Pyrethroid

A pyrethroid is an

Pyrethroids are classified based on their mechanism of biological action, as they do not share a common chemical structure. Many are 2,2-dimethylcyclopropanecarboxylic acid derivatives, like

Pyrethroids are toxic to insects such as

Pyrethroid absorption can happen via skin, inhalation or ingestion.[11] Pyrethroids often do not bind efficiently to mammalian sodium channels.^[12] They also absorb poorly via skin and human liver is often able to metabolize them relatively efficiently. Pyrethroids are thus much less toxic to humans than to insects.^[13]

It is not well established if chronic exposure to small amounts of pyrethroids is hazardous or not.^[14] However, large doses can cause acute poisoning, which is rarely life threatening. Typical symptoms include facial paresthesia, itching, burning, dizziness, nausea, vomiting and more severe cases of muscle twitching. Severe poisoning is often caused by ingestion of pyrethroids and can result in a variety of symptoms like seizures, coma, bleeding or pulmonary edema.^[11] There is an association of pyrethroids with poorer early social-emotional and language development.^[4]

Pyrethroids are very toxic to

The use of pyrethroids as insecticides has led to the development of widespread resistance to them among some insect populations, especially mosquitoes.[18]

Pyrethroids have been used against bedbugs, but resistant populations have developed to them.

Knockdown resistance (kdr) is one of the stronger kinds of resistance.^[27] kdr mutations confer target-site resistance to DDT and pyrethroids and cross-resistance to DDT.^[27] Most kdr mutations are within or proximate to the two arthropod sodium channel genes.^[27]

Pyrethroids were introduced by a team of Rothamsted Research scientists in the 1960s and 1970s following the elucidation of the structures of pyrethrin I and II by Hermann Staudinger and Leopold Ružička in the 1920s.^[28] The pyrethroids represented a major advancement in the chemistry that would synthesize the analog of the natural version found in pyrethrum. Its insecticidal activity has relatively low mammalian toxicity and an unusually fast biodegradation. Their development coincided with the identification of problems with DDT use. Their work consisted firstly of identifying the most active components of pyrethrum, extracted from East African chrysanthemum flowers and long known to have insecticidal properties. Pyrethrum rapidly knocks down flying insects but has negligible persistence — which is good for the environment but gives poor efficacy when applied in the field. Pyrethroids are essentially chemically stabilized forms of natural pyrethrum and belong to IRAC MoA group 3 (they interfere with sodium transport in insect nerve cells).^[29]

The first-generation pyrethroids, developed in the 1960s, include bioallethrin, tetramethrin, resmethrin, and bioresmethrin. They are more active than the natural pyrethrum but are unstable in sunlight. With the 91/414/EEC review,^[30] many 1st-generation compounds have not been included on Annex 1, probably because the market is not big enough to warrant the costs of re-registration (rather than any special concerns about safety).

By 1974, the Rothamsted team had discovered a second generation of more persistent compounds notably: