No-See-Ums: What They Are, Why You Cannot See Them, and What Keeps Them Off
No-see-ums are biting midges, tiny flies in the family Ceratopogonidae. Most are between 1 and 2.5 millimeters long, small enough to pass through ordinary window screens. The females bite because they need a protein meal to make eggs. The CDC's advice is an EPA-registered repellent whose label names flies, biting flies, or Culicoides.
What exactly is a no-see-um?
It is a fly. The insects people call no-see-ums are biting midges, and the CDC treats the two names as the same thing, describing Oropouche virus as spread mostly by "tiny biting midges, sometimes called 'no-see-ums.'" Depending where you are standing, you may hear punkies, granny nippers, sand flies, or simply gnats.
They belong to the family Ceratopogonidae. The genus that matters to anyone with itchy ankles is Culicoides, which a 2023 review in Microorganisms describes as "the most notorious," noting that "many of its 1347 species are hematophagous," meaning they feed on blood.
Why can I feel the bite but not see the insect?
Because they are roughly the size of a grain of coarse pepper. The same review puts it plainly: biting midges are "tiny culicomorph insects, usually measuring 1 to 2.5 mm in length." A mosquito is several times larger and announces itself. A midge does not.
Size is also why the usual defenses fail. The CDC is direct about this: "Because of their small size, biting midges can crawl through most screens that otherwise keep out mosquitoes." A porch screen sized for mosquitoes is, to a no-see-um, an open door.
Why do the bites itch so much for something so small?
Only the females bite, and they are not feeding for themselves. In Culicoides and related genera "the females usually need a proteinaceous meal to produce eggs." The bite is a means to reproduction, which is why swatting one insect rarely ends the problem: you are standing where they breed.
Where do they come from?
Wet ground, broadly defined. Their larvae "develop in all types of aquatic, semiaquatic and terrestrial humid habitats, with many species using the entire range of these." That covers salt marsh, pond margin, damp sand, mud, and the soggy corner of a lawn. It is why coastal holidays and lakeside evenings have a reputation, and why there is no single container to tip out the way there is with mosquitoes.
What time of day do no-see-ums bite?
There is no reliable answer, and that is the useful thing to know. CDC medical entomologist Roxanne Connelly puts it this way: "Biting midges can be out and around at different times of day, the time varies among different species." Dawn and dusk are a common experience, but planning your evening around a fixed window is not a strategy.
Community-level control is also thinner than it is for mosquitoes. The CDC notes that "there's less known about controlling biting midges in the community." Which pushes the problem back to what you personally put on.
Do no-see-ums spread disease?
Some biting midges carry viruses. The reason the CDC has been writing about them recently is Oropouche virus, and the agency reports that "a biting midge species that could carry the virus lives in the eastern United States." That is a statement about the insect's range, not a forecast, and most no-see-um encounters in the United States are an itching problem rather than an illness one. If you develop fever, joint pain, or body aches after travel, that is a conversation for a clinician, not a web page.
Does spraying the yard get rid of them?
Not reliably, and the reason is timing and location rather than the chemistry. The CDC notes that "chemicals used to spray for mosquitoes in the community will also kill biting midges, but those are typically used when mosquitoes are most active." Midges do not necessarily keep mosquito hours.
There is also an open question about whether treatment reaches them at all. As Connelly puts it: "We know the equipment and products used to control mosquitoes will also work on biting midges. But what's not been evaluated is how effective the products are in getting to where the biting midges are." For now, what you wear and what you put on your skin is the part you control.
What actually repels no-see-ums?
The CDC's guidance is specific, and the specificity is the point: "Look for EPA-registered repellents labeled for flies, biting flies, or Culicoides (biting midges, punkies, granny nippers, and no-see-ums)." A product that only names mosquitoes and ticks has not been evaluated against midges. The label is the evidence.
The CDC adds that "when used as directed, EPA-registered insect repellents are proven safe and effective," and pairs repellent with two physical measures: "wear loose-fitting, long-sleeved shirts and pants" and "stay in air-conditioned places to keep insects outside."
Among the registered active ingredients, picaridin is one that carries biting-fly coverage. The National Pesticide Information Center states that "picaridin can be used on human skin or clothing to repel mosquitoes, biting flies, ticks, fleas, and chiggers." It is "a human-made (synthetic) compound" made "to resemble the natural compound piperine, which is found in the group of plants that are used to produce black pepper," and it has been sold in the United States since 2005.
How do I check a product before I buy it?
Read the label, and read it for your pest. EPA runs a search tool that lets you filter by "mosquitoes, ticks or both; protection time; active ingredient; or other product-specific information," and the agency's standing instruction is that "for the safe and effective use of pesticide products, always read the product label before using the product." If no-see-ums are your problem, the words to look for are flies, biting flies, or Culicoides. If they are not printed, assume the product was not tested on them.
Because EPA's tool filters on mosquitoes and ticks rather than midges, we built something that answers this exact question. The Repellent Index holds all 631 EPA-registered skin repellents, and you can filter it by pest, including no-see-ums. Of those 631, 47 carry a no-see-um protection time we could read off their federal label. Every figure links back to the label it came from, ours included.
Longsleeve repels no-see-ums for up to 8 hours. That is a registered claim, not a marketing line: the accepted federal label reads "Repels biting flies, stable flies, black flies, gnats, chiggers, sand flies and Scottish biting midges (no see-ums) for up to 8 hours." Scottish biting midges is the label's own name for the insect; no-see-ums is the parenthetical it gives alongside.
Longsleeve is an EPA-registered picaridin lotion, EPA Reg. No. 101457-1. It repels mosquitoes and ticks for up to 14 hours, it is fragrance-free, and all nine of its ingredients are named on the tube.
Common questions
Are no-see-ums the same as gnats?
"Gnat" is a loose word covering several small flies, most of which do not bite. No-see-ums are biting midges in the family Ceratopogonidae, and the females do bite. If something you could barely see drew blood, midge is the likelier answer.
Will a window screen keep no-see-ums out?
Usually not. The CDC states that "because of their small size, biting midges can crawl through most screens that otherwise keep out mosquitoes." Finer mesh exists and is sold for this purpose.
Do no-see-ums bite at a particular time of day?
It varies. Per the CDC, "biting midges can be out and around at different times of day, the time varies among different species." Treat the whole outdoor stretch as exposure rather than one window.
Does a mosquito repellent automatically work on no-see-ums?
Not automatically. The CDC directs people to repellents "labeled for flies, biting flies, or Culicoides." Coverage is established on the label, product by product, so check yours rather than assuming.
Why do no-see-ums bite me and not the person next to me?
Attraction varies between individuals and between midge species, and the published work on midges is thinner than it is for mosquitoes. The CDC itself notes that less is known about biting midges in the community than about mosquitoes.
Sources
- CDC. Oropouche Puts Biting Midges Under the Microscope. https://www.cdc.gov/oropouche/stories/midges-under-the-microscope.html
- CDC. Preventing Oropouche. https://www.cdc.gov/oropouche/prevention/index.html
- Kampen H, Werner D. Biting Midges (Diptera: Ceratopogonidae) as Vectors of Viruses. Microorganisms, 2023;11(11):2706. https://pmc.ncbi.nlm.nih.gov/articles/PMC10673010/
- National Pesticide Information Center. Picaridin Fact Sheet. https://npic.orst.edu/factsheets/PicaridinGen.html
- US EPA. Find the Repellent that is Right for You. https://www.epa.gov/insect-repellents/find-repellent-right-you
Last reviewed 2026-08-24 · every claim on this page is sourced above