Apyrethroid is anorganic compound similar to the naturalpyrethrins, which are produced by the flowers ofpyrethrums (Chrysanthemum cinerariaefolium andC. coccineum). Pyrethroids are used as commercial and householdinsecticides.^[1]

In household concentrations pyrethroids are generally harmless to humans.^[1] However, pyrethroids are toxic to insects such asbees,dragonflies,mayflies,gadflies, and some otherinvertebrates, including those that constitute the base of aquatic and terrestrialfood webs.^[2] Pyrethroids are toxic toaquatic organisms, especially fish.^[3] They have been shown to be an effective control measure for malaria outbreaks, through indoor applications.^[4]

Pyrethroids areexcitotoxic toaxons. They act by preventing the closure of thevoltage-gated sodium channels in the axonalmembranes. The sodium channel is amembrane protein with ahydrophilic interior. This interior is shaped precisely to allowsodium ions to pass through the membrane, enter the axon, and propagate anaction potential. When the toxin keeps the channels in their open state, the nerves cannotrepolarize, leaving the axonal membrane permanentlydepolarized, thereby paralyzing the organism.^[5] Pyrethroids can be combined with the synergistpiperonyl butoxide, a knowninhibitor ofmicrosomal P450 enzymes which are important in metabolizing the pyrethroid. By that means, the efficacy (lethality) of the pyrethroid is increased.^[6] It is likely that there are other mechanisms of intoxication also.^[7] Disruption ofneuroendocrine activity is thought to contribute to their irreversible effects on insects, which indicates a pyrethroid action onvoltage-gated calcium channels (and perhaps othervoltage-gated channels more widely).^[7]

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, likechrysanthemic acid,esterified with analcohol. However, thecyclopropyl ring does not occur in all pyrethroids.Fenvalerate, which was developed in 1972, is one such example and was the first commercialized pyrethroid without that group.

Pyrethroids which lack an α-cyano group are often classified astype I pyrethroids and those with it are calledtype II pyrethroids. Pyrethroids that have a common name starting with "cy" have a cyano group and are type II. Fenvalerate also contains an α-cyano group.

Some pyrethroids, likeetofenprox, also lack the ester bond found in most other pyrethroids and have anether bond in its place.Silafluofen is also classified as a pyrethroid and has asilicon atom in the place of the ester. Pyrethroids often havechiral centers and only certain stereoisomers work efficiently asinsecticides.^[8]

Pyrethroids are toxic to insects such asbees,dragonflies,mayflies,gadflies, and some otherinvertebrates, including those that constitute the base of aquatic and terrestrialfood webs.^[2] They are toxic to aquatic organisms including fish.^[3]

Pyrethroids are usuallybroken apart bysunlight and the atmosphere in one or two days, however when associated with sediment they can persist for some time.^{[better source needed][9]}

Pyrethroids are unaffected by conventional secondary treatment systems at municipalwastewater treatment facilities. They appear in the effluent, usually at levels lethal to invertebrates.^{[better source needed][10]}

Pyrethroid absorption can happen via skin, inhalation or ingestion.^[11] Pyrethroids often do not bind efficiently to mammaliansodium 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 without liver problems 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 facialparesthesia, 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 orpulmonary edema.^[11] There is an association of pyrethroids with poorer early social-emotional and language development.^[4]

Pyrethroids are very toxic tocats, but not todogs. Poisoning in cats can result in seizures, fever,ataxia and even death. Poisoning can occur if pyrethroid containingflea treatment products, which are intended for dogs, are used on cats. The livers of cats detoxify pyrethroids viaglucuronidation more poorly than dogs, which is the cause of this difference.^[15] Aside from cats, pyrethroids are typically not toxic tomammals orbirds.^[16] They are often toxic tofish,reptiles andamphibians.^[17]

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.^{[19][20][21][22]} Populations ofdiamondback moths have also commonly developed resistance to pyrethroids^{[23][better source needed]} – including in U.S. statesNorth Dakota^[24] andWisconsin^[25] while pyrethroids are still recommended inCalifornia.^[26] Various mosquito populations have been discovered to have a high level of resistance, includingAnopheles gambiae s.l. inWest Africa by Chandre et al 1999 through Pwalia et al 2019,A. arabiensis inSudan by Ismail et al 2018 andThe Gambia by Opondo et al 2019, andAedes aegypti inSouth East Asia by Amelia-Yap et al 2018,Papua New Guinea by Demok et al 2019, and various other locations by Smith et al 2016.^[18]

Knockdown resistance (kdr) is one of the stronger kinds of resistance.^[27]kdrmutations confertarget-site resistance toDDT and pyrethroids andcross-resistance to DDT.^[27] Mostkdr mutations are within or proximate to the two arthropodsodium channel genes.^[27]

Pyrethroids were introduced by a team ofRothamsted Research scientists in the 1960s and 1970s following the elucidation of the structures of pyrethrin I and II byHermann Staudinger andLeopold Ružička in the 1920s.^[28] The pyrethroids represented a major advancement in the chemistry that would synthesize the analog of the natural version found inpyrethrum. Its insecticidal activity has relatively lowmammaliantoxicity and an unusually fast biodegradation. Their development coincided with the identification of problems withDDT use. Their work consisted firstly of identifying the most active components ofpyrethrum, 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]

Thefirst-generation pyrethroids, developed in the 1960s, includebioallethrin,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 asecond generation of more persistent compounds notably:permethrin,cypermethrin anddeltamethrin. They are substantially more resistant to degradation by light and air, thus making them suitable for use inagriculture, but they have significantly higher mammalian toxicities. Over the subsequent decades these derivatives were followed with other proprietary compounds such asfenvalerate,lambda-cyhalothrin and beta-cyfluthrin. Most patents have now expired, making these compounds cheap and therefore popular (although permethrin and fenvalerate have not been re-registered under the 91/414/EEC process).

• Pyrethroids
• Household chemicals

• Cite Q - cites a work with an erratum
• Webarchive template wayback links
• CS1 maint: DOI inactive as of July 2025
• All articles with dead external links
• Articles with dead external links from December 2024
• Articles with permanently dead external links
• Articles with short description
• Short description is different from Wikidata
• All articles lacking reliable references
• Articles lacking reliable references from October 2024
• Articles lacking reliable references from January 2022