The most effective fly repellents for humans contain DEET or picaridin, but which one you choose depends on the type of fly you’re dealing with. Black flies, no-see-ums, and biting midges respond well to standard repellents. Horse flies and deer flies, on the other hand, are notoriously difficult to repel with any product on the market.
Which Repellents Work on Which Flies
Not all biting flies respond to repellents the same way, and this is the single most important thing to understand before you buy a product. DEET works well against black flies, biting midges (also called punkies or no-see-ums), and fleas. It does not work well against horse flies, deer flies, greenheads, or snipe flies. No repellent truly does.
Picaridin performs similarly to DEET against most species and is effective against mosquitoes, biting midges, and chiggers. Products with lower picaridin concentrations are labeled for biting flies and chiggers. For most fly species, picaridin is as effective as DEET, and some people prefer it because it’s odorless and doesn’t damage plastics or synthetic fabrics the way DEET can.
IR3535, the active ingredient in several Avon products, works moderately well on mosquitoes and is registered for use against biting midges, black flies, gnats, no-see-ums, and sand flies. It’s a reasonable option for light outdoor activity but generally provides shorter protection than DEET or picaridin at comparable concentrations.
Oil of lemon eucalyptus (PMD) is the strongest plant-based option and has provided protection times similar to low-concentration DEET in clinical testing. One important distinction: the CDC recommendation applies only to EPA-registered products containing refined PMD, not to pure lemon eucalyptus essential oil, which hasn’t undergone the same validated testing for safety or efficacy.
How Concentration Affects Protection Time
Higher concentrations of DEET don’t repel flies more aggressively, but they do last longer on your skin. A product with 10% DEET provides about 2 hours of protection. Bump that to 30% and you get roughly 5 hours. Anything above 50% DEET doesn’t extend protection further, so there’s no benefit to using the highest concentrations available.
This matters when you’re planning a day outdoors. A short hike calls for a lower-concentration product. An all-day fishing trip near a marsh full of black flies calls for 30% DEET or 20% picaridin, applied more than once if you’re sweating heavily or getting wet.
Treating Clothing With Permethrin
Permethrin is an insecticide, not a repellent, and it works differently from anything you spray on your skin. Applied to clothing and gear, it kills flies, ticks, black flies, and other biting insects on contact when they land on treated fabric. It’s labeled for mosquitoes, ticks, fleas, black flies, chiggers, and lice. For flies that don’t respond well to skin-applied repellents (like deer flies and horse flies), permethrin-treated clothing is one of the few strategies that helps, since it irritates these flies enough to make them leave almost immediately after landing.
A 0.5% permethrin spray applied to clothing lasts through 5 to 7 washes. You can also buy pre-treated clothing or send your gear to a professional treatment service for longer-lasting protection. A few rules to follow:
- Never apply permethrin directly to skin. It can be harmful if absorbed and is designed only for fabric and gear.
- Apply outdoors. Wear gloves and let treated items dry completely before wearing them.
- Keep it away from cats. Wet permethrin is toxic to cats. Store treated clothing and spray bottles where cats can’t reach them.
- Wash treated clothing separately from untreated items, and re-treat as needed after multiple washes.
Combining permethrin-treated clothing with a DEET or picaridin product on exposed skin gives you the broadest protection, especially in areas with multiple biting fly species.
How Fly Repellents Actually Work
Flies find you by smell. Odors from your skin float through the air and contact tiny hairs on a fly’s antennae. Those odors pass through small pores in the hairs and bind to specialized smell receptors on nerve cells. Repellents like DEET interfere with this process, essentially scrambling the chemical signals flies use to locate a blood meal. Research at Rockefeller University identified a single receptor in fruit flies, called Or83b, that is essential for the entire sense of smell. Without it, the flies couldn’t detect a wide range of odors because their other 61 smell receptors never reached the nerve endings where they normally pick up incoming scents. This kind of research helps explain why broad-spectrum repellents can work against many insects at once: they target a fundamental part of the smell machinery rather than blocking one specific odor.
Application Tips That Affect Performance
If you’re using sunscreen and repellent together, apply sunscreen first and let it dry completely before putting on the repellent. Reversing this order buries the repellent under sunscreen and reduces its effectiveness. Avoid combination sunscreen-repellent products, since sunscreen needs reapplication every two hours while repellent should be applied less frequently.
Apply repellent only to exposed skin and clothing, not under clothing. Spray it onto your hands first, then rub it on your face, avoiding eyes and mouth. Reapply after swimming or heavy sweating, but don’t overdo it. More product doesn’t mean more protection once you’ve covered exposed skin evenly.
Repellent Safety for Children
The American Academy of Pediatrics and the CDC both recommend EPA-registered repellents containing DEET, picaridin, or oil of lemon eucalyptus for children. Infant and toddler skin absorbs chemicals differently than adult skin, so DEET should be applied sparingly on children under 2. Products containing oil of lemon eucalyptus or PMD should not be used on children younger than 3.
For young children, permethrin-treated clothing is a practical alternative that avoids putting any chemical directly on skin. Dressing kids in long sleeves and pants made of tightly woven fabric, then treating those garments with permethrin, gives solid protection without the need for repeated skin applications throughout the day.
What to Do About Horse Flies and Deer Flies
These are the flies that make people most desperate for a repellent, and they’re the hardest to deter. Horse flies and deer flies are large, aggressive, and painful biters. Standard skin repellents like DEET provide some protection (several hours per application, according to the University of Kentucky’s entomology department) but are far less reliable against these species than against black flies or no-see-ums.
Permethrin-treated clothing is your best tool here. These flies are highly sensitive to the chemical and tend to fly away almost immediately after landing on treated fabric. Covering as much skin as possible with treated long sleeves, pants, and a hat reduces bites significantly. Light-colored clothing also helps, since horse flies and deer flies are more attracted to dark colors. Physical barriers, including head nets and tightly woven fabrics, fill in the gaps where chemical repellents fall short.