Only a few types of gnats actually bite. The two main culprits are biting midges (also called no-see-ums) and black flies. Most of the tiny flies people call “gnats,” including fungus gnats and non-biting midges, are completely harmless. They may swarm around your face or hover near fruit on your counter, but they lack the mouthparts to break skin.
Biting Midges (No-See-Ums)
Biting midges belong to the fly family Ceratopogonidae, and the most common genus is Culicoides. Their nickname, “no-see-ums,” is well earned: adults measure just 1 to 2.5 millimeters long, small enough to pass through standard window screens. You’ll often feel the bite before you ever spot the insect responsible.
Only the females bite. They need a blood meal to develop their eggs, and they get it by slicing into your skin with blade-like mouthparts, then feeding from the tiny pool of blood that forms. Their saliva contains compounds that keep your blood flowing and prevent clotting, and it’s this saliva that triggers the itchy, red welts afterward. Biting midges breed in moist soil and mud around streams, ponds, and marshes, so they’re most common near water. They tend to be most active at dawn and dusk.
Black Flies
Black flies (family Simuliidae) are slightly larger than no-see-ums but still small, typically 2 to 5 millimeters. They’re closely related to biting midges and feed the same way: the female cuts a hole in your skin and drinks the blood that pools in the wound. She injects anticoagulants and even a mild painkiller to stay undetected while feeding.
Black fly bites tend to be more painful and dramatic than no-see-um bites. Reactions range from a small puncture wound to swelling the size of a golf ball, depending on your sensitivity. A cluster of bites can trigger what’s sometimes called “black fly fever,” with headache, nausea, low-grade fever, and swollen lymph nodes in the neck. Adults of most species are active from mid-May through July, and they prefer to breed near running water rather than still ponds. In North America, black flies are not known to transmit diseases to humans, so the main concern is the allergic reaction to the bite itself.
Sand Flies
Sand flies (family Psychodidae) are another small biting fly sometimes lumped under the “gnat” label, especially in coastal and tropical areas. They use chisel-like mouthparts to cut through the outer layers of skin and create a small hemorrhagic pool to feed from. Like biting midges, they inject saliva loaded with anticoagulants and vasodilators to keep blood flowing. Sand flies are more of a concern in tropical regions, where certain species can transmit parasitic diseases. In most of the continental United States, they’re far less common than no-see-ums or black flies.
Gnats That Don’t Bite
The vast majority of insects people call gnats are harmless. Fungus gnats, the tiny dark flies that hover around houseplants, feed on fungi and decaying organic matter in potting soil. They never bite. Non-biting midges (family Chironomidae) look remarkably like mosquitoes and can form enormous swarms near lakes and rivers, but they don’t feed at all as adults. They live just long enough to mate, lay eggs, and die. Eye gnats, another common nuisance species, are attracted to moisture around your eyes and face but lack biting mouthparts.
If you’re dealing with tiny flies indoors, they’re almost certainly a non-biting species. Biting gnats are outdoor insects tied to wet habitats, and they rarely establish themselves inside homes.
How to Tell If a Gnat Bit You
Biting gnat bites produce small, raised lumps on the skin that are intensely itchy. They look similar to mosquito bites but are often smaller and grouped in clusters, since these insects tend to bite multiple times in the same area. If you’re particularly sensitive, the bites can develop into fluid-filled blisters or raised circular welts around the puncture site.
Black fly bites are easier to identify because they often leave a visible puncture wound at the center of the swollen area, and the swelling can be surprisingly large relative to the size of the insect. No-see-um bites are usually smaller but can itch for days.
Treating Gnat Bites
Most biting gnat reactions are mild and resolve on their own. Wash the area gently with soap and water, then apply a cold, damp cloth or ice wrapped in fabric for 10 to 20 minutes to bring down swelling. If the itch is persistent, calamine lotion, a baking soda paste, or a low-strength hydrocortisone cream applied several times a day helps. An over-the-counter antihistamine like cetirizine or loratadine can also reduce itching and swelling from the inside.
The hardest part is resisting the urge to scratch. Scratching breaks the skin and introduces bacteria, which is the most common way gnat bites turn into something more serious. If a bite site grows increasingly red, warm, or swollen over the following days, or begins oozing, that suggests a secondary infection rather than a normal reaction.
Reducing Your Exposure
Because biting gnats breed in wet soil and near water, your risk goes up dramatically near marshes, streams, and lakeshores, especially during warm months. No-see-ums are most active at dawn and dusk, while black flies often bite during daylight hours. Wearing long sleeves and light-colored clothing helps, since dark colors attract black flies. Fine-mesh head nets are effective in heavily infested areas where standard insect repellent isn’t enough.
For no-see-ums specifically, replacing standard window screens with finer mesh (at least 16×16 threads per inch) can keep them from entering your home. Reducing standing water and soggy soil around your property limits local breeding habitat, though biting gnats can fly in from nearby waterways regardless of how dry you keep your yard.