The best fly repellent depends on which flies you’re dealing with. For common biting flies like black flies, gnats, and no-see-ums, DEET at 20-30% concentration is the most reliable option. For deer flies and horse flies, no repellent works particularly well on its own, but combining a skin-applied repellent with permethrin-treated clothing gives you the best shot at protection.
What Works on Which Flies
Not all flies respond to repellents the same way, and this is where most people get frustrated. DEET, the gold standard for mosquitoes, performs well against black flies, biting midges (no-see-ums), and fleas. It does not work well against deer flies, horse flies, or greenheads. No skin-applied repellent truly does. If you’re being swarmed by horse flies at the lake, even a generous coating of high-concentration DEET will only provide partial relief.
Picaridin, the other widely recommended active ingredient, is labeled for mosquitoes, biting flies, and chiggers at lower concentrations. It’s a good alternative if you dislike the greasy feel of DEET, and it won’t damage plastics or synthetic fabrics the way DEET can. Oil of lemon eucalyptus (sometimes listed as PMD on labels) covers black flies, gnats, mosquitoes, and no-see-ums, with protection roughly comparable to 10% DEET. IR3535, found in some Avon products, carries similar labeling for biting midges, black flies, gnats, and sandflies.
For no-see-ums and sandflies specifically, mineral oil-based lotions can be surprisingly effective. They create a physical barrier that prevents the insect’s short mouthparts from reaching your skin. These won’t stop a horse fly, whose mouthparts are much longer, but they’re a solid choice for beach or marsh environments where tiny biting flies are the main problem.
Concentration Matters, but Only to a Point
Higher concentrations of DEET don’t repel flies more effectively. They last longer. A product with 10% DEET might protect you for one to two hours, while 20-30% can last several hours before you need to reapply. Products above 50% DEET offer no additional benefit over 50%, so there’s no reason to reach for the maximum-strength version. The CDC recommends at least 20% DEET for meaningful protection.
The same principle applies to picaridin and oil of lemon eucalyptus. A higher percentage means longer duration, not a stronger repellent effect. For a short walk or outdoor lunch, a lower-concentration product is fine as long as you reapply when it wears off. For a full day of hiking or working outside, go with 20% or higher of your chosen active ingredient.
Permethrin-Treated Clothing: The Best Add-On
Permethrin is the single most effective tool against biting flies when used on clothing. Unlike skin-applied repellents that simply discourage flies from landing, permethrin is toxic to insects on contact, quickly knocking down mosquitoes, biting flies, fleas, ticks, and chiggers. You never apply it to skin. It goes on shirts, pants, socks, hats, and gear.
You can buy pre-treated clothing or treat your own using EPA-registered permethrin spray products. Apply it outdoors in a ventilated area, let the fabric dry completely before wearing it, and wash treated items separately from the rest of your laundry. When applied correctly, permethrin binds tightly to fabric and holds up through multiple washes with minimal transfer to skin.
For anyone spending serious time in fly-heavy environments (farming, trail work, fishing, camping near water), the combination of permethrin-treated clothing plus a skin repellent like DEET or picaridin on exposed areas is the most effective approach available. This layered strategy is especially important for deer flies and horse flies, since skin repellents alone provide limited protection against those species.
Natural and Plant-Based Options
Oil of lemon eucalyptus is the only plant-derived active ingredient that the EPA and CDC recommend alongside DEET and picaridin. It provides real, measurable protection against black flies, gnats, and no-see-ums, though its duration is shorter than higher-concentration DEET products. Look for products listing PMD (p-menthane-3,8-diol) as the active ingredient at 8-10% or higher.
Pure essential oils like citronella, lemongrass, and peppermint are popular but offer much shorter protection windows, often under an hour. They require frequent reapplication and simply cannot match the performance of EPA-registered active ingredients for any sustained outdoor activity. If you prefer natural products, oil of lemon eucalyptus is your best bet by a wide margin.
Fly Repellents for Horses and Livestock
If you searched “best fly repellent” while thinking about your horses, the active ingredients are similar but the formulations differ. Most equine fly sprays use pyrethrins (extracted from chrysanthemum flowers) combined with piperonyl butoxide, a synergist that boosts their effectiveness. These products knock down flies quickly but break down in sunlight, so protection typically lasts only a few hours before you need to reapply.
Synthetic pyrethroids like cypermethrin and permethrin are more stable than natural pyrethrins and provide protection lasting several days per application. Many popular equine fly sprays combine both: pyrethrins for immediate knockdown and a pyrethroid for longer residual protection. Spot-on treatments with 45% permethrin offer the longest-lasting option, applied between the shoulders or along the back and effective for weeks rather than hours.
Sweat-resistant formulations matter for horses. Standard water-based sprays wash off quickly with perspiration, so look for products specifically labeled as sweat-resistant if your horse is working or turned out in hot weather.
Safety for Children
DEET is safe for children over two months of age at concentrations up to 30%. Do not use DEET on infants younger than two months. For kids, apply the repellent to your own hands first, then rub it on the child’s skin, avoiding their hands, eyes, and mouth. Picaridin and oil of lemon eucalyptus are also options, though oil of lemon eucalyptus is not recommended for children under three years old.
For any age group, the practical advice is the same: use the lowest concentration that covers your time outdoors, reapply when protection fades, and wash treated skin with soap and water when you come inside.
Choosing the Right Product
- For black flies, gnats, and no-see-ums: DEET (20-30%), picaridin, or oil of lemon eucalyptus applied to exposed skin. All three work well for these smaller biting flies.
- For deer flies and horse flies: Permethrin-treated clothing is your primary defense. Add DEET at 20-30% on exposed skin and apply it liberally. Expect reduced, not eliminated, biting.
- For all-day outdoor work or recreation: Permethrin on clothing plus 20-30% DEET or picaridin on skin. This combination covers the widest range of fly species for the longest duration.
- For short outings under two hours: A lower-concentration product (10-15% DEET or picaridin) is sufficient and feels lighter on the skin.
- For a natural option: Oil of lemon eucalyptus at 10% PMD or higher. Expect to reapply more often than with DEET.