The most effective outdoor repellents for both flies and mosquitoes contain one of four EPA-registered active ingredients: DEET, picaridin, IR3535, or oil of lemon eucalyptus (OLE). Each offers reliable mosquito protection, but their effectiveness against flies varies significantly, and no repellent works well against every type of biting fly. Choosing the right product depends on what’s biting you, how long you’ll be outside, and who’s using it.
What Works Best Against Mosquitoes
DEET remains the most studied and widely available mosquito repellent. Products with concentrations above 20% provide the longest protection, but even low concentrations of 5 to 10% will keep mosquitoes away for one to three hours. Controlled-release formulations stretch protection further: a 34% DEET lotion has been shown to provide 12 hours of protection, matching what you’d get from a 75% DEET alcohol-based spray. A 20% controlled-release lotion performs equally to 50% DEET in alcohol. In short, formulation matters as much as concentration.
Picaridin at 10 to 20% concentrations performs about as well as DEET against mosquitoes in lab and field tests. It has a lighter feel on skin, doesn’t damage plastics or synthetic fabrics the way DEET can, and has almost no odor. Products with lower concentrations (around 7.5%) are available but may not hold up as long outdoors.
Oil of lemon eucalyptus, listed on labels as PMD (p-menthane-3,8-diol), is the strongest plant-derived option with EPA registration. At concentrations of 10% or higher, it provides meaningful protection, though generally shorter than what you’d get from 20%+ DEET or picaridin. IR3535 is another option, but at the 7.5% concentration found in many consumer products, protection can be short-lived, sometimes as little as 10 to 60 minutes in lab tests and roughly two hours at best. At 20%, IR3535 approaches the effectiveness of the other active ingredients.
A general rule across all repellents: products with less than 10% of any active ingredient typically protect for only one to two hours.
Which Repellents Actually Work on Flies
Here’s where expectations need adjusting. DEET works well against black flies, biting midges (no-see-ums), and fleas, but it does not work well against deer flies, horse flies, greenheads, or snipe flies. No topical repellent really does. Those larger biting flies are aggressive feeders that aren’t easily deterred by chemicals on skin.
Picaridin handles mosquitoes, biting midges, and chiggers effectively, and products with lower picaridin concentrations are labeled for mosquitoes, biting flies, and chiggers. Oil of lemon eucalyptus products list black flies, gnats, mosquitoes, and no-see-ums on their labels. IR3535-based products like Avon’s Skin So Soft Bug Guard cover biting midges, black flies, gnats, mosquitoes, no-see-ums, and sandflies.
If your main problem is horse flies or deer flies, physical barriers like long sleeves, hats, and head nets will do more than any spray. For black flies and no-see-ums, DEET and picaridin are your best bets.
Treating Clothing With Permethrin
Permethrin is not a skin repellent. It’s an insecticide that you spray onto clothing, gear, sleeping bags, and tents. When a mosquito or fly lands on treated fabric, it’s either repelled or killed on contact. This makes it an excellent companion to whatever you’re putting on exposed skin.
A single application of 0.5% permethrin spray lasts through six washings or about six weeks, whichever comes first. One 24-ounce bottle treats roughly seven full outfits (shirts, pants, and socks). You need to spray and fully dry the clothing before heading outdoors. Some manufacturers also sell pre-treated clothing that lasts through dozens of washes.
Layering permethrin-treated clothing with a skin-applied repellent on exposed areas gives you the broadest protection, especially in heavily infested environments like wooded trails or marshy areas at dusk.
How to Apply Repellent Correctly
If you’re also wearing sunscreen, apply it first and let it dry completely before putting on repellent. Repellent goes on top so it stays at the skin’s surface where insects encounter it. Combo sunscreen-repellent products exist but aren’t ideal because sunscreen needs reapplication more frequently than repellent, and re-dosing the combo means layering on more repellent than you need.
Apply repellent only to exposed skin and outer clothing. There’s no benefit to putting it under your shirt. Use just enough to create a thin, even layer. Heavier application doesn’t increase protection. When you come indoors, wash treated skin with soap and water.
Safety for Children and Pregnant Women
DEET, picaridin, IR3535, and OLE are all considered safe for use during pregnancy when applied as directed. For children, the guidelines are more specific. DEET can be used on infants and toddlers, but the American Academy of Pediatrics recommends applying it sparingly, especially on children under two, whose skin absorbs chemicals differently. Parents of newborns and premature infants should be particularly cautious.
Oil of lemon eucalyptus and PMD should not be used on children under three years old. For any repellent, avoid applying it to a child’s hands, since kids frequently touch their eyes and mouths. Instead of spraying a child’s face directly, spray a small amount onto your own hands first and then rub it on, keeping clear of the eyes and mouth.
Choosing the Right Product for Your Situation
For a short walk or an hour in the backyard, a lower-concentration product (10 to 15% DEET or picaridin) will do the job without over-applying. For a full day of hiking, fishing, or camping, look for 20%+ DEET or picaridin, and consider treating your clothing with permethrin ahead of time. If you prefer plant-based options, oil of lemon eucalyptus at 10% or higher is the only botanical with strong enough evidence to recommend, but plan on reapplying more often than you would with DEET or picaridin.
For environments with aggressive biting flies like horse flies and deer flies, shift your strategy toward physical protection: long pants, long sleeves, light-colored clothing (dark colors attract many fly species), and head nets. No spray on the market reliably stops those insects. For mosquitoes, black flies, and no-see-ums, the topical repellents described above are highly effective when matched to the right concentration and reapplied on schedule.