Flies find you mostly by what you give off: the carbon dioxide (CO2) you breathe out, your sweat, and the chemicals skin bacteria make from it. Up close they switch to sight and body heat to pick a landing spot. Two kinds of fly are at work here, with different goals. Nuisance flies want to lick what’s on your skin, and biting flies want your blood.1Pursuit, University of Melbourne. Why flies and mosquitoes are irritating little pests
Your Breath Is the First Signal
Biting flies switch from random searching to actively seeking a host when they pick up signals like carbon dioxide, the gas in your breath.2International Journal of Tropical Insect Science. Distance orientation of biting flies to their hosts They then follow an invisible trail of breath and odour drifting downwind to reach you.3International Journal of Tropical Insect Science. Distance orientation of biting flies to their hosts Each sensory hair on a fly’s antenna carries smell-detecting nerve cells tuned to particular chemicals,4UC San Diego Today. Detecting Odors on the Edge: Researchers Decipher How Insects Smell More with Less and these sensors can pick up a meal from a long way off.5Live Science. Why are flies attracted to humans?
The breath plume mixes with air and drifts downwind as a patchy trail, and flies zigzag through it toward the source. Still air keeps the plume close to you, while wind spreads it out and breaks it up. When a fly loses the scent it often flies crosswind to pick it up again, which is why its path to you looks erratic. It generally moves upwind while tracking, so a fly downwind of you can follow your plume more easily.
When people’s exhaled breath was taken away, black flies found them only about 15% as attractive as before.6PubMed. Human individuals vary in attractiveness for host-seeking black flies (Diptera: Simuliidae) based on exhaled carbon dioxide Adding back each person’s own carbon dioxide level restored about 90% of their attractiveness.7PubMed. Human individuals vary in attractiveness for host-seeking black flies (Diptera: Simuliidae) based on exhaled carbon dioxide Breath alone isn’t the whole story, though: other smells boosted trap catches well beyond carbon dioxide alone.8PubMed Central. Identification of Human Semiochemicals Attractive to the Major Vectors of Onchocerciasis
That helps explain why bigger or more active people may draw more biting flies. Larger bodies and exercise mean more breathing, and panting after a hike or run sends out a stronger plume. Children and smaller adults typically exhale less.
When a biting fly has locked onto you, you often notice it circling your head or trailing behind you for a long time. Flies tend to cruise around the face and shoulders before landing lower, so early buzzing around your head means you’ve been detected, not that the fly is lost. Walking away doesn’t reliably shake it, because you carry your breath and odour with you and the fly keeps following the trail you leave behind.
Sweat and Skin Smells That Draw Flies In
Flies taste with their feet, and they are drawn to human sweat.9Pursuit, University of Melbourne. Why flies and mosquitoes are irritating little pests Sweat carries protein, salts, sugars, dead skin cells and the bacteria that grow in it, all of which flies feed on.10Pursuit, University of Melbourne. Why flies and mosquitoes are irritating little pests Tsetse flies have taste sensors near their feet that respond to human sweat, and they try to feed more on surfaces coated with it.11Chemical Senses. Taste stimuli for tsetse flies on the human skin
That’s also why the same fly lands on your arm again and again. Swatting chases it off but leaves the salty film it found, so the reward is still there when it returns. Wiping or rinsing the spot removes the food it was tasting, which works better than swatting, though in hot weather you keep sweating and the film builds back up.
Much of the smell comes from bacteria living on your skin. They give off ammonia, which attracts stable flies, and a blend of other bacterial odours made ammonia even more attractive.12Frontiers in Ecology and Evolution. Staphylococcus microbes in the bovine skin microbiome attract blood-feeding stable flies Skin oil produces many of the chemicals that make up your personal body odour.13PubMed. Sebaceous origins of human odor Blends of skin-type chemicals plus carbon dioxide caught two to three times more biting black flies than carbon dioxide alone.14PubMed Central. Identification of Human Semiochemicals Attractive to the Major Vectors of Onchocerciasis For some flies, like the canyon fly, sweat added nothing without the gas in your breath present.15PubMed. Evaluation of ammonia, human sweat, and bovine blood as attractants for the female canyon fly, Fannia conspicua (Diptera: Muscidae), in southern California
Fresh sweat is mostly water and salt and is nearly odourless. Much of the odour comes later, as skin bacteria feed on it over hours and turn it into stronger smells. Damp socks, waistbands and skin folds stay moist and warm, giving bacteria ideal conditions.
Heat, Colour and Movement Up Close
Stable flies are most attracted to objects that both look like a host and feel warm.16Entomologia Experimentalis et Applicata. Host‐foraging stable flies, <i>Stomoxys calcitrans</i>, are preferentially attracted to objects with both visual and thermal host‐like characteristics Heat mostly works at close range, prompting a fly to land rather than guiding it from afar.17Entomologia Experimentalis et Applicata. Host‐foraging stable flies, <i>Stomoxys calcitrans</i>, are preferentially attracted to objects with both visual and thermal host‐like characteristics Well-fed flies ignored the heat, so hunger strongly affects this response.18Entomologia Experimentalis et Applicata. Host‐foraging stable flies, <i>Stomoxys calcitrans</i>, are preferentially attracted to objects with both visual and thermal host‐like characteristics Horseflies prefer fine stripes of warmer temperature on skin, like those above blood vessels.19PubMed Central. Sunlit zebra stripes may confuse the thermal perception of blood vessels causing the visual unattractiveness of zebras to horseflies Warm, thin-skinned spots like ankles, neck and wrists are popular, as are sweaty areas like the hairline and exposed skin over visible veins.
Colour matters too. Some biting flies, like the deer ked, landed most on blue and least on yellow.20PubMed Central. Colour Preference of the Deer Ked Lipoptena Fortisetosa (Diptera: Hippoboscidae) Zebra stripes appear to confuse horseflies’ heat sensing, making stripes less attractive.21PubMed Central. Sunlit zebra stripes may confuse the thermal perception of blood vessels causing the visual unattractiveness of zebras to horseflies Dark fabric absorbs sunlight and gets warmer, so it can look and feel more like a warm animal to a fly nearing it. Preferences differ between species, so no single colour keeps every fly away. Some flies see up to 250 flashes a second, about four times what people perceive,22Florida International University. Flies evade your swatting thanks to sophisticated vision and neural shortcuts so a fly sees your hand coming in slow motion, which is why swatting so often misses.
Why Flies Pick Some People Over Others
Some people are bitten more than others mainly because of their odour or appearance.23PubMed Central. Why are some people bitten more than others? Each person’s skin oil and bacteria also give a distinct odour signature,24PubMed. Sebaceous origins of human odor so two people sitting side by side can send very different signals.
Drinking can change your smell. In a study of mosquitoes rather than flies, people who drank beer drew more malaria mosquitoes toward them than after drinking water.25PubMed Central. Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes The rise wasn’t explained by more breath or higher body temperature.26PubMed Central. Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes
Some of this you can change and some you can’t. Your basic skin chemistry, including the oils and bacteria behind your odour signature, is fixed for practical purposes. You can cut the build-up of sweat and the smell that follows by rinsing off and changing damp clothes. Breathing calmly instead of panting lowers how much carbon dioxide you send out, though only a little.
Some biting flies don’t really prefer people at all. Sand flies feed on whatever host is handy and bite people mostly when other animals aren’t around.
Nuisance Flies Versus Biting Flies
Non-biting flies feed on skin secretions or seek rotting material for its bacteria.27Pursuit, University of Melbourne. Why flies and mosquitoes are irritating little pests Flies’ smell receptors are especially tuned to fruit and social odours.28PubMed Central. Smelling on the fly: sensory cues and strategies for olfactory navigation in Drosophila Biting flies such as tsetse flies have receptors that detect chemicals in human body odour and breath, such as acetone.29PubMed Central. The molecular and cellular basis of olfactory response to tsetse fly attractants Some flies lean mostly on smell while others lean more on heat to find targets.30Kensington. Which Menace is It? Scent-Seeking Flies vs Heat-Seeking Flies Explained
You can often tell which kind you’re dealing with. House flies and similar nuisance flies walk and dab at your skin without causing pain, because they are tasting rather than biting. Horseflies and stable flies give a sharp bite you feel straight away. Biting flies are often found near water, livestock or woods, while nuisance flies gather around food, bins and pets. The place and the bite together are better clues than either alone.
Wind, light, temperature, humidity and time of day all change how many biting black flies come looking for hosts.31The Canadian Entomologist. EFFECT OF TRAP SITE, TIME OF DAY, AND METEOROLOGICAL FACTORS ON ABUNDANCE OF HOST-SEEKING MAMMALOPHILIC BLACK FLIES (DIPTERA: SIMULIIDAE) Calm, warm, humid conditions favour many biters, while a breeze scatters breath plumes and weak fliers. Checking the local peak hours for the flies in your area helps you pick your timing.
How to Make Yourself Less Attractive to Flies
Start with the food and smell. Rinse or wipe sweat off during hot activity, since sweat is the meal nuisance flies are after. Change out of damp clothes, which feed skin bacteria that make attractive ammonia. Skip beer before time outdoors in biting-insect country, given its effect on mosquitoes. Keep food, drinks and rubbish covered, since flies also come for rotting material.
Then cut the visual and heat cues. Long, loose clothing hides the warm skin and veins that guide landings. Avoid dark clothing in strong sun where horseflies are common. Sitting in a breeze or using a fan scatters your breath and odour trail. An insect repellent registered for biting flies on exposed skin adds another layer.
Apply repellent to exposed skin rather than under clothing, and follow the label. Reapply after heavy sweating or swimming, since sweat and water wash it off. Follow the label for children, because age limits and suitable products differ. Where biters are heavy, treat clothing with permethrin, which is made for fabric and is not for use on skin.
Most fly bites itch and fade within days. Watch for spreading redness, warmth or pus, and seek care for swelling of the face or trouble breathing.