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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 in low doses but can harm sensitive individuals.[1] They are usually broken apart by sunlight and the atmosphere in one or two days, and do not significantly affect groundwater quality.[2]

Pyrethroids are toxic to fish and other aquatic organisms. At extremely small levels, such as 2 parts per trillion,[3] pyrethroids are lethal to mayflies, gadflies, and invertebrates that constitute the base of many aquatic and terrestrial food webs.[4]

Pyrethroids have been found at acutely toxic levels in sediments and waterways in California; the chemical is able to pass through secondary treatment systems at municipal wastewater treatment facilities causing the chemical to be commonly found in the final effluent, usually at levels lethal to invertebrates.[5]

Pyrethroids are axonic poisons and cause paralysis of an organism. The chemical causes paralysis by keeping the sodium channels open in the neuronal membranes of an organism. 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 organism 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 [6]:

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.[9]

Pollution of Streams and Rivers

A study by Donald P. Weston and Michael J. Lydy have shown pyrethroids in Californian waterways at levels 10 times the lethal dose for aquatic invertebrates such as Hyalella azteca. Their study found that residential water run-off had much more pyrethroid contamination than agricultural run-off. The study also found that pyrethroids are not removed during secondary waste treatment at municipal water treatment plants, often finding pyrethroids in the final effluent.[5]

Bed Bugs impervious to Pyrethroids

Bed bugs have been a problem for humans since biblical times. Up until the 1950's, they were almost completely wiped out due to the use of DDT. After DDT was banned, pyrethroids became more commonly used against bed bugs; as a result, as of 2010 a super strain of bedbugs has evolved nerve cell mutations impervious to pyrethroids. This has caused an international bed bug epidemic due to ineffective treatment methods.[10]


  1. ^ Pyrethroids fact sheet from the Illinois Department of Public Health.
  2. ^ "Permethrin, Resmethrin, Sumithrin: Synthetic Pyrethroids For Mosquito Control". United States Environmental Protection Agency. April 17, 2002. Retrieved 2008-04-04. 
  3. ^ Bhanoo, Sindya (2/4/2010). "Household Pesticide Is Finding Its Way Into California Rivers, Study Suggests" (in english). Green Inc. Energy, Environment, and the Bottom Line. New York Times. Retrieved 5 February 2010. 
  4. ^ Zaveri, Mihir (Thursday, February 4, 2010). "Study Links Pesticides to River Contamination" (in English). The Daily Californian (The Daily Californian). Retrieved 5 February 2010. 
  5. ^ a b Weston, Donald P.; Michael J. Lydy (February 2, 2010). "Urban and Agricultural Sources of Pyrethroid Insecticides to the Sacramento-San Joaquin Delta of California". Environmental Science and Technology (Department of Integrative Biology, University of California, Berkeley: American Chemical Society). Retrieved February 4, 2010. 
  6. ^ A synthesis of chrysanthemic ester: An undergraduate experiment. Kelly, Lawrence F. J. Chem. Educ. 1987, 64, 1061.
  7. ^
  8. ^ prallethrin data sheet
  9. ^
  10. ^ Cappiello, Dina (14 April 2009). "Bedbugs on the Rebound, Feds Take Note". Discovery News. 

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