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A pyrethroid is a synthetic chemical compound similar to the natural chemical pyrethrins produced by the flowers of pyrethrums (Chrysanthemum cinerariaefolium and C. coccineum). Pyrethroids now constitute a major proportion of the synthetic insecticide market and are common in commercial products such as household insecticides. In the concentrations used in such products, they may also have insect repellent properties and are generally harmless to human beings but can harm sensitive individuals. They are usually broken apart by sunlight and the atmosphere in one or two days, and do not significantly affect groundwater quality except for being toxic to fish.

Pyrethroids are axonic poisons that work by keeping the sodium channels open in the neuronal membranes of insects. The sodium channel consists of a membrane protein with a hydrophilic interior; this interior is effectively a tiny hole which is shaped exactly right to strip away the partially charged water molecules from a sodium ion and create a thermodynamically favorable way for 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 de-excite, so the insect is paralyzed.

Pyrethroids are usually combined with piperonyl butoxide, a known inhibitor of key microsomal oxidase enzymes. This prevents these enzymes from clearing the pyrethroid from the body of the insect, and assures the pyrethroid will be lethal and not merely a paralyzing agent. Combined, pyrethroids are toxic to most beneficial insects such as bees and dragonflies.

The pyrethroid chrysanthemic acid is produced industrially in a cyclopropanation reaction of a diene as a mixture of cis- and trans isomers followed by hydrolysis of the ester :

chrysanthemic ester synthesis

The compound is the starting material for many derivatives by re-esterfication.


Pyrethroids were introduced thirty years ago by a team of Rothamsted Research scientists led by M. Elliott, and represented a major advancement in activity and relatively-low mammalian toxicity. Their development was especially timely 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 a low mammalian toxicity and 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 1st 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. Activity of pyrethrum and 1st generation pyrethroids is often enhanced by addition of the synergist piperonyl butoxide (which is not itself biologically active). With the 91/414/EEC review, many 1st generation compounds have not been included on Annex 1, probably because the market is simply 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 2nd 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 were followed with other proprietary compounds such as fenvalerate, lambda-cyhalothrin and beta-cyfluthrin, but most patents have now expired, making them cheap and therefore popular (although permethrin and fenvalerate have not been re-registered under the 91/414/EEC process). One of the less desirable characteristics, especially of 2nd generation pyrethroids is that they can be irritant to the skin and eyes, so special formulations such as capsule suspensions (CS) have been developed.

Commercial Pyrethroid Insecticides/Repellants

Possible link to Autism

A recent study (2008), led by lead author Irva Hertz-Picciotto, professor in the Department of Public Health Sciences at University of California, Davis, suggested a correlation between pyrethrins and autism. In the study of 500 children (with or without autism), the 138 children with autism and their mothers were twice as likely to "report using pet shampoos and other household products containing pyrenthrins than other mothers". However, no causal link was established and further studies were called for.


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