Black Fly Bites: Why They Bleed, Why They Swell, and What Helps
Black fly bites bleed and swell more than mosquito bites, and Illinois DPH says they are often itchy and slow to heal, aimed at the head and wherever clothing fits tightly. Treat them with CDC bite care: soap and water, ice for 10 minutes, anti-itch cream, and no scratching.
If a spring hike near a river left you with bloody, swollen bites along your hairline and collar, you met black flies. They are the meanest biters most people in the northern half of the country will encounter, and they punch far above their eighth-of-an-inch weight. Here is what they are, what their bites do, and what actually helps.
What are black flies?
The Illinois Department of Public Health describes them precisely: adult black flies are small, no more than 1/8 inch long, with broad wings and a humpbacked appearance. They are also known as "buffalo gnats," and they are creatures of moist environments, usually encountered near creeks and rivers, where their larvae attach to submerged stones.
That running-water detail explains the classic black fly experience: you can be miles from any river and still get swarmed, because, per Illinois DPH, "black flies will fly up to 10 miles in search of blood." Their season is concentrated, typically late spring and early summer, when they can be present in very large numbers.
Why are the bites so bad?
Illinois DPH does not soften it: "Black fly bites often cause considerable swelling and bleeding, may be itchy and slow to heal." And they are targeted: black flies "prefer to attack the head and where clothing fits tightly," which is why the bites line up along hairlines, ears, collars and waistbands.
At the extreme, the department notes that when black flies are present in very large numbers, injury from their bites can threaten the lives of livestock and even people, with deaths reported from allergic reactions and blood loss. That is the mass-swarm scenario, not the day hike, but it is why northern outbreak years make the news.
One piece of genuinely good news, again from Illinois DPH: black flies "do not transmit disease to humans in the United States." The bite is the whole problem. That puts the black fly in a different category from the mosquito and the tick: what you are managing is pain, swelling and healing time, not an infection watch. It also explains why the prevention advice leans so hard on physical barriers. There is no disease calculus to weigh; there is only whether the fly reaches skin.
How do you treat black fly bites?
The CDC's insect bite-care steps are the playbook:
- Wash the bite and surrounding area with soap and water.
- Ice it. An ice pack for 10 minutes reduces swelling and itching, and with black fly bites the swelling is usually the worst part. Reapply as needed.
- Use an over-the-counter anti-itch or antihistamine cream, following the product label directions.
- Do not scratch. The CDC's warning: "Do not scratch bites. They can become infected." Since black fly bites are slow healers already, scratching sets you back the most here.
Watch the infected-bite signs the CDC lists: redness, warmth, or a red streak spreading outward. See a healthcare provider if symptoms worsen. The bump and itch themselves are your body reacting to the fly's saliva, the same mechanism the CDC describes for mosquito bites, just delivered by a messier bite.
How do you keep black flies off?
The Illinois DPH prescription, in order:
- Repellent. Those containing DEET or picaridin "are best," with the caveat that repellents can be less effective against some biting flies than against mosquitoes, so do not make repellent the whole plan.
- Clothing that denies the target. Heavy-duty, light-colored clothing with long sleeves, long pants and hats. Black flies aim for the head and tight clothing lines, so a hat is not an accessory here, it is armor.
- The head net, when it is bad. The department's own escalation: when black flies are numerous and unavoidable, "netting that covers the head, like the 'bee bonnets' used by beekeepers, can provide protection." Every northern trail town sells them for a reason.
- Timing. Late spring and early summer near running water is the peak. Avoiding the riverside at the height of the hatch is the cheapest protection there is.
Longsleeve is an EPA-registered picaridin lotion (EPA Reg. No. 101457-1). It repels mosquitoes and ticks for up to 14 hours, and its accepted federal label carries the claim "Repels biting flies, stable flies, black flies, gnats, chiggers, sand flies and Scottish biting midges (no see-ums) for up to 8 hours." It is fragrance-free, with nine disclosed ingredients. Of the 631 EPA-registered skin repellents in our Repellent Index, 130 carry a stated fly protection time on their federal label.
Short answers
Why do black fly bites bleed?
Illinois DPH describes black fly bites as often causing considerable swelling and bleeding, and notes they may be itchy and slow to heal. That is the observed character of the bite; the department's practical advice is to prevent them with repellent, clothing and, when swarms are heavy, a head net.
How long do black fly bites take to heal?
Longer than you want. Illinois DPH describes the bites as slow to heal on top of the swelling and bleeding, and the CDC's no-scratching rule is what protects that slow healing from becoming an infected bite: redness, warmth, or a spreading red streak means a provider visit.
Do black flies carry disease?
Per Illinois DPH, black flies do not transmit disease to humans in the United States. The concern is the bites themselves: swelling, bleeding, slow healing, and in mass-swarm conditions, allergic reactions.
Why do the bites swell so much?
The swelling is your body's reaction to the fly's saliva, the same mechanism the CDC describes for mosquito bites: saliva goes in as the insect feeds, and the reaction produces the bump and the itch. Black fly bites add bleeding and a slower heal on top, per Illinois DPH, which is why the CDC's ice-pack step earns its place here.
Why do black flies go for my head?
Illinois DPH: they prefer to attack the head and wherever clothing fits tightly. A hat and, in bad stretches, a head net are the department's answer.
When is black fly season?
Late spring and early summer, per Illinois DPH, when they can appear in very large numbers near the creeks and rivers where their larvae grow. They range up to 10 miles from the water, so distance is only partial protection.
Sources
- Illinois Department of Public Health, Biting Flies. https://dph.illinois.gov/topics-services/environmental-health-protection/structural-pest-control/biting-flies.html
- CDC, About Mosquito Bites. https://www.cdc.gov/mosquitoes/about/about-mosquito-bites.html
Last reviewed 2026-08-26 · every claim on this page is sourced above