THE SHORT ANSWER
A no-see-um is a biting midge — a fly in the genus Culicoides, often smaller than a millimetre. They breed in damp soil and rotting organic matter rather than standing water, and they pass straight through ordinary window screens. That combination is why the usual mosquito measures do nothing.
WHAT IT IS
Culicoides biting midge
SIZE
Under 1/8 inch
BREEDS IN
Damp soil, not water
SCREEN MESH NEEDED
20×20 or finer
What is a no-see-um?
A tiny biting fly in the family Ceratopogonidae, almost always the genus Culicoides. Depending on where you are it goes by punkies, sand gnats, sand flies or biting midges — all the same insect.
The name is literal. Colorado State Extension puts adults at under an eighth of an inch; NC State notes some species are only half a millimetre. They are roughly a quarter the size of a mosquito, which is why you register the bite before you register anything doing the biting.
Only the female bites, and only because she needs a blood meal to develop eggs. Males feed on nectar and are harmless. She finds you the same way a mosquito does — carbon dioxide, body heat, skin odour — and she works at dusk and dawn, with activity continuing through the night in some conditions. Damp, still, overcast evenings are peak.
Why the bite feels different from a mosquito bite
Because it is a different mechanism entirely, and this is worth knowing if you are trying to work out what bit you.
A mosquito inserts a fine proboscis and draws blood from a capillary. A midge does not. Montana State Extension describes the process as pool feeding: she lacerates the skin and siphons up the blood that collects in the wound. It is a cut rather than a puncture, which is why the sting is sharper at the moment it happens and why the reaction that follows is often more intense and longer-lasting than a mosquito bite of the same size.
The pattern gives it away too. Midge bites tend to arrive in clusters and in lines along exposed skin — ankles, wrists, the hairline, around a collar — because a swarm is working the same area at once. Mosquito bites are usually more scattered and fewer.
University of Arizona Extension notes that people who call about these bites frequently believe they have fleas or chiggers, and only work out it was midges by elimination. If you were outside at dusk near anything damp and came in with a row of small, intensely itchy welts you did not feel arrive, that is the likely answer.
One reassurance: in North America, biting midges are not considered vectors of human disease. They do transmit bluetongue virus among livestock, and true sand flies in tropical regions carry other pathogens — but that is a different insect in a different part of the world.
They do not breed in standing water
This is the single fact that explains why nothing you have tried has worked.
Every piece of mosquito advice you have ever read starts with emptying standing water — the birdbath, the plant saucers, the sagging tarp. That advice is correct, for mosquitoes, whose larvae are aquatic and need water to develop in.
Midge larvae are not aquatic in that sense. They develop in moist soil rich in organic matter. Colorado State Extension lists damp ground at the margins of ponds, swamps and manure lagoons as the favoured sites. NC State adds mud, intertidal sand, wet soil around seepage areas, and — on the coast — decaying seaweed thrown up after a storm. Closer to home that means soggy leaf litter, wet compost, permanently damp mulch, a boggy corner, the saturated edge where a lawn meets a ditch.
So you can empty every container on the property and change nothing. The habitat you are looking for is wet dirt, not open water.
There is a harder truth attached. Colorado State states plainly that larval control with insecticide is not possible, because the breeding sites are inaccessible — and that those sites can be miles from where the biting is happening. Montana State reports host-seeking females travelling an average of about a mile and a half. Which means for a great many people the source is not on their land at all, and nothing done to their own yard will remove it.
Your screens are the wrong mesh
Standard insect screening is about 16 mesh — sixteen openings per inch. That stops mosquitoes comfortably. It does not stop a midge, which walks through it.
Clemson Extension gives the working specification: 20×20 mesh or finer is what excludes biting midges. Camping suppliers sell the same thing as no-see-um netting, and it is the reason those tents exist as a separate product.
If you have a screened porch that keeps mosquitoes out and still fills with something biting you at dusk, you have not got a hole in the screen. You have got the wrong gauge of screen, and no amount of patching changes it. Rescreening a porch in fine mesh is the single most reliable thing a homeowner can do about this insect.
The trade-off is airflow. A 20×20 weave restricts breeze noticeably more than standard screen, which matters on a hot evening. That is a genuine cost and worth knowing before you order.
What actually reduces them
The best tool on the list is a fan, and it costs almost nothing.
Midges are weak fliers. A ceiling fan or an oscillating floor fan aimed across a seating area keeps them out of it, because they cannot make headway against moving air. For a deck, a patio or a porch, that is more effective than most sprays and it works instantly.
Carbon-dioxide-baited traps are the method most often recommended for adults, and they do catch host-seeking females. Set expectations accordingly — a trap reduces the number reaching a specific area, it does not clear a property, and if the breeding ground is a marsh half a mile away it never will.
Treating screens with a permethrin product is a further option where midges are getting through existing mesh and rescreening is not immediately possible.
WHERE THIS STOPS BEING A DIY JOB
You cannot find the damp ground. The larval habitat is diffuse and often invisible — seepage at a property line, a shaded area that never dries, ground under decking. Someone who does this work knows where to look, and identifying the site is most of the job.
You have already tried the mosquito treatment and nothing changed. That is the expected result rather than a failure of the product, and it usually means the target was wrong. A different survey is needed, not a stronger spray.
Bites are causing a genuine reaction. Midge bites can produce swelling well out of proportion to the insect. Persistent, spreading or infected-looking bites are a matter for a doctor, not a pest company.
The source is next door or on public ground. Nothing on this page fixes that, and it is worth knowing early so you spend the money on screening and fans instead of on treating your own yard repeatedly.
Tried mosquito treatment and still getting bitten at dusk? Call (877) 360-4657
Common questions
What do no-see-um bites look like?
Small red welts that itch intensely, usually in clusters or lines on exposed skin rather than scattered singly. They often appear hours after you were outside, because the insect is too small to notice at the time.
Are no-see-ums and sand flies the same thing?
In the United States, yes — sand fly, sand gnat, punkie and biting midge all refer to Culicoides. True sand flies in the tropics are a separate group of insects, which is why disease information from overseas does not transfer.
Why do no-see-ums get through my screens?
Because standard screening is about 16 mesh and they are small enough to pass through it. Clemson Extension specifies 20×20 mesh or finer to exclude biting midges. The screen is not damaged — it is the wrong gauge.
How do I get rid of no-see-ums in my yard?
Realistically you reduce exposure rather than eliminate them. Fans, fine mesh, repellent and avoiding dusk work. Larval control is impractical because the breeding sites are diffuse damp soil and are often not on your property.
Do no-see-ums carry disease?
Not to people in North America. They are significant vectors of bluetongue virus in livestock, and related midges transmit disease in other parts of the world, but human transmission is not a concern here.
When are no-see-ums most active?
Dusk and dawn, with biting continuing overnight in some conditions. They are worst on damp, still, overcast evenings and largely absent when there is any real breeze.
Sources. Identification, size and larval habitat follow NC State Extension and Colorado State University Extension. Pool feeding, host-seeking range and disease status follow Montana State Extension. Screen mesh specification follows Clemson Cooperative Extension. Misidentification with fleas and chiggers follows University of Arizona Cooperative Extension.
Related
- Mosquito control — the insect these are constantly confused with
- Professional mosquito control — when the work is habitat rather than spraying
- Mosquito killers and traps — including the CO2-baited traps mentioned above
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