100 vs 156 vs 200 HPI: Which Mesh Count Fails the Cone Bioassay

The cone bioassay is the test that settles arguments. It does not care what the spec sheet says, what the denier is, or how the fabric looks. It puts susceptible mosquitoes in contact with a treated net sample for a fixed time and counts how many are knocked down and how many die. A 100 HPI net fails it. A 156 HPI net passes. A 200 HPI net passes with room to spare — and costs you airflow for margin you may not need.

What the cone bioassay measures

The test exists because insecticidal netting works through contact. The chemistry is bonded into the yarn or coated onto it, and a mosquito that lands on the surface picks up a dose through its tarsi. If the mosquito never lands, or lands briefly and leaves, the dose is too small to matter.

Geometry therefore sits upstream of chemistry. An aperture that lets the insect through, or lets it avoid the surface, defeats a perfectly dosed net. That is what the cone test detects: not whether the fabric contains insecticide, but whether the fabric delivers it.

This is the reason the test is specified in tenders rather than left to factory self-declaration. A laboratory can confirm the dosage on a sample and still miss a weave that prevents contact. The bioassay closes that gap.

The test in brief

The standard arrangement uses clear cones with the net sample covering the base. A small number of susceptible, non-blood-fed female mosquitoes — typically a few days old, from a reference colony — are introduced into each cone and confined against the fabric for a short, fixed exposure.

Two endpoints are recorded: knock-down, observed shortly after exposure, and mortality, scored after a holding period of around 24 hours. The acceptance thresholds in common use sit in the region of 80 percent mortality, or 95 percent knock-down plus mortality combined. Always confirm the exact figures against the protocol version your buyer cites, because the wording has been revised over time.

What matters for a sourcing conversation is the shape of the test, not the decimal places: short contact, live insects, functional outcome. A fabric can pass on dosage and fail on geometry in the same run.

100 HPI: where it fails

A 100 HPI weave is coarse. The openings are large enough that Aedes aegypti — a relatively small, agile species — can pass through the mesh or move around inside the cone without settling on the treated surface long enough to take up a lethal dose.

Mortality then lands below the threshold, and the net fails. The insecticide content can be exactly as specified. The denier can be exactly as specified. The failure is purely geometric, and no amount of chemistry fixes it.

This is the trap that catches buyers who specify by denier alone. A “150D” net built on a 100 HPI weave is an internally consistent product that cannot pass a tender. The yarn is right, the weave is wrong, and the label does not distinguish between them.

For untreated netting there is no bioassay, and the equivalent failure is simpler: 100 HPI does not reliably stop small mosquito species. Coarse tourist netting sold as mosquito protection is usually in this range, and its reputation problem in the market is deserved.

156 HPI: the pass line

156 holes per square inch is where the geometry and the chemistry meet. The openings are small enough that the target species cannot pass through and tend to settle on the surface, which delivers the dose the product was designed around. Mortality clears the threshold, and the net behaves as a barrier rather than a filter with gaps.

It is a pass line, not a comfortable one. The margin between 156 and failure is real but not generous, which is why weave consistency matters as much as the nominal count. A fabric that averages 156 but contains patches at 130 — from a worn reed, uneven tension or a missed yarn — can fail on the strength of its worst section rather than its average.

Two quality controls protect that margin:

• Reed and tension control during weaving. Consistent warp spacing is what keeps the aperture uniform across the roll.
• Aperture measurement at several points on the roll. Not one reading from the centre. Edge-to-edge variation is where the failures hide.

200 HPI: over-spec and its cost

At 200 HPI the protection margin is comfortable. The aperture is finer, the insect cannot pass, and contact is more likely because the surface presents more yarn per unit area. For untreated netting, 200 HPI also gives real protection against sandflies, which 156 does not reliably do.

The costs are concrete and predictable:

The decision rule is market-driven, not aspirational. If your customers camp in mosquito country, 156 with tight weave control is the efficient choice. If they camp where sandflies or midges are the problem, going finer is not over-spec — it is the correct spec, and the airflow penalty has to be solved structurally.

What you should not do is specify 200 HPI as a generic upgrade. It raises fabric cost, cuts airflow, and does nothing for the insect groups that 156 already stops. Over-specification is a way of paying for margin you will never use.

Reading a bioassay report

When a supplier sends a bioassay report, four fields decide whether it means anything:

• Species and strain. Aedes aegypti and Anopheles gambiae behave differently on the same fabric. A report against a susceptible laboratory strain is not a field performance guarantee, but it is the standard reference.
• Exposure time and holding period. Short exposures and 24-hour holding are the usual pattern; deviations need an explanation.
• Number of replicates and cones. A single cone is an anecdote. The protocol calls for multiple replicates, and the report should show them individually, not only as an average.
• Wash state. A net tested once at zero washes tells you about the factory, not about the product after a season of use. Ask for results after the wash cycles your label claims.

If the report shows only a pass mark and none of those four, it is a marketing document. Ask for the underlying sheet.