The most effective fly repellents use one of four active ingredients: DEET, picaridin, IR3535, or permethrin (for clothing only). DEET has been the gold standard for over 60 years, but newer options like IR3535 actually outperform it against certain biting flies, including black flies and sand flies. Choosing the right repellent depends on which flies you’re dealing with, how long you need protection, and whether you’re protecting skin, clothing, or a larger outdoor space.
How Fly Repellents Actually Work
Flies find you by following chemical signals your body releases, primarily carbon dioxide, body heat, and skin odors. Repellents don’t create a force field. Instead, they interfere with the smell receptors flies use to zero in on you during the final stages of approaching a host. At the concentrations found in commercial products, repellent molecules activate those receptors at levels far higher than normal, essentially overwhelming the fly’s navigation system and causing abnormal responses that disrupt its ability to land and bite.
This is why repellents work better at close range than at a distance. A fly may still buzz around your general area, but a well-applied repellent prevents it from completing the host-seeking behavior that leads to a bite.
DEET, Picaridin, and IR3535 Compared
DEET remains the most widely used insect repellent in the world and works against flies, mosquitoes, chiggers, and ticks. Products with 20% DEET or higher are what the CDC recommends, and concentrations above 50% don’t add any extra protection. They just last marginally longer. For most outdoor situations involving biting flies, a 20-30% DEET product covers several hours of activity.
Picaridin at 20% performs comparably to DEET at the same concentration for mosquitoes and is effective against flies, chiggers, and ticks. Its vapor repels flies on contact, and it has the advantage of not damaging plastics or synthetic fabrics the way DEET can. A 20% picaridin product protects for 8 to 14 hours, while 10% concentrations last 3.5 to 8 hours.
IR3535 is the least familiar name on this list, but it deserves attention if biting flies are your main concern. Originally marketed as a skin moisturizer, it turned out to be as effective as DEET against biting midges (no-see-ums) and actually more effective than DEET against black flies and sand flies, with longer-lasting protection against both. A 20% IR3535 product protects against most mosquito species for 7 to 10 hours. If you spend time near rivers, marshes, or wooded areas where black flies are a problem, IR3535 is worth seeking out.
Natural Options and Their Limits
Oil of lemon eucalyptus (OLE) is the most credible plant-based repellent. Its active compound, PMD, initially provided protection similar to low-concentration DEET in testing against mosquitoes. The catch: that protection dropped off quickly after the first few minutes, making it far less durable than synthetic options. Commercial products contain 8-10% PMD and are labeled for black flies, gnats, mosquitoes, and no-see-ums.
Products with less than 10% of any active ingredient generally protect for only 1 to 2 hours. That’s fine for a short walk but inadequate for a day of hiking, fishing, or yard work. If you prefer a natural repellent, plan on reapplying frequently and carrying backup.
One important restriction: oil of lemon eucalyptus products should not be used on children under three years old.
Permethrin for Clothing and Gear
Permethrin takes a completely different approach. Instead of masking your scent, it’s a contact insecticide that kills or disables flies, mosquitoes, ticks, black flies, fleas, and chiggers when they land on treated fabric. You spray it on clothing, shoes, hats, backpacks, and tent material, never directly on skin.
A single application of 0.5% permethrin spray lasts through 5 to 7 washes, making it practical for outdoor work clothes or hiking gear you use regularly. Pairing permethrin-treated clothing with a skin-applied repellent like DEET or picaridin gives you the strongest possible protection, covering both exposed and clothed areas.
Permethrin and Pet Safety
Permethrin is commonly used in fly repellent products for dogs and horses, but it is extremely dangerous to cats. Cats metabolize permethrin far more slowly than dogs or people, and exposure to products with high concentrations (45-65%) can cause anxiety, loss of coordination, muscle tremors, seizures, and death. Even dogs and cats with permethrin on their skin may twitch, flick their paws, or roll on the ground. Animals that lick treated surfaces often drool excessively or smack their lips.
If you use permethrin-treated clothing or gear in a home with cats, store those items where cats can’t rub against them. Never apply a dog-labeled permethrin product to a cat, and keep cats separated from recently treated dogs until the product dries completely.
Choosing a Repellent for Tough Biting Flies
Horse flies and deer flies are notoriously difficult to repel because they’re large, aggressive, and strongly motivated by visual cues as much as scent. No repellent works perfectly against them, but high-concentration DEET (30% or above) and permethrin-treated clothing together provide the best available defense. Wearing light-colored clothing also helps, since horse flies are more attracted to dark colors and movement.
Black flies respond better to repellents overall. IR3535 outperforms DEET specifically for this species, and permethrin-treated hats and collars help protect the head and neck areas black flies favor. For sand flies and no-see-ums, both DEET and IR3535 at 20% work well, though IR3535 again edges ahead in duration.
Traps for Indoor and Outdoor Fly Control
When the goal is reducing fly populations rather than protecting your skin, traps serve a different purpose. Three main types exist, each suited to different situations.
- Light traps use ultraviolet or green-wavelength light to attract flies. Green light is especially attractive to house flies. These come in two forms: electric grid models that zap insects on contact, and glueboard models that catch them passively. Both are widely used in restaurants, food facilities, and kitchens for ongoing monitoring and control.
- Baited traps use food-based attractants like vinegar, banana, or apple to lure flies into bags or jars they can’t escape. These work well outdoors, particularly for house flies and fruit flies, and are inexpensive enough to place around patios, barns, or garbage areas.
- Sticky traps are low-cost options useful for monitoring fly activity levels and catching smaller species. They’re less effective as a primary control method but work well in combination with other approaches.
For outdoor gatherings, baited jar traps placed 15 to 20 feet away from your seating area draw flies away from the space you’re using. Positioning them downwind helps carry the attractant scent toward incoming flies before they reach you.
Application Tips That Affect Performance
Repellents only protect the skin they actually cover. Flies will bite any untreated patch, so even application matters more than using a higher concentration. Spray repellents onto your hands first, then rub them onto your face, ears, and neck rather than spraying directly at your head.
Sweat, rain, and swimming all reduce how long a repellent lasts. If you’re active outdoors and sweating heavily, reapply every 2 to 3 hours regardless of what the label says about duration. When using both sunscreen and repellent, apply sunscreen first, let it absorb for 10 to 15 minutes, then apply repellent on top.
For children older than two months, DEET-based products are considered safe by both the CDC and the American Academy of Pediatrics, though the AAP recommends keeping concentrations at 30% or below for kids. Children under two months should not use any insect repellent. Adults should apply repellent to their own hands and then rub it onto a child’s skin, avoiding the hands (children touch their eyes and mouths) and any irritated or broken skin.