Black flies are aggressive daytime biters drawn primarily by your body odor, and the most effective way to repel them combines the right clothing, timing, and repellents. Unlike mosquitoes, black flies don’t pierce skin with a needle-like proboscis. They slash it open and feed on the pooling blood, which is why their bites tend to swell, itch intensely, and sometimes trigger a full-body reaction. Here’s how to keep them off you.
What Draws Black Flies to You
Understanding what attracts black flies makes it easier to counteract them. The single strongest attractant is smell. Chemicals naturally present on human skin are so powerful that when researchers removed them with vigorous washing and petroleum jelly, fly numbers dropped dramatically. Carbon dioxide from your breath is the second major signal: adding CO2 to traps produced a four-fold increase in catches near worn clothing and an 18-fold increase near clean clothing. The takeaway is that your skin odor does most of the work pulling flies in, and your exhaled breath amplifies the signal from a distance.
Vision plays a supporting role. Field studies consistently show black flies prefer dark colors, especially dark blue, and they favor shaded surfaces over sunlit ones. If you’re hiking in a navy blue shirt on a cloudy afternoon near a stream, you’re essentially a black fly magnet.
When and Where Black Flies Are Worst
Black flies bite only during the day and are completely inactive at night. Biting peaks in two windows: roughly 9:00 to 11:00 a.m. and again from 4:00 to 7:00 p.m., tapering off once the sun drops below the horizon. They’re most aggressive on humid, overcast days and in the hours just before a storm. If you can schedule outdoor work or recreation for midday on a sunny, breezy afternoon, you’ll encounter far fewer of them.
Black flies breed in fast-moving, well-oxygenated water. Shallow mountain streams are prime habitat, though some species use larger rivers or even temporary creeks fed by snowmelt. The closer you are to running water during peak season (late spring through early summer in most of North America), the more intense the pressure will be. Moving your campsite even a few hundred yards from a stream can make a noticeable difference.
Clothing as Your First Line of Defense
Because black flies land and crawl before biting, physical coverage is one of the most reliable defenses. Long sleeves, long pants, and closed-toe shoes block the majority of bites. Tuck pant legs into socks or boots and button collars and cuffs, since black flies are skilled at finding gaps and crawling underneath fabric edges.
Color matters. Choose light-colored clothing in white, tan, or light gray. Dark blue and black are the most attractive colors to black flies, and field research on biting flies confirms that lighter, matte surfaces draw significantly fewer landings than dark ones.
For heavy infestations, a head net is essential. Black flies target the face, ears, and hairline relentlessly. Standard mosquito netting may have holes large enough for smaller black fly species to squeeze through. Look for ultra-fine mesh with at least 2,000 holes per square inch (310 holes per square centimeter), which is tight enough to block even no-see-ums and sand flies. Wearing a wide-brimmed hat underneath keeps the mesh off your skin and improves airflow.
Permethrin-Treated Clothing
Treating your clothing with permethrin adds a chemical barrier on top of the physical one. Permethrin is an insecticide that kills or repels biting flies on contact with treated fabric. The EPA has reviewed factory-treated permethrin clothing and confirmed it is effective against target pests. You can buy pre-treated shirts, pants, and socks, or apply a permethrin spray to your own gear.
Factory treatments generally last through 70 or more wash cycles. DIY spray treatments are less durable, typically holding up for about six washes. Small amounts of permethrin do come off in the laundry, so wash treated items separately from untreated clothing. Permethrin should only go on fabric, not directly on skin.
Skin-Applied Repellents
DEET remains the most widely tested repellent against black flies. Concentrations of 20 to 30 percent provide several hours of protection and are suitable for most outdoor activities. Higher concentrations last longer but don’t repel more effectively at any given moment. Apply it to all exposed skin, paying special attention to the neck, ears, and forehead, the areas black flies target most.
Picaridin at 20 percent concentration is a comparable alternative that feels less greasy and won’t damage synthetic fabrics or plastics the way DEET can. Oil of lemon eucalyptus (the refined version registered as a repellent, not the raw essential oil) offers moderate protection but typically needs reapplication every two hours. Citronella, lavender, and most other plant-based oils provide only brief, unreliable relief against black flies and aren’t worth relying on in heavy infestations.
Layering a skin repellent with permethrin-treated clothing creates a two-zone defense: the clothing kills or repels flies on contact, and the repellent protects exposed areas like your hands and face.
Reducing Odor Signals
Since body odor is the primary attractant, reducing it can meaningfully lower how many flies zero in on you. Shower before heading outdoors and wear clean clothing. Avoid scented lotions, shampoos, and sunscreens when possible, not because the fragrance attracts flies, but because it doesn’t mask the skin chemicals they’re actually tracking. Some people find that applying a thin layer of petroleum jelly to exposed skin (especially the back of the neck and behind the ears) reduces bites, which aligns with research showing that sealing off skin odors cuts fly attraction substantially.
Wind and Fans as Natural Barriers
Black flies are weak fliers. Even a moderate breeze of 5 to 10 miles per hour makes it difficult for them to land and feed. If you’re working or relaxing in a fixed location, a portable fan aimed at your upper body can replicate this effect. Position it so airflow crosses your head and shoulders, where black flies concentrate. This won’t eliminate every fly, but it cuts the number that successfully land and bite.
What Happens After a Bite
Black fly bites typically produce a small, swollen welt that itches intensely for several days. The bite site often bleeds initially because of how the fly feeds. Swelling can be substantial, especially around the eyes and ears, and some people develop bruising around the wound.
In rare cases, a large number of bites can trigger a systemic reaction called black fly fever. Symptoms include headache, nausea, joint pain, high fever (potentially reaching 104°F), weakness, and swollen lymph nodes. In a documented case, skin lesions healed fully within 10 to 14 days, but the systemic symptoms required medical treatment. If you develop fever, widespread swelling, or feel generally ill after heavy exposure to black flies, that warrants prompt medical attention rather than waiting it out.
For ordinary bites, washing the area with soap and water and applying a cold compress reduces swelling. Oral antihistamines help with itching. Resist scratching, since broken skin from black fly bites is especially prone to secondary infection.