“Gnat” isn’t a scientific term. It’s a catch-all name for several species of tiny flying insects, and the one buzzing around your kitchen is probably not the same species swarming near a lake. The most common types you’ll encounter are fungus gnats, drain flies (also called moth flies), biting midges, black flies, eye gnats, and fruit flies. Identifying which type you’re dealing with is the first step to getting rid of them, because each breeds in a completely different environment.
Why “Gnat” Means Different Things
There’s no scientific consensus on what counts as a gnat. Entomologists at the University of Kentucky consider only non-biting flies to be true gnats, while those at the University of Nebraska include fungus gnats in the category. The Smithsonian Institution adds eye gnats to the list. In everyday language, people use “gnat” for virtually any tiny fly that shows up uninvited. All gnats belong to the order Diptera (true flies), but they span multiple families with very different habits, habitats, and risks.
Fungus Gnats
Fungus gnats are the ones you’ll most often see hovering around houseplants. They’re dark, delicate flies about 1/16 to 1/8 inch long with slender legs and segmented antennae longer than their heads. Their wings are light gray to clear. They look a bit like tiny mosquitoes, but they don’t bite.
Females lay eggs in moist organic material, especially potting soil, compost, decomposing grass clippings, and mulch. The larvae are small, legless, whitish worms with shiny black heads. They primarily feed on fungi and decaying matter in the soil, but they also chew plant roots. In small numbers, the damage is negligible. In large populations, though, larvae can stunt seedlings, kill young plants, and cause established houseplants to wilt. A wilting plant that has moist soil may actually have a fungus gnat larva problem rather than a watering issue. In severe infestations, larvae leave slime trails on the soil surface that resemble tiny snail tracks.
Their lifecycle is fast. At about 75°F, eggs hatch in 3 days, larvae develop over roughly 10 days, pupae emerge as adults after another 4 days. That’s a full generation in around 17 days, and warmer temperatures speed things up even further. This rapid turnover is why a small problem can become a big one quickly if conditions stay moist.
Drain Flies
Drain flies, sometimes called moth flies, are easy to recognize once you know what to look for. They’re light gray, up to 1/8 inch long, and densely covered in fine hair that gives them a fuzzy, moth-like appearance. Their wings are large and oval-shaped, and when resting, they hold their wings over their body in a roof-like position. They don’t bite.
These flies breed in the slimy organic buildup inside drainpipes, septic tanks, sewage systems, and damp soil beneath leaking pipes. If you’re finding tiny moth-like insects near a bathroom or kitchen sink, drain flies are the likely culprit. Their lifecycle runs 8 to 24 days total: eggs hatch within about 2 days, larvae feed for 8 to 15 days, and the pupal stage lasts just 1 to 2 days before adults emerge. A single female lays 30 to 100 eggs. They’re more of a nuisance than a health risk, but their presence usually signals a drain that needs cleaning.
Biting Midges (No-See-Ums)
Biting midges are the smallest gnats on this list, just 1/32 of an inch long. They go by several names: no-see-ums, punkies, or simply “gnats.” Their size is so small they can pass through standard window and door screens, which is part of what makes them so frustrating.
Unlike fungus gnats and drain flies, biting midges feed on blood. Females need blood protein for egg production, and they’ll bite during the day or at night. They’re most common along the shores of oceans, lakes, ponds, rivers, and near marshes and lagoons. Their larvae are tiny, worm-like, and live in moist sand, soil, rotting vegetation, or tree holes at water’s edge.
The bites are disproportionately painful for such a small insect. In the United States, biting midges can transmit blue tongue virus to livestock, though human disease transmission is less of a concern domestically. Several species also feed on the blood of other insects, including mosquitoes.
Black Flies (Buffalo Gnats)
Black flies are also called buffalo gnats because of their distinctive humpbacked shape. Adults are small, no more than 1/8 inch long, with broad wings. They’re creatures of moist environments, found most often near creeks and rivers where their larvae attach to submerged rocks.
Black flies are aggressive blood feeders. They’ll fly up to 10 miles searching for a host, and they prefer to bite around the head and wherever clothing fits tightly against the body. Their bites cause considerable swelling and bleeding, tend to be very itchy, and are slow to heal. They don’t transmit diseases to humans in the United States, but in large swarms (typically in late spring and early summer), they can pose real danger. Deaths have been reported from severe allergic reactions, significant blood loss from numerous bites, and even from inhaling the flies when swarms are dense enough.
Eye Gnats
Eye gnats are non-biting flies no bigger than a few grains of salt, but they’re distinctly unpleasant. They’re attracted to the moisture around your eyes, wounds, and sweat. Female eye gnats feed on animal sweat, sebaceous secretions, pus, or blood to get proteins for egg production. They don’t bite or break the skin, but they have sponge-like mouthparts that soak up bodily secretions, and curved spurs on their hind legs that can scrape skin to increase the flow of those secretions.
What sets eye gnats apart from other tiny flies is their persistence. A female will repeatedly attempt to feed around your eyes and any open wounds. This persistent contact with mucous membranes and broken skin is what makes them a potential disease vector. Eye gnats can spread organisms that cause bacterial infections, including conjunctivitis.
Fruit Flies
Fruit flies are the tiny flies that appear near ripening fruit, open drinks, and compost bins. They’re about 1/8 inch long with a yellowish to pale brown body, a few black bands on the abdomen, and, in the most common species, distinctly red eyes. Their antennae have a feathery appearance. They’re not technically classified as gnats by most entomologists, but they’re lumped in so often that leaving them off this list would be misleading.
Fruit flies breed on fermenting organic matter. Overripe fruit, vegetable scraps, spilled juice, and even the residue inside empty bottles can sustain a population. They reproduce extremely quickly, which is why a few fruit flies on Monday can become dozens by Friday.
How to Tell Them Apart
Location is your best clue. Tiny flies near houseplants are almost certainly fungus gnats. Fuzzy, moth-like insects near drains are drain flies. Flies drawn to your face and eyes outdoors are eye gnats. Anything biting you near water is likely a biting midge or black fly, depending on size (midges are nearly invisible, black flies are about 1/8 inch and humpbacked). Red-eyed flies near your fruit bowl are fruit flies.
- Fungus gnats: Dark, mosquito-like, long legs, found near potted plants and moist soil
- Drain flies: Fuzzy, gray, moth-shaped wings, found near sinks and drains
- Biting midges: Nearly invisible at 1/32 inch, bite painfully, found near water
- Black flies: Humpbacked, 1/8 inch, aggressive biters near rivers and creeks
- Eye gnats: Grain-of-salt sized, persistently target eyes and wounds
- Fruit flies: Yellowish-brown with red eyes, found near ripe or fermenting fruit
Each type requires a different approach to control, because each breeds in a different substrate. Letting soil dry out between waterings handles fungus gnats. Cleaning drain buildup eliminates drain flies. Biting midges and black flies are outdoor pests best managed with fine-mesh screens (smaller than standard window screen for midges) and avoiding peak activity times near water. Fruit flies disappear when you remove their food source. Identifying the right type saves you from wasting effort on the wrong fix.