Black flies breed exclusively in running water. Some species prefer large, fast-flowing rivers while others thrive in small, sluggish streams. Almost any kind of permanent or semi-permanent flowing water can support them, but biting species tend to breed most abundantly at the outlets of ponds and lakes, likely because of warmer temperatures and more food for developing larvae.
The timing of emergence depends on how a given species overwinters. Some overwinter as eggs submerged in streams, hatching in spring once water temperatures reach about 40° to 50°F. Others overwinter as larvae, developing through the cold months and emerging as adults from late April through May. In northern climates, peak black fly season runs from May into July, though the exact window varies by region and species.
Black flies are strictly daytime biters. They do not bite at night. Activity peaks tend to hit twice a day: around 9:00 to 11:00 in the morning and again from 4:00 to 7:00 in the late afternoon, continuing until the sun drops below the horizon. They’re most aggressive on humid, overcast days and just before storms. Bright, breezy afternoons tend to suppress their activity.
How Black Flies Bite
Unlike mosquitoes, which pierce the skin with a needle-like proboscis, black flies use blade-like mouthparts to lacerate the skin and create a small wound. They then feed on the pool of blood that forms. Their saliva contains a powerful anticoagulant that keeps the blood flowing freely while they feed. This saliva is also what triggers the intense immune reaction most people experience afterward.
You may not feel the initial cut. Black flies often crawl into hairlines, under clothing edges, and around ears and ankles before biting, so the first sign can be a trickle of blood or a growing itch rather than a sharp sting.
Bite Symptoms and Black Fly Fever
Reactions range widely from person to person. At the mild end, you’ll see a small puncture wound with moderate redness. At the more severe end, the bite area can swell dramatically, sometimes reaching the size of a golf ball. A raised lesion often forms at the bite site, and itching can be intense and persistent, lasting days.
When someone receives many bites in a short period, a cluster of systemic symptoms called “black fly fever” can develop. This includes headache, nausea, fever, and swollen lymph nodes in the neck. Black fly fever isn’t an infection; it’s your immune system reacting to the volume of foreign proteins injected through the saliva. It typically resolves on its own within a few days, but it can be miserable while it lasts.
People who are new to an area with heavy black fly populations tend to react more severely than long-term residents. Over years of repeated exposure, some people develop a degree of tolerance, though the bites never become entirely harmless.
Diseases Spread by Black Flies
In sub-Saharan Africa and parts of Central and South America, black flies transmit a parasitic worm that causes onchocerciasis, commonly known as river blindness. The flies breed in fast-flowing rivers and streams, and when an infected fly bites a person, it deposits immature larvae into the skin. These larvae mature into adult worms that can live in the body for years, producing millions of microscopic offspring that migrate through the skin and eyes.
Symptoms of river blindness include severe, unrelenting itching, disfiguring skin changes, and progressive visual impairment that can lead to permanent blindness. Early exposure in children has also been linked to epilepsy. The disease cycle perpetuates when another black fly bites an infected person, ingests the microscopic larvae, and carries them to the next host.
In North America and Europe, black flies are not known to transmit diseases to humans. The primary concern in these regions is the bite reaction itself and, in rare cases, secondary bacterial infection from scratching.
Treating Black Fly Bites
Start by washing the bite with soap and water to lower the risk of infection. Apply a cloth-wrapped ice pack for at least 20 minutes to reduce swelling. If you can, keep the bitten area elevated.
For itching, an over-the-counter antihistamine can help significantly. Anti-itch creams containing hydrocortisone also provide relief. If the bite is painful, a standard anti-inflammatory painkiller can take the edge off. The most important thing is to avoid scratching. Black fly bites itch intensely, and broken skin from scratching is the main route for bacterial infection.
Watch for signs of infection in the days following a bite. These include increasing redness or red streaks spreading outward from the bite, skin that feels unusually warm to the touch, pus or drainage, a fluid-filled blister forming around the wound, fever and chills, swollen lymph nodes near the bite, or unusual fatigue. Any of these warrant medical attention, as they can indicate cellulitis, a bacterial skin infection that needs treatment.
Prevention and Repellents
Clothing is your first line of defense. Black flies tend to crawl into gaps, so tuck pants into socks, button collars, and wear long sleeves when you’re near streams during peak season. Light-colored clothing makes it easier to spot flies before they find exposed skin. Head nets are standard gear in areas with heavy black fly pressure, particularly for hikers, anglers, and anyone working near water.
For chemical repellents, DEET remains the benchmark. Products with up to 30% DEET provide strong protection. Picaridin, available in the U.S. since 2005, performs comparably to DEET against most biting fly species and tends to feel less greasy on the skin. Products with lower picaridin concentrations are labeled for mosquitoes, biting flies, and chiggers, while higher concentrations add tick protection. IR3535, found in some Avon and Sawyer products, is another option specifically registered for black flies, though it’s less widely available.
Timing your outdoor activities can also help. Since black flies peak in the mid-morning and late afternoon, midday is generally the lowest-risk window. They also avoid deep shade and windy conditions, so choosing exposed, breezy spots for rest breaks can keep them at bay. At night, you’re completely safe from black flies, as they simply don’t bite after dark.