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A pyrethroid is an
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
similar to the natural pyrethrins, which are produced by the flowers of pyrethrums ('' Chrysanthemum cinerariaefolium'' and '' C. coccineum''). Pyrethroids are used as commercial and household insecticides. In household concentrations pyrethroids are generally harmless to humans. However, pyrethroids are toxic to insects such as
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfami ...
s, dragonflies, mayflies, gadflies, and some other invertebrates, including those that constitute the base of aquatic and terrestrial
food webs A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one ...
. Pyrethroids are toxic to
aquatic organism An aquatic animal is any animal, whether invertebrate or vertebrate, that lives in water for most or all of its lifetime. Many insects such as mosquitoes, mayflies, dragonflies and caddisflies have aquatic larvae, with winged adults. Aquatic anim ...
s, especially fish.Pyrethroids fact sheet
from the Illinois Department of Public Health.
They have been shown to be an effective control measure for malaria outbreaks, through indoor applications.


Mode of action

Pyrethroids are axonic excitotoxins, the toxic effects of which are mediated through preventing the closure of the voltage-gated sodium channels in the axonal membranes. The sodium channel is a membrane protein with a hydrophilic interior. This interior is shaped precisely to allow
sodium ions Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable isot ...
to pass through the membrane, enter the axon, and propagate an action potential. When the toxin keeps the channels in their open state, the nerves cannot repolarize, leaving the axonal membrane permanently depolarized, thereby paralyzing the organism. Pyrethroids can be combined with the synergist piperonyl butoxide, a known
inhibitor Inhibitor or inhibition may refer to: In biology * Enzyme inhibitor, a substance that binds to an enzyme and decreases the enzyme's activity * Reuptake inhibitor, a substance that increases neurotransmission by blocking the reuptake of a neurotra ...
of microsomal cytochrome P450
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s which are important in metabolizing the pyrethroid. By that means, the efficacy (lethality) of the pyrethroid is increased. It is likely that there are other mechanisms of intoxication also. Disruption of neuroendocrine activity is thought to contribute to their irreversible effects on insects, which indicates a pyrethroid action on voltage-gated calcium channels (and perhaps other voltage-gated channels more widely).


Chemistry and classification

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 chrysanthemic acid, esterified with an alcohol. However, the cyclopropyl 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 as ''type I pyrethroids'' and those with it are called ''type 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 Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...
. Some pyrethroids, like etofenprox, also lack the ester bond found in most other pyrethroids and have an ether bond in its place. Silafluofen is also classified as a pyrethroid and has a silicon atom in the place of the ester. Pyrethroids often have chiral centers and only certain stereoisomers work efficiently as insecticides.


Examples

* Allethrin, the first pyrethroid synthesized * Bifenthrin, active ingredient of ''Talstar'', ''Capture'', ''Ortho Home Defense Max'', and ''Bifenthrine'' *
Cyfluthrin Cyfluthrin is a pyrethroid insecticide and common household pesticide. It is a complex organic compound and the commercial product is sold as a mixture of isomers. Like most pyrethroids (MoA 3a), it is highly toxic to fish and invertebrates, but ...
, an active ingredient in
Baygon Baygon is a pesticide brand produced by S. C. Johnson & Son. It is an insecticide used for extermination and control of household pests such as crickets, roaches, ants, carpenter ants, spiders, silverfish and mosquitoes. In 1975, Baygon intro ...
, Temprid, Fumakilla Vape Aerosol, Tempo SC, and many more, dichlorovinyl derivative of pyrethrin *
Cypermethrin Cypermethrin (CP) is a synthetic pyrethroid used as an insecticide in large-scale commercial agricultural applications as well as in consumer products for domestic purposes. It behaves as a fast-acting neurotoxin in insects. It is easily degrade ...
, including the resolved isomer alpha-cypermethrin, dichlorovinyl derivative of pyrethrin. Commonly found in crawling insect killers and some mosquito sprays. * Cyphenothrin, active ingredient of K2000 Insect spray sold in Israel. Mostly used in some aerosols as a Cypermethrin substitute in developing countries. * Deltamethrin, dibromovinyl derivative of pyrethrin * Dimefluthrin * Esfenvalerate * Etofenprox * Fenpropathrin * Fenvalerate * Flucythrinate *
Flumethrin Flumethrin is a pyrethroid insecticide. It is used externally in veterinary medicine against parasitic insects and ticks on cattle, sheep, goats, horses, and dogs, and the treatment of parasitic mites in honeybee colonies. Chemistry Flumethrin ...
*
Imiprothrin Imiprothrin is a synthetic pyrethroid insecticide. It is an ingredient in some commercial and consumer insecticide products for indoor use. It has low acute toxicity to humans through the inhalation and dermal routes, but to insects it acts as a n ...
* lambda-Cyhalothrin *
Metofluthrin Metofluthrin is a pyrethroid used as an insect repellent. The vapors of metofluthrin are highly effective and capable of repelling up to 97% of mosquitoes in field tests. Metofluthrin is used in a variety of consumer products, called emanator ...
* Permethrin, dichlorovinyl derivative of pyrethrin and most widely used pyrethroid. * Phenothrin (Sumithrin), active ingredient of Anvil *
Prallethrin Prallethrin is a pyrethroid insecticide. Prallethrin 1.6% w/w liquid vaporizer is a repellent insecticide which is generally used for the control of mosquitoes in the household. Products It is marketed as a mosquito repellent by Godrej as "Goo ...
* Resmethrin, active ingredient of ''Scourge'' * Silafluofen * tau-Fluvalinate * Tefluthrin *
Tetramethrin Tetramethrin is a potent synthetic insecticide in the pyrethroid family. It is a white crystalline solid with a melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from ...
* Tralomethrin * Transfluthrin, an active ingredient in Baygon and other products.


Environmental effects

Pyrethroids are toxic to insects such as
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfami ...
s, dragonflies, mayflies, gadflies, and some other invertebrates, including those that constitute the base of aquatic and terrestrial
food webs A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one ...
. They are toxic to aquatic organisms including fish.


Biodegradation

Pyrethroids are usually broken apart by sunlight and the atmosphere in one or two days, however when associated with sediment they can persist for some time. Pyrethroids are unaffected by conventional secondary treatment systems at municipal wastewater treatment facilities. They appear in the effluent, usually at levels lethal to invertebrates.


Safety


Humans

Pyrethroid absorption can happen via skin, inhalation or ingestion. Pyrethroids often do not bind efficiently to mammalian sodium channels. 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. It is not well established if chronic exposure to small amounts of pyrethroids is hazardous or not. 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. There is an association of pyrethroids with poorer early social-emotional and language development.


Other organisms

Pyrethroids are very toxic to cats, but not to dogs. Poisoning in cats can result in seizures, fever, ataxia and even death. Poisoning can occur if pyrethroid containing flea treatment products, which are intended for dogs, are used on cats. The livers of cats detoxify pyrethroids via glucuronidation more poorly than dogs, which is the cause of this difference. Aside from cats, pyrethroids are typically not toxic to mammals or birds. They are often toxic to fish, reptiles and amphibians.


Resistance

The use of pyrethroids as insecticides has led to the development of widespread resistance to them among some insect populations, especially mosquitoes. . Although
bedbug Bed bugs are insects from the genus ''Cimex'' that feed on blood, usually at night. Their bites can result in a number of health impacts including skin rashes, psychological effects, and allergic symptoms. Bed bug bites may lead to skin changes ...
s were almost eradicated in North America through the use of DDT and organophosphates, populations of bedbugs resistant to both have developed. The use of DDT for this purpose was banned, and its reintroduction would not offer a solution to the problem of bedbugs, due to resistance. Pyrethroids became more commonly used against bedbugs, but resistant populations have now developed to them as well.Voiland, Adam.
"You May not be Alone"
U.S. News & World Report 16 July 2007, Vol. 143, Issue 2, p53–54.
Populations of Diamondback moths have also commonly developed resistance to pyrethroids including in U.S. states North Dakota and Wisconsin while pyrethroids are still recommended in
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
. Various mosquito populations have been discovered to have a high level of resistance, including '' Anopheles gambiae s.l.'' in West Africa by Chandre et al 1999 through Pwalia et al 2019, '' A. arabiensis'' in Sudan by Ismail et al 2018 and The Gambia by Opondo et al 2019, and ''
Aedes aegypti ''Aedes aegypti'', the yellow fever mosquito, is a mosquito that can spread dengue fever, chikungunya, Zika fever, Mayaro and yellow fever viruses, and other disease agents. The mosquito can be recognized by black and white markings on its l ...
'' in South East Asia by Amelia-Yap et al 2018,
Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...
by Demok et al 2019, and various other locations by Smith et al 2016.


History

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. 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). The ''first-generation pyrethroids'', developed in the 1960s, include bioallethrin,
tetramethrin Tetramethrin is a potent synthetic insecticide in the pyrethroid family. It is a white crystalline solid with a melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from ...
, resmethrin, and bioresmethrin. They are more active than the natural pyrethrum but are unstable in sunlight. With the 91/414/EEC review, 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: permethrin,
cypermethrin Cypermethrin (CP) is a synthetic pyrethroid used as an insecticide in large-scale commercial agricultural applications as well as in consumer products for domestic purposes. It behaves as a fast-acting neurotoxin in insects. It is easily degrade ...
and deltamethrin. They are substantially more resistant to degradation by light and air, thus making them suitable for use in
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
, but they have significantly higher mammalian toxicities. Over the subsequent decades these derivatives were followed with other proprietary compounds such as fenvalerate, lambda-cyhalothrin and beta-
cyfluthrin Cyfluthrin is a pyrethroid insecticide and common household pesticide. It is a complex organic compound and the commercial product is sold as a mixture of isomers. Like most pyrethroids (MoA 3a), it is highly toxic to fish and invertebrates, but ...
. 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).


References

{{Insecticides Pyrethroids Household chemicals