Black Fly Repellent: How to Pick One for Buffalo Gnat Season
Illinois DPH names DEET and picaridin as the actives to look for against biting flies, paired with light-colored long sleeves, pants and a hat, and a head net when swarms are unavoidable. Black flies target the head, so what you wear matters as much as what you apply.
Black fly season is short and it is vicious: a few weeks in late spring and early summer when the rivers hatch and the "buffalo gnats" come looking for blood, up to 10 miles from the water they grew in. There is a well-established playbook for it, straight from public health guidance, and it has more moving parts than a bottle.
What does the guidance say to use?
The Illinois Department of Public Health, in its biting flies guidance: repellents "containing the active ingredient DEET (N,N-diethyl-meta-toluamide) or picaridin are best." Then the sentence every honest black fly page has to include: "While effective against mosquitoes, repellents have been found to be less effective against some types of biting flies," and their use "should be supplemented with other preventive methods, such as avoiding areas inhabited by the flies, avoiding peak biting times, and wearing heavy-duty, light-colored clothing including long-sleeve shirts, long pants and hats."
Translation: repellent is the foundation, not the whole building. Against black flies, the clothing half of the plan carries real weight.
Why does clothing matter so much for black flies?
Because of where they bite. Illinois DPH: black flies "prefer to attack the head and where clothing fits tightly." A hat covers the preferred target. Loose, heavy, light-colored fabric removes the tight-fit lines they work along. And when they are at their worst, the department's own escalation is the beekeeper move: when black flies are "numerous and unavoidable, netting that covers the head, like the 'bee bonnets' used by beekeepers, can provide protection."
The season and geography tell you when to bother: late spring and early summer, near creeks and rivers where the larvae attach to submerged stones, with adults ranging up to 10 miles out. One mercy, per the same guidance: black flies do not transmit disease to humans in the United States.
What should you check on the repellent label?
Fly claims are registered pest by pest, and EPA's consumer search tool only filters by mosquitoes and ticks, so a black-fly answer has to come off the product's own federal label. EPA's rule: "always read the product label before using the product."
We read all of them. The Repellent Index holds the 631 EPA-registered skin-applied repellents with the pest claims on each federal label: 130 of them carry a stated fly protection time. The statistics page has the full breakdown, and the dataset is free to download.
Then apply it properly, per EPA: follow the label directions, be sure you understand how much to apply, and apply only to exposed skin and/or clothing, never under clothing. For children, EPA's rule is specific: do not allow children to handle the product and do not apply it to their hands; apply to your own hands first, then put it on the child. On a black fly trip that matters doubly, because the skin children most need covered, the hairline, ears and neck, is exactly where an adult's hands-first application does the neatest job.
Why the plan has so many parts
Because the agency guidance itself is layered. Illinois DPH does not present repellent as sufficient for biting flies; it presents it as the base layer of a plan that includes avoiding the areas the flies inhabit, avoiding their peak biting times, and dressing against them. Black flies concentrate that logic: they are seasonal (late spring and early summer), geographic (near running water, with a 10-mile reach), and targeted (the head, and tight clothing lines). Each of those facts maps to a countermeasure, and the countermeasures stack. A hat plus repellent beats either alone; both plus a loose light-colored shirt beats that; and the head net is the ceiling for the days when nothing else is enough.
The buffalo gnat checklist
- Repellent with DEET or picaridin on exposed skin, per Illinois DPH.
- A label that mentions flies. 130 of 631 registered products state a fly protection time.
- Hat on. The head is their preferred target.
- Light-colored, heavy-duty, loose clothing: long sleeves, long pants.
- Head net in the pack for the days they are unavoidable.
- Timing: the hatch peaks late spring to early summer near running water. Plan the riverside lunch for July.
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.
Short answers
What is the best black fly repellent?
No agency ranks products. Illinois DPH names DEET and picaridin as the actives to look for against biting flies, notes repellents can be less effective on some biting flies than on mosquitoes, and pairs them with clothing, hats and head nets. Check the individual product's federal label for a fly claim.
Do black flies bite through clothing?
Their pattern, per Illinois DPH, is to attack the head and wherever clothing fits tightly. The department's guidance is heavy-duty, loose, light-colored clothing, which removes the tight-fit lines they exploit.
When do I actually need a head net?
Illinois DPH's threshold: when black flies are numerous and unavoidable. In a bad hatch year near running water, that describes most of late spring.
Why is black fly season so short and so intense?
The population peaks when the rivers hatch, and Illinois DPH notes they can be present in very large numbers in late spring and early summer, in concentrations that can threaten livestock and even people through allergic reactions and sheer volume of bites. The same concentration means the season burns out: by midsummer the pressure drops sharply.
How far from a river can black flies find me?
Up to 10 miles, per Illinois DPH. Distance from water helps, but inside that radius the clothing-plus-repellent plan is what protects you.
Sources
- Illinois Department of Public Health, Biting Flies. https://dph.illinois.gov/topics-services/environmental-health-protection/structural-pest-control/biting-flies.html
- EPA, Using Insect Repellents Safely and Effectively. https://www.epa.gov/insect-repellents/using-insect-repellents-safely-and-effectively
- EPA, Find the Repellent that is Right for You. https://www.epa.gov/insect-repellents/find-repellent-right-you
Last reviewed 2026-08-26 · every claim on this page is sourced above