No-See-Ums Are Small Enough to Walk Through a Window Screen

An intensely itchy, painful bite appears with no visible insect anywhere nearby, usually on a calm evening close to water. This is the classic no-see-um encounter — a biting midge small enough that most people never actually spot the insect responsible, only the bite it leaves behind. Despite the disproportionate reaction they cause, no-see-ums are genuinely tiny, breed nothing like mosquitoes do, and call for a different set of prevention steps than the ones that work for other biting insects.

How small is small

No-see-ums belong to the family Ceratopogonidae, with Culicoides being the main biting genus in the US. Just how small they actually are varies somewhat by source and species — UF/IFAS describes many species as under roughly 1/24 inch (about a millimeter), while other extension material puts some species closer to 1/8 to 1/4 inch. The practical takeaway holds either way: they're tiny enough, often under 1/8 inch, that spotting one before or during a bite is genuinely difficult, which is a large part of why the bite itself tends to be the first and only sign something was there at all.

Why ordinary window screens don't stop them

University of Arizona Cooperative Extension is direct on this point: no-see-ums are small enough to pass straight through standard window screen mesh, which is sized to stop mosquitoes and larger insects but has gaps too wide to exclude something this tiny. Arizona Extension's practical fix is replacing standard screening with a tighter mesh specifically rated for biting midges, or treating existing screens with a permethrin-based product, rather than assuming a porch or window screen provides the same protection against no-see-ums that it does against mosquitoes.

Where they actually breed

This is the detail that separates no-see-um control from mosquito control most clearly. Mosquitoes need open standing water to breed, which is why eliminating puddles, clogged gutters, and other standing water is the standard mosquito-prevention advice. No-see-ums don't follow that pattern at all — University of Arizona Extension describes their larvae developing in wet, organic-rich soil rather than open water: heavily saturated ground with a lot of decaying organic matter, areas affected by heavy manure loads, over-irrigated mulch beds or sod, water trapped in tree holes, and even the consistently damp soil in indoor potted plants. Draining a visible puddle does nothing for a no-see-um population breeding in damp, organically rich soil nearby, which is why a yard that looks mosquito-prevention-ready on paper can still have an active no-see-um problem.

When and why they bite

As with mosquitoes, only female no-see-ums bite — they need a blood meal to develop eggs, while males feed on plant nectar and pollen instead. Both UF/IFAS and University of Arizona Extension describe peak activity around dawn and dusk, and on calm, cloudy days specifically; no-see-ums are weak fliers, so even a modest breeze noticeably reduces how active and effective they are at finding a target, which is part of why a sheltered, wind-protected porch or patio near water can see much heavier activity than a more exposed outdoor space just a short distance away.

The bite itself is disproportionate to the insect's size. A 2026 piece from University of Arkansas Extension entomologist Emily McDermott explains that no-see-um saliva contains a mix of anticoagulants, proteases, vasodilators, and analgesics (the exact combination varies by species), which is part of why the bite often isn't felt as it happens, followed by a stronger, longer-lasting itchy reaction once the body's immune response to those proteins kicks in. UF/IFAS separately describes the bites as irritating and painful, sometimes leaving longer-lasting lesions — reaction severity varies meaningfully from person to person.

Where they're most commonly a problem

UF/IFAS documents roughly 170 no-see-um species in the US, with about 50 found in Florida alone and the greatest diversity concentrated in the northern part of the state — framing them as persistent, abundant pests specifically for people near US coastal areas, including campers, beachgoers, and anglers. They're not exclusive to any one region, but coastal and wetland-adjacent areas with the kind of consistently damp, organically rich soil described above tend to see the heaviest activity.

• No-see-ums are tiny biting midges (family Ceratopogonidae) often under 1/8 inch, small enough to pass straight through standard window screen mesh.
• They breed in wet, organically rich soil — saturated ground, over-irrigated mulch or sod, tree holes, damp potting soil — not in open standing water the way mosquitoes do.
• Only females bite, for a blood meal; they're weak fliers most active at dawn and dusk on calm, cloudy days, since even light wind disrupts them.
• Their saliva contains anticoagulant and analgesic compounds that mask the initial bite, which is part of why the itchy reaction that follows feels disproportionate to such a small insect.
• Fine mesh screening or permethrin-treated screens, plus fans on patios and porches, are the practical steps extension sources point to — standard mosquito-focused standing-water removal doesn't address their actual breeding sites.

A yard with no standing water can still have an active no-see-um problem if damp, organically rich soil is going unaddressed nearby. A free Sevacorp inspection can help identify what's actually driving biting-insect activity on a property.

• Sources:
• University of Florida/IFAS — "Biting Midges: Biology and Public Health Risk" (EENY-349/IN626) — https://edis.ifas.ufl.edu/publication/IN626
• University of Arizona Cooperative Extension — "No-See-Ums" — https://extension.arizona.edu/sites/extension.arizona.edu/files/barnesm/No-See-Um.pdf
• University of Arkansas Extension — "Ark... Summer Itch" (June 2026) — https://uaex.uada.edu/media-resources/news/2026/june/06-29-2026-ark-summer-itch.aspx