Permethrin on textiles: how it works, and what the tests measure

Permethrin is a synthetic pyrethroid, modelled on the pyrethrins found in chrysanthemum flowers. It acts on the nervous system of insects, causing paralysis and death. In textile sprays and treated nets it binds to the fabric, so that insects touching it pick up a dose.

Contact, not distance

Permethrin works through contact. A mosquito can land on a treated fabric, and even bite, before the insecticide takes effect. That is a key difference from a repellent, which acts on whether the insect lands at all.

What the standard test measures

Treated netting is evaluated with a cone test. Mosquitoes are kept in contact with a piece of treated net for three minutes; knock-down is recorded after an hour and mortality after a day. The test measures what the insecticide does to insects that are made to stay in contact with it.

That is the right test for a tool designed to reduce mosquito numbers across a community. It says less about whether one person under a net or in treated clothing gets bitten, because in real life a mosquito can touch the fabric and leave before the dose takes effect.

Side effects

Permethrin is very harmful to fish and aquatic invertebrates, and it binds to soil and sediment, from where it can be washed into waterways. It is harmful to bees, and dangerous for cats, which break it down far more slowly than dogs or people do. If you keep cats, read the label of any permethrin product with care.

A different principle

Repeltec works differently.