No-See-Ums: The Biting Gnats You Cannot See
No-see-ums are biting midges, flies about 1 to 3 mm long in the family Ceratopogonidae, whose females bite people for blood at dawn, dusk and into the night, especially near coastal marshes, mangroves and other wet ground. They are small enough to crawl through ordinary window screen, so the defences that work are finer-mesh screening, moving air (they are weak fliers), timing, covering up and an EPA-registered repellent labelled for biting flies or midges. In the US they are a nuisance rather than a disease risk to people.
The name is literal. Purdue’s public health entomologists describe them as minute to tiny flies with mouthparts carrying four minute cutting blades that lacerate the skin and inflict a sharp, burning pain, which is why people feel the bite and then find nothing there. They go by many local names: punkies in the Northeast, sand gnats or sand flies on the coast, and, in Florida and Alabama, “five-O’s”, after their habit of biting around 5 p.m.
Do gnats bite?
Some do, and most of the “gnats” people meet indoors do not. That distinction decides what you should do next.
| Small fly | Bites people? | Size | Where you meet it | What it tells you |
|---|---|---|---|---|
| No-see-um (biting midge) | Yes, females only | 1 to 3 mm | Outdoors near marsh, mangrove, ponds, wet soil; dusk and night; can come indoors through screens | A location and timing problem, not a house infestation |
| Black fly (buffalo gnat) | Yes, females, by day | Small, robust, arched back | Near flowing streams and rivers, where larvae develop; bites head and upper body; does not enter buildings to feed | Seasonal outdoor nuisance |
| Fungus gnat | No | About 3 mm | Around damp houseplant soil | Overwatering |
| Fruit fly | No | 1 to 3 mm | Kitchens, fruit bowls, bins, drains | Fermenting food or residue |
| Drain fly (moth fly) | No | 2 to 4 mm | Bathroom and kitchen walls near drains | Organic film in a drain |
Colorado State University Extension says adult fungus gnats are harmless and do not bite or feed. Iowa State says moth flies do not bite or sting. Oregon State’s school pest management guide groups fruit flies, drain flies and fungus gnats together as nonbiting flies, and describes fruit flies as having sponging mouthparts. If tiny flies are drifting around a houseplant or hovering over the fruit bowl and you are getting bitten, the flies are not the biters. Our guides to fruit flies, drain flies and fungus gnats and small flying bugs in the house cover those.
Black flies do bite. Purdue says females of most species feed by day, usually on the upper body and head, and unlike certain mosquitoes and biting midges, they do not enter buildings to seek blood. So a bite indoors at night points towards midges or mosquitoes, not black flies.
What no-see-ums are
Biting midges belong to the family Ceratopogonidae in the true flies. UF/IFAS points out that most species in the family do not bite at all; the blood feeders are a minority, and the most important in the US is the genus Culicoides, with about 170 species in the country and around 50 in Florida. Leptoconops species bite in the West, including the Valley black gnat of California’s Sacramento Valley.
UF/IFAS gives adults as usually 1.0 to 2.0 mm, about 0.04 to 0.08 inch, with Florida species averaging 1.5 mm; Purdue gives 1 to 3 mm for the group. They are greyish, more reddish when full of blood, and many have spotted wings. Only females bite, because they need blood to develop eggs; males and females both feed on nectar.
Where they breed and live
The larvae are not fully aquatic but need constant moisture. UF/IFAS lists the edges of streams, marshes, ponds, puddles and water-filled tree holes, with several Florida pest species breeding in coastal marshes, swamps and mangroves, where they “can reach incredible densities”. University of Arizona Extension adds urban sources: over-irrigated mulch or sod, tree holes, construction sites, indoor plant pots and even water from air conditioners.
Purdue identifies the places where they most limit outdoor life: the Atlantic Coast, the Gulf Coast, the San Francisco Bay region and the southwestern deserts. In Florida, UF/IFAS notes diversity is greatest in the northern half of the state, while coastal species produce the worst swarms. Our guide to common house bugs in Florida puts them alongside the state’s other regulars.
How far they travel
Not far, for most. Purdue says females of most Culicoides species disperse no more than half a mile to a mile from where they developed, while one western Leptoconops species ranges up to about 10 miles. Arizona notes that high numbers of bites are likely close to where they emerge. If your yard is bad and a friend’s a few streets inland is not, that is why.
When no-see-ums are active
Most Culicoides do not begin to feed until dusk and continue at night, Purdue says, and Culicoides that bite people are attracted to light and readily enter dwellings. UF/IFAS notes that many species bite at dusk, at night or in the morning, but that in cold weather and in coastal areas many will bite in broad daylight. Leptoconops species feed by day. Arizona says numbers are highest in summer, but in places with warm winters they can occur year round.
Our analysis of iNaturalist research-grade records shows the same summer bulge for the whole family: records of biting midges, as a share of everything people recorded, peak in June, with a second high in August and September. The sample is small, 309 records in all, so read it as a rough shape rather than a precise calendar, and note that it includes the many non-biting midges in the family.
What happens when no-see-ums bite you?
A female uses her cutting mouthparts to open the skin and feed on blood, and her saliva sets off the reaction. Purdue describes a burning sensation with reactions ranging from a small reddish welt to local allergic reactions with significant itching. Observant people may even see tiny red specks on the skin: midges filling with blood. Arizona describes small red welts or water-filled blisters that itch intensely and can break and bleed when scratched.
How long it lasts varies widely. UF/IFAS says discomfort usually lasts minutes to hours, while people who are hypersensitive may itch for two to three days. The western Valley black gnat is worse: UC Davis entomologists describe a single bite welting into a spot one or two inches across that lasts about two weeks, and they warn that scratching can make bites last twice as long and lead to infected sores.
Why do noseeums itch so badly?
Two reasons. The bite itself is a cut rather than a fine puncture, which is why it burns, and the itch that follows is your immune system’s reaction to the saliva, which is stronger in some people than others. Bites also tend to come in numbers: Arizona notes that high numbers of bites are likely close to where midges emerge.
Are no-see-um bites worse than mosquitoes?
In the moment, many people find them worse, because the sharp, burning bite is distinctive and bites tend to come in clusters. For health risk, the answer runs the other way in the US. Purdue says no biting midges are known to transmit disease agents to people in the United States, whereas CDC calls West Nile virus the leading cause of mosquito-borne disease in the contiguous United States. Our piece on mosquito spraying and disease risk covers that side.
Abroad, the picture differs. UF/IFAS notes that biting midges spread Oropouche virus in South America and the Caribbean, and CDC has reported a handful of cases in travellers. CDC also says a midge species that could carry the virus lives in the eastern US. If you feel unwell with fever after travel, tell a doctor where you have been. For bites in general, our bite identification guide explains why the mark alone rarely identifies the insect.
How do I stop being bitten by noseeums?
The measures that work are physical and behavioural. Extension sources disagree about repellents, so do not rely on spray alone.
- Time your outdoor hours. Purdue calls scheduling activities to avoid the daily biting peaks the most effective way to avoid bites. For Culicoides, that means dusk, evening and early morning, and still, humid evenings near water.
- Use moving air. Purdue says biting midges are weak fliers and that flight is greatly reduced or curtailed in windy conditions. That is the logic behind a fan blowing across a porch seating area or a bed: it is cheap and works only where the air is moving.
- Cover up. CDC advises loose-fitting long-sleeved shirts and trousers; Arizona suggests light colours and a hat with fine mesh netting for the face and neck in bad areas.
- Choose a repellent labelled for them. CDC says to use an EPA-registered repellent labelled for flies, biting flies or Culicoides. UF/IFAS says DEET is the main repellent used against biting midges and reports lemon eucalyptus oil performing well in laboratory tests. Purdue, by contrast, says DEET products and treated clothing give only limited protection, and UC Davis entomologists say repellents are not effective against the Valley black gnat. Follow the label.
- Fix the screens. See the next section; this is what stops bites indoors.
- Dry out local breeding spots if you have them. Arizona says that holding off irrigation and letting moist spots drain and dry kills eggs and larvae in over-watered turf, mulch and poorly drained areas. This does nothing about a salt marsh, and UF/IFAS notes that altering wetland habitat runs into cost and wetland regulations.
No-see-ums in the house and in bed
Ordinary window screen does not keep them out. CDC says midges can crawl through most screens that keep out mosquitoes, and UF/IFAS states plainly that regular window screens do not exclude them. Arizona is specific: no-see-ums pass through standard 16 by 16 or 18 by 16 mesh, and it advises replacing these with 30 by 30 mesh where possible. Purdue says screens with a mesh smaller than normal are much more effective at reducing bites than insecticides.
Mesh counts are wires per inch, so the arithmetic is simple. In 16 mesh the wires sit about 1.6 mm apart centre to centre, so each gap is a little smaller than that once the wire is subtracted. A midge 1 to 2 mm long, with a body narrower than its length, fits through. At 30 mesh the spacing drops to about 0.85 mm. UF/IFAS notes that no-see-um screen exists but is not typically fitted to windows, so it is often something you have to ask for.
Indoors, keep doors shut at dusk, as Arizona advises, and remember Purdue’s point that the biting species are attracted to light: a lit room with an open, standard-screened window is an invitation. A fan over the bed helps for the same reason it helps on a porch.
Spraying: what it can and cannot do
Purdue describes control of biting midge larvae and adults as very difficult, with insecticide suppression of adults enjoying limited success. Evening aerial applications along the Atlantic and Gulf coasts have given temporary relief. UF/IFAS reports Florida field work in which ultra-low-volume permethrin applications killed field-collected Culicoides furens, and a bifenthrin barrier spray at the maximum label rate reduced their numbers. In practice, area spraying for midges is mostly the work of public mosquito control programmes, and a yard barrier spray is a short-term measure at best; our guide to mosquito control cost explains what those services involve. Any product must be labelled for the site and used as the label directs.
Frequently asked questions
Why are they called no-see-ums?
Because people feel the bite and then cannot find the culprit. Adults are usually 1 to 2 mm long and greyish, and most bite at dusk or at night when they are hardest to spot. Purdue notes they also go by punkies, sand gnats and, in Florida and Alabama, five-O’s, after their habit of biting around 5 p.m.
What does a gnat bite look like?
A no-see-um bite usually starts as a burning prick and becomes a small red welt, sometimes a tiny water-filled blister, that itches intensely; bites often come in groups on exposed skin. Black fly bites range from a small puncture to swelling as large as a golf ball, according to Purdue. Fungus gnats, fruit flies and drain flies do not bite, so they leave no marks.
How to get rid of gnats biting you?
First confirm the biter is a midge or black fly, not the harmless house gnats. Then avoid the dusk and dawn peaks near water, cover up, use an EPA-registered repellent labelled for biting flies or midges, run a fan where you sit or sleep, and replace standard screens with finer mesh. Area spraying gives temporary relief at best, according to Purdue.
Can gnats bite you in your sleep?
No-see-ums can. Purdue says the Culicoides species that bite people feed from dusk into the night, are attracted to light, and readily enter homes, and University of Arizona Extension says they occasionally bite people indoors after passing through ordinary screens. Black flies do not enter buildings to feed, and fungus gnats, fruit flies and drain flies do not bite at all. A fan over the bed and finer screens help.
What kills gnats instantly?
For biting midges, nothing practical gives lasting results: Purdue says insecticide sprays have had limited success and give only temporary relief, because new midges keep arriving from wet habitat nearby. Prevention works better than killing. For non-biting house gnats, the fix is removing the source, such as overwatered plant soil, fermenting fruit or a dirty drain, rather than spraying the adults.
Sources
- UF/IFAS Featured Creatures EENY-349 / EDIS IN626: Biting Midges, No-See-Ums, Culicoides spp. (Burkett-Cadena, Cooper, Addae, Connelly and Buckner).
- Purdue University Public Health and Medical Entomology, Biting Midges; and Black Flies.
- University of Arizona Cooperative Extension, Managing No-See-Ums (IPM short, 2018).
- University of California Agriculture and Natural Resources, Bug Squad: No-See-Ums? You Feel ‘Em (Valley black gnat, Bohart Museum, UC Davis).
- CDC, Midges Under the Microscope (Oropouche), March 2025; CDC, About West Nile Virus.
- Colorado State University Extension, Fungus Gnats as Houseplant and Indoor Pests.
- Iowa State University Extension, Moth Fly.
- Oregon State University Extension, PNW 770: Integrated Pest Management of Nonbiting Flies in Schools.
- The Pest Control Brief analysis of iNaturalist research-grade records, pulled October 2026.
