Pyrethroid

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Chemical structure of Allethrin isomers Allethrin 2D.svg
Chemical structure of Allethrin isomers
Chemical structure of Permethrin isomers Permethrin-2D-skeletal.png
Chemical structure of Permethrin isomers

A pyrethroid is an organic compound 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. [1]

Contents

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

Mode of action

Pyrethroids are excitotoxic to axons. They act by 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 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. [5] Pyrethroids can be combined with the synergist piperonyl butoxide, a known inhibitor of microsomal 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 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). [7]

Chemistry and classification

(1R,3R)- or (+)-trans-chrysanthemic acid. (R,R)-(+)-trans-chrysanthemic acid.svg
(1R,3R)- or (+)-trans-chrysanthemic acid.

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.

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. [8]

Examples

Environmental effects

Pyrethroids are toxic to insects such as bees, dragonflies, mayflies, gadflies, and some other invertebrates, including those that constitute the base of aquatic and terrestrial food webs. [2] They are toxic to aquatic organisms including fish. [3]

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. [9]

Pyrethroids are unaffected by conventional secondary treatment systems at municipal wastewater treatment facilities. They appear in the effluent, usually at levels lethal to invertebrates. [10]

Safety

Humans

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]

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. [15] Aside from cats, pyrethroids are typically not toxic to mammals or birds. [16] They are often toxic to fish, reptiles and amphibians. [17]

Resistance

The use of pyrethroids as insecticides has led to the development of widespread resistance to them among some insect populations, especially mosquitoes. [18] Although bedbugs 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. [19] Pyrethroids became more commonly used against bedbugs, but resistant populations have now developed to them as well. [20] [21] [22] [23] Populations of Diamondback moths have also commonly developed resistance to pyrethroids [24] [ better source needed ]– including in U.S. states North Dakota [25] and Wisconsin [26] while pyrethroids are still recommended in California. [27] 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 in South 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. [28] kdr mutations confer target-site resistance to DDT and pyrethroids and cross-resistance to DDT. [28] Most kdr mutations are within or proximate to the two arthropod sodium channel genes. [28]

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. [29] 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). [30]

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, [31] 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 and deltamethrin. They are substantially more resistant to degradation by light and air, thus making them suitable for use in agriculture, 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. 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).

Related Research Articles

<span class="mw-page-title-main">DDT</span> Organochloride known for its insecticidal properties

Dichlorodiphenyltrichloroethane, commonly known as DDT, is a colorless, tasteless, and almost odorless crystalline chemical compound, an organochloride. Originally developed as an insecticide, it became infamous for its environmental impacts. DDT was first synthesized in 1874 by the Austrian chemist Othmar Zeidler. DDT's insecticidal action was discovered by the Swiss chemist Paul Hermann Müller in 1939. DDT was used in the second half of World War II to limit the spread of the insect-borne diseases malaria and typhus among civilians and troops. Müller was awarded the Nobel Prize in Physiology or Medicine in 1948 "for his discovery of the high efficiency of DDT as a contact poison against several arthropods". The WHO's anti-malaria campaign of the 1950s and 1960s relied heavily on DDT and the results were promising, though there was a resurgence in developing countries afterwards.

<span class="mw-page-title-main">Insecticide</span> Pesticide used against insects

Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. Insecticides are used in agriculture, medicine, industry and by consumers. Insecticides are claimed to be a major factor behind the increase in the 20th-century's agricultural productivity. Nearly all insecticides have the potential to significantly alter ecosystems; many are toxic to humans and/or animals; some become concentrated as they spread along the food chain.

<span class="mw-page-title-main">Cypermethrin</span> Chemical compound

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 degraded on soil and plants but can be effective for weeks when applied to indoor inert surfaces. Exposure to sunlight, water and oxygen will accelerate its decomposition. Cypermethrin is highly toxic to fish, bees and aquatic insects, according to the National Pesticides Telecommunications Network (NPTN). It is found in many household ant and cockroach killers, including Raid, Ortho, Combat, ant chalk, and some products of Baygon in Southeast Asia.

Pyrethrum was a genus of several Old World plants now classified as Chrysanthemum or Tanacetum which are cultivated as ornamentals for their showy flower heads. Pyrethrum continues to be used as a common name for plants formerly included in the genus Pyrethrum. Pyrethrum is also the name of a natural insecticide made from the dried flower heads of Chrysanthemum cinerariifolium and Chrysanthemum coccineum. The insecticidal compounds present in these species are pyrethrins.

<span class="mw-page-title-main">Piperonyl butoxide</span> Chemical compound

Piperonyl butoxide (PBO) is a pale yellow to light brown liquid organic compound used as a synergist component of pesticide formulations. That is, despite having no pesticidal activity of its own, it enhances the potency of certain pesticides such as carbamates, pyrethrins, pyrethroids, and rotenone. It is a semisynthetic derivative of safrole.

<span class="mw-page-title-main">Pyrethrin</span> Class of organic chemical compounds with insecticidal properties

The pyrethrins are a class of organic compounds normally derived from Chrysanthemum cinerariifolium that have potent insecticidal activity by targeting the nervous systems of insects. Pyrethrin naturally occurs in chrysanthemum flowers and is often considered an organic insecticide when it is not combined with piperonyl butoxide or other synthetic adjuvants. Their insecticidal and insect-repellent properties have been known and used for thousands of years.

<span class="mw-page-title-main">Bifenthrin</span> Chemical compound

Bifenthrin is a pyrethroid insecticide. It is widely used against ant infestations.

<span class="mw-page-title-main">Permethrin</span> Medication and insecticide

Permethrin is a medication and an insecticide. As a medication, it is used to treat scabies and lice. It is applied to the skin as a cream or lotion. As an insecticide, it can be sprayed onto outer clothing or mosquito nets to kill the insects that touch them.

Persian powder is an insecticide powder. It is also known as Persian pellitory and insect powder.

<span class="mw-page-title-main">Allethrins</span> Class of synthetic chemicals used as insecticides

The allethrins are a group of related synthetic compounds used in insecticides. They are classified as pyrethroids, i.e. synthetic versions of pyrethrin, a chemical with insecticidal properties found naturally in Chrysanthemum flowers. They were first synthesized in the United States by Milton S. Schechter in 1949. Allethrin was the first pyrethroid.

<span class="mw-page-title-main">Deltamethrin</span> Chemical compound

Deltamethrin is a pyrethroid ester insecticide. Deltamethrin plays a key role in controlling malaria vectors, and is used in the manufacture of long-lasting insecticidal mosquito nets; however, resistance of mosquitos and bed bugs to deltamethrin has seen a widespread increase.

<span class="mw-page-title-main">Phenothrin</span> Chemical compound

Phenothrin, also called sumithrin and d-phenothrin, is a synthetic pyrethroid that kills adult fleas and ticks. It has also been used to kill head lice in humans. d-Phenothrin is used as a component of aerosol insecticides for domestic use. It is often used with methoprene, an insect growth regulator that interrupts the insect's biological lifecycle by killing the eggs.

<span class="mw-page-title-main">Cyhalothrin</span> Synthetic pyrethroid used as insecticide

Cyhalothrin is the ISO common name for an organic compound that, in specific isomeric forms, is used as a pesticide. It is a pyrethroid, a class of synthetic insecticides that mimic the structure and properties of the naturally occurring insecticide pyrethrin which is present in the flowers of Chrysanthemum cinerariifolium. Pyrethroids such as cyhalothrin are often preferred as an active ingredient in agricultural insecticides because they are more cost-effective and longer acting than natural pyrethrins. λ-and γ-cyhalothrin are now used to control insects and spider mites in crops including cotton, cereals, potatoes and vegetables.

A fogger is any device that creates a fog, typically containing an insecticide for killing insects and other arthropods. Foggers are often used by consumers as a low cost alternative to professional pest control services. The number of foggers needed for pest control depends on the size of the space to be treated, as stated for safety reasons on the instructions supplied with the devices. The fog may contain flammable gases, leading to a danger of explosion if a fogger is used in a building with a pilot light or other naked flame.

<span class="mw-page-title-main">Bed bug control techniques</span>

Bed bugs, or Cimicidae, are small parasitic insects. The term usually refers to species that prefer to feed on human blood.

<span class="mw-page-title-main">Tefluthrin</span> Synthetic pyrethroid used as insecticide

Tefluthrin is the ISO common name for an organic compound that is used as a pesticide. It is a pyrethroid, a class of synthetic insecticides that mimic the structure and properties of the naturally occurring insecticide pyrethrin which is present in the flowers of Chrysanthemum cinerariifolium. Pyrethroids such as tefluthrin are often preferred as active ingredients in agricultural insecticides because they are more cost-effective and longer acting than natural pyrethrins. It is effective against soil pests because it can move as a vapour without irreversibly binding to soil particles: in this respect it differs from most other pyrethroids.

<span class="mw-page-title-main">Antifeedant</span>

Antifeedants are organic compounds produced by plants to repel herbivores through distaste or toxicity. These chemical compounds are typically classified as secondary metabolites in that they are not essential for the metabolism of the plant, but instead confer longevity. Antifeedants exhibit a wide range of activities and chemical structures as biopesticides. Examples include rosin, which inhibits attack on trees, and many alkaloids, which are highly toxic to specific insect species, such as quassinoids against the diamondback moth. Samadera indica also has quassinoids used for insect antifeedant uses.

<span class="mw-page-title-main">Fenpropathrin</span> Chemical compound

Fenpropathrin, or fenopropathrin, is a widely used pyrethroid insecticide in agriculture and household. Fenpropathrin is an ingestion and contact synthetic pyrethroid. Its mode of action is similar to other natural (pyrethrum) and synthetic pyrethroids where in they interfere with the kinetics of voltage gated sodium channels causing paralysis and death of the pest. Fenpropathrin was the first of the light-stable synthetic pyrethroids to be synthesized in 1971, but it was not commercialized until 1980. Like other pyrethroids with an α-cyano group, fenpropathrin also belongs to the termed type II pyrethroids. Type II pyrethroids are a more potent toxicant than type I in depolarizing insect nerves. Application rates of fenpropathrin in agriculture according to US environmental protection agency (EPA) varies by crop but is not to exceed 0.4 lb ai/acre.

<span class="mw-page-title-main">Michael Elliott (chemist)</span>

Michael Elliott, was a chemist and Lawes Trust Senior Fellow at Rothamsted Experimental Station who invented and commercialised the development of novel insecticides known as pyrethroids.

References

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