What Mesh Size Stops No-See-Ums and Biting Midges?

There is a specific failure that repeats every season in midge country: a camper buys a net tent rated for mosquitoes, sleeps badly, and concludes the product is defective. It is not defective. It was built to a mosquito specification, and a biting midge is not a mosquito. The aperture that stops one does not stop the other, and the gap between them is roughly a factor of two.

Why midges walk through mosquito mesh

Mosquito netting is engineered around a body-girth threshold: about 1.2 mm is the working target for the species that carry malaria, dengue and yellow fever, and 156 holes per square inch is the reference density that delivers it. A biting midge — the family Ceratopogonidae, called no-see-ums in North America, sandflies in some regions, and simply “midges” across northern Europe — sits well below that threshold.

The practical result is not that midges occasionally find a gap. It is that a 1.2 mm weave is open to them, the way a 10 mm gap in a fence is open to a cat. They pass through, they feed, and the net provides no protection at all in the hours around dusk and dawn when midge activity peaks.

This is why the phrase “mosquito net” is a claim with a boundary. Buyers who sell into midge regions and use mosquito-grade language are making a claim their product cannot honour, and the returns follow.

Size, not species

The instinctive fix — buy a net rated for “smaller insects” — does not work, because the relevant number is not body length. A midge is 1–3 mm long, which sounds comparable to a mosquito. What matters is the cross-section it has to squeeze through, and that is a fraction of a millimetre.

Insects also do not cooperate with the geometry. They probe, flatten, and work at gaps with mouthparts and legs before the body follows. A net that is marginal on paper is unreliable in practice, which is why the working midge specification carries a margin rather than sitting exactly on the measured body width.

The useful way to think about it: you are not selecting a net that excludes a species. You are selecting an aperture with enough margin that an insect under pressure cannot find a way through.

The 0.6 mm threshold and the 500+ grade

Midge-grade netting is normally specified at around 0.6 mm clear opening, and sold under names like “no-see-um mesh”, “midge-proof” or “marine grade”. In mesh-count terms it is usually quoted as 500+ holes per square inch, which is roughly three times the density of mosquito-grade fabric.

Three practical notes on that grade:

• The names are not standardised. “Midge-proof” has no formal definition. Ask for the aperture in millimetres, not for the marketing term.
• Fineness changes the fabric, not just the hole. At 0.6 mm the open area drops sharply, which means less airflow, more weight, and reduced visibility through the mesh. Those are real costs, not side notes.
• Quality control gets harder. Finer yarns are less tolerant of weaving faults. A single missed yarn in a 0.6 mm weave produces a gap several times the intended aperture, and one gap is enough to ruin the product's claim.

Sandflies: the middle case

Sandflies are the case most often missed in product planning. They are smaller than mosquitoes and larger than midges, and the practical consequence is that a coarse mosquito weave does not stop them. A 1.8 mm opening — the kind of fabric that results when mesh count and yarn diameter are specified independently — loses reliable sandfly protection entirely.

For markets in the Mediterranean, the Middle East, parts of South America and riverine Africa, a 1.0–1.2 mm aperture is the sensible target. It overlaps the mosquito grade, which is convenient: one fabric can serve both, as long as you specify the aperture rather than a mesh count that happens to land in the right place.

Midges sit below sandflies again, and there the overlap ends. No single fabric covers mosquitoes and midges without becoming a midge-grade product, and a midge-grade product has airflow characteristics that make it a poor choice for hot, humid markets where midges are not the problem.

What finer mesh costs you

This is the trade-off that decides whether midge-grade netting belongs in your range at all. At a fixed yarn specification, halving the aperture cuts the open area dramatically, and air permeability falls with it.

Below roughly 800 L/m²/s, a sleeping net starts to feel like a plastic bag in warm, humid conditions. Carbon dioxide accumulates, humidity rises, and the occupant opens the net — at which point the protection is gone regardless of the aperture.

Four design responses make midge-grade netting usable rather than merely correct:

• More internal volume. A midge-grade bivvy needs to hold the mesh away from the face and body. Contact defeats any aperture.
• Ventilation through structure, not through mesh. Mesh doors on two sides, a vented ridge, a raised foot end. Move air where the weave cannot.
• Lighter yarn. Finer yarn at a fine aperture keeps weight and open area as high as the geometry allows.
• Honest labelling. State the aperture and the insect group on the packaging. A customer who knows they bought midge-grade netting will judge it as midge-grade netting.

How to specify and test midge-grade netting

Write the order in three lines: aperture, weight, permeability. Then verify each one on delivery.

• Aperture: “0.6 mm ±0.05 mm clear opening, measured on finished fabric.” Not “500 mesh” — the count convention is ambiguous and the millimetres are not.
• Weight: midge-grade fabric usually runs lighter in yarn and higher in density; state the gsm target and the tolerance.
• Permeability: set a floor, and expect it to be lower than mosquito-grade. If you cannot reach a comfortable figure, the answer is structural ventilation, not a looser weave.
• Weaving faults: specify an inspection standard for missed yarns. In a 0.6 mm weave, one defect is a hole several times the target aperture, and it will be found by a customer before it is found by your inspector unless you look for it.

Under 10× magnification, a 0.6 mm weave gives roughly 25 openings per linear inch. If you count closer to 20, you are looking at a sandfly-grade fabric being sold as midge-grade, and the difference will show up as bites in the first week of the season.