DEET and picaridin are the most effective chemical repellents against biting flies, and wearing light-colored clothing while avoiding blue and dark colors makes a measurable difference. But biting flies are tougher to repel than mosquitoes, so the best protection combines repellents, the right clothing, and environmental tricks like fans and traps.
Why Biting Flies Are Harder to Repel
Mosquitoes locate you primarily by smell, which is why chemical repellents work so well against them. Biting flies, including horse flies, deer flies, black flies, and stable flies, rely heavily on vision and heat in addition to chemical cues. They spot you by detecting movement and contrast against the landscape, then zero in using body heat and carbon dioxide. This means a repellent spray alone won’t always do the job the way it does for mosquitoes. You need a layered approach.
DEET: The Strongest Chemical Option
DEET remains the gold standard for repelling biting flies. A field study on horses in Switzerland found that a 15 to 17% DEET formulation provided at least 80% protection against horse flies for three hours, with full protection (zero flies landing) in half the treated subjects. By the four-hour mark, efficacy dropped to around 71%. For humans spending extended time outdoors, concentrations of 20 to 30% offer longer protection without needing to reapply as often.
DEET works by creating a vapor barrier near the skin that interferes with the fly’s ability to detect you as a host. It’s effective against horse flies, deer flies, black flies, and stable flies, though large horse flies are the most persistent and may still land briefly before being deterred.
Picaridin: A Less Greasy Alternative
Picaridin performs similarly to DEET against biting flies and has the advantage of being odorless and non-greasy. It won’t damage plastics or synthetic fabrics the way DEET can. A 20% picaridin product provides 8 to 12 hours of protection, while a 5% concentration lasts only 3 to 4 hours. If you’re dealing with aggressive horse flies or deer flies, go with the higher concentration. Products below 10% aren’t worth the trade-off in duration.
Oil of Lemon Eucalyptus and Other Botanicals
Oil of lemon eucalyptus (the refined version, not the essential oil) is the only plant-based repellent that performs in the same general range as DEET and picaridin against mosquitoes and ticks. Against biting flies specifically, it’s less studied and generally less effective. It also requires more frequent reapplication, typically every two hours.
Citronella, peppermint oil, and other essential oil blends provide minimal protection against biting flies. These products may reduce mosquito landings in low-pressure situations, but horse flies and deer flies are far more determined feeders and will push through weak repellent barriers. If biting flies are your primary concern, stick with DEET or picaridin.
Clothing Color Matters More Than You Think
Biting flies are strongly attracted to blue and dark-colored objects. Research published in Proceedings of the Royal Society B used artificial neural networks to model how flies see the world, and the findings explain a long-standing puzzle. Flies have five types of photoreceptors sensitive to UV, blue, and green wavelengths. To distinguish an animal from a leafy background, their visual system compares blue and green signals. When the blue signal is relatively stronger, the fly classifies the object as a potential host.
Blue surfaces trigger this animal-detection mechanism powerfully, which is why blue traps are so effective at catching horse flies and tsetse flies. In the study, the neural networks consistently misclassified blue objects as animals. Dark colors also attract biting flies because they absorb more heat, creating a thermal signal that mimics a warm-blooded host.
The practical takeaway: wear white, khaki, or other light-colored clothing outdoors. Avoid blue, black, and dark red. This single change won’t eliminate bites, but it reduces how visible you are to flies scanning for a meal.
Permethrin-Treated Clothing
Permethrin is an insecticide, not a repellent, but it’s one of the most effective tools against biting flies when applied to clothing. It doesn’t stop flies from approaching, but flies that land on treated fabric are knocked down or killed within minutes. Defence research found that permethrin-treated jackets provided good protection against black flies and mosquitoes, with the biting fly population near the wearer dropping significantly after about 10 minutes of exposure as flies contacted the treated surface and were disabled.
You can buy pre-treated clothing or treat your own gear with a permethrin spray kit. Treated clothing remains effective through roughly 6 washes for spray-on treatments and up to 70 washes for factory-treated garments. Apply permethrin to shirts, pants, hats, and socks, then use DEET or picaridin on exposed skin. This combination of a contact killer on fabric and a spatial repellent on skin is the most complete protection available.
Fans and Wind as a Barrier
Biting flies are weak fliers relative to their size. Mosquitoes avoid wind speeds approaching their own flight speed of 0.9 to 3.6 miles per hour, and most biting flies have similar limitations. A standard oscillating fan on a patio or deck creates enough airflow to significantly reduce fly landings. Position fans to blow across seating areas at roughly waist to chest height, since that’s where most flies approach.
For outdoor dining, even a pair of small clip-on fans can make a noticeable difference. The moving air disrupts the plume of carbon dioxide and body odor that guides flies to you, while also making it physically difficult for them to land and feed. This approach works best in combination with repellents, since flies that manage to get close will still be deterred by the chemical barrier on your skin.
Traps to Reduce Local Populations
Blue and black traps exploit the same visual attraction that makes dark clothing a bad choice. Commercial horse fly traps typically use a dark sphere or blue panel to attract flies, sometimes paired with a heat source. Flies approach what they perceive as an animal, contact a sticky surface or funnel into a collection chamber, and are removed from the population.
These traps won’t eliminate biting flies from your property, but placing several around the perimeter of a yard or pasture can meaningfully reduce their numbers over the course of a season. Position traps in sunny areas away from where people gather, since the goal is to draw flies toward the trap and away from you.
Timing and Habitat Awareness
Most biting flies are most active during warm, sunny, calm conditions, typically from late morning through late afternoon. Horse flies and deer flies peak on hot days with little wind. Black flies prefer moving water and are worst near streams and rivers during late spring and early summer. Stable flies breed in decaying organic material like old hay, grass clippings, and manure.
If you can time outdoor activities for early morning, evening, or breezy days, you’ll encounter fewer biting flies. Removing breeding habitat also helps: clear standing organic debris, manage manure, and keep vegetation trimmed near gathering areas. These environmental steps reduce the number of flies you need to repel in the first place.