Several types of flies bite humans, and they all do it for the same reason: females need a blood meal to produce eggs. The most common biting flies in North America are horse flies, deer flies, stable flies, black flies, biting midges (no-see-ums), and sand flies. Each one varies in size, habitat, peak season, and how much the bite hurts.
Horse Flies
Horse flies are the largest biting flies you’ll encounter, ranging from half an inch to over an inch long. Some are entirely black, while others, called “greenheads,” are light brown with shiny green eyes. They’re strong, fast fliers and are most active in mornings and evenings, resting in shady areas during the heat of the day.
Horse fly bites are painful because of how the fly feeds. Instead of piercing skin with a needle-like mouthpart, horse flies use two pairs of cutting blades that lacerate the skin and cause blood to flow from the wound. The fly then laps up the pooling blood with a sponge-like mouthpart. This slicing action is why the bite feels more like a cut than a sting, and why it often continues bleeding after the fly leaves. Their larvae develop in moist or wet soil, and the full life cycle can take up to two years to complete.
Deer Flies
Deer flies are smaller relatives of horse flies, about a quarter inch long, roughly the size of a house fly. They’re typically yellow-brown to black with distinctive dark bands on their wings, and some have iridescent green eyes. They’re most common in spring and use the same slicing mouthparts as horse flies, so their bites are similarly painful and prone to bleeding.
Deer flies are one of the few fly species in the United States that transmit disease to people. They can occasionally carry tularemia, a bacterial illness also known as “rabbit fever.” While tick bites and contact with infected animals are more common routes of transmission, the deer fly Chrysops discalis is a recognized carrier.
Black Flies
Black flies, also called buffalo gnats, are small (about an eighth of an inch) with broad wings and a distinctive humpbacked shape. They breed in clean, flowing water, so you’ll find them near creeks, rivers, and streams. They will fly up to 10 miles from these breeding sites to find a host, and they’re most problematic in late spring and early summer.
Black fly bites often cause significant swelling and bleeding that can be slow to heal. They tend to target the head and areas where clothing fits tightly against the skin. Some people develop strong reactions to the fly’s saliva, including headache, fever, nausea, and swollen lymph nodes in the neck. In very large numbers, black flies can threaten the lives of livestock and, rarely, people through allergic reactions, blood loss, or even inhalation of the flies. They do not transmit disease to humans in the United States.
Stable Flies
Stable flies look almost identical to house flies, about a quarter inch long and gray with four dark stripes behind the head. The key difference is a pointed, needle-like mouthpart that protrudes from beneath the head. Despite the name, stable flies aren’t limited to barns. They’re a common pest on recreational beaches (earning the nickname “beach fly”), in the Great Lakes area, along the Atlantic seaboard, and on the Gulf Coast.
Stable flies typically bite the lower legs and ankles, delivering a sharp, stabbing pain. They’re most active in early morning and late afternoon, and most abundant in late summer and fall. Their larvae breed in moist, decaying organic matter like rotting vegetation and mud around livestock feeding areas, and adults can fly several miles from these breeding sites.
Biting Midges (No-See-Ums)
Biting midges are tiny, just 1 to 3 millimeters long, which is why they’re commonly called no-see-ums. You’ll often feel the bite before you ever spot the insect. Their mouthparts lacerate the skin, creating a sharp, burning sensation. Reactions vary from a small red welt to intense local itching and allergic swelling.
Different species feed at different times. Some are active during daylight, while others begin feeding at dusk and continue through the night. Despite being extremely annoying, biting midges in the United States are not known to transmit any disease to humans.
Sand Flies
Sand flies are a concern primarily for travelers. In the United States, only one species bites humans, and it isn’t known to transmit disease. Internationally, the picture is very different. Sand flies in parts of Central and South America, the Middle East, East Africa, North Africa, and South Asia carry leishmaniasis, a parasitic disease found in about 88 countries.
The most common form, cutaneous leishmaniasis, produces skin sores that develop where the fly fed. These lesions change over time, often forming a raised edge with a sunken center, sometimes covered by a scab. The more serious form, visceral leishmaniasis, causes fever, weight loss, and organ enlargement. Over 90 percent of visceral cases occur in India, Bangladesh, Nepal, Sudan, and Brazil.
How to Treat a Fly Bite
For most fly bites, basic first aid is enough. Wash the area gently with soap and water, then apply a cold, damp cloth or ice wrapped in cloth for 10 to 20 minutes to reduce pain and swelling. Calamine lotion, baking soda paste, or a low-strength hydrocortisone cream applied several times a day can help with itching. An over-the-counter antihistamine like cetirizine or loratadine can also reduce the itch from the inside out.
Watch for signs of infection: increasing swelling, warmth, redness spreading from the bite, or pus. Black fly bites in particular can swell dramatically and heal slowly, which is normal for that species but worth monitoring. Seek emergency help if you experience difficulty breathing, facial or throat swelling, dizziness, a rapid weak pulse, or hives, as these are signs of a severe allergic reaction.
Repellents That Actually Work on Flies
Not all repellents are equally effective against all biting flies, and for some species, no repellent works particularly well. DEET is effective against biting midges (no-see-ums) and black flies, but it doesn’t work well on deer flies, horse flies, or greenheads. Picaridin, at concentrations labeled for biting flies, offers protection against mosquitoes, biting flies, and chiggers. Oil of lemon eucalyptus products are labeled for black flies, gnats, mosquitoes, and no-see-ums.
For horse flies and deer flies, physical barriers tend to be more effective than chemical repellents. Wearing long sleeves, long pants, and light-colored clothing (dark colors attract these flies) can reduce bites. Avoiding wet, marshy areas during peak morning and evening hours also helps, since these flies rest in shady, moist environments during the day and actively hunt at the edges of those periods.