A pyrethroid is an organic compound similar to the natural , which are produced by the flowers of ( Chrysanthemum cinerariaefolium and C. coccineum). Pyrethroids are used as commercial and household insecticides.
In household concentrations pyrethroids are generally harmless to humans.[ 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
excitotoxin to
. They act by preventing the closure of the voltage-gated sodium channels in the axonal
. 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
repolarization, leaving the axonal membrane permanently
depolarization, thereby paralyzing the organism.
Pyrethroids can be combined with the synergist piperonyl butoxide, a known
enzyme inhibitor of microsomal P450 enzymes 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,
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 .
Examples
-
Allethrin, the first pyrethroid synthesized
-
Bifenthrin, active ingredient of Talstar, Capture, Ortho Home Defense Max, and Bifenthrine
-
Cyfluthrin, an active ingredient in Baygon, Temprid, Fumakilla Vape Aerosol, Tempo SC, and many more, dichlorovinyl derivative of pyrethrin
-
Cypermethrin, 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
-
Imiprothrin
-
lambda-Cyhalothrin
-
Metofluthrin
-
Permethrin, dichlorovinyl derivative of pyrethrin and most widely used pyrethroid.
-
Phenothrin (Sumithrin), active ingredient of Anvil
-
Prallethrin
-
Resmethrin, active ingredient of Scourge
-
Silafluofen
-
tau-Fluvalinate
-
Tefluthrin
-
Tetramethrin
-
Tralomethrin
-
Transfluthrin, an active ingredient in Baygon and other products.
Safety
Environmental effects
Pyrethroids are toxic to insects such as
,
dragonflies,
mayflies,
Horse-fly, and some other
invertebrates, including those that constitute the base of aquatic and terrestrial
food webs.
[ They are toxic to aquatic organisms including fish.][
]
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.
Humans
Pyrethroid absorption can happen via skin, inhalation or ingestion.
It is not well established if chronic exposure to small amounts of pyrethroids is hazardous or not.
Other organisms
Pyrethroids are very toxic to , but not to . 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.
Resistance
The use of pyrethroids as insecticides has led to the development of widespread resistance to them among some insect populations, especially mosquitoes.
Pyrethroids have been used against bedbugs, but resistant populations have developed to them.[Voiland, Adam. "You May not be Alone" U.S. News & World Report 16 July 2007, Vol. 143, Issue 2, p53–54.]
(''kdr'') is one of the stronger kinds of resistance.
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 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,
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).
