Editing Pyrethroid (section)

==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.<ref name="StaudingerRuzicka1924">{{cite journal |doi=10.1002/hlca.19240070124 |title=Insektentötende Stoffe I. Über Isolierung und Konstitution des wirksamen Teiles des dalmatinischen Insektenpulvers |trans-title=Insecticidal substances I. On isolation and constitution of the active part of the Dalmatian insect powder |journal=[[Helvetica Chimica Acta]]|volume=7 |issue=1 |pages=177–201 |year=1924 |last1=Staudinger |first1=H |last2=Ruzicka |first2=L }}</ref> The pyrethroids represented a major advancement in the chemistry that would synthesize the analog of the natural version found in [[Chrysanthemum#Insecticidal uses|pyrethrum]]. Its insecticidal activity has relatively low [[mammal]]ian [[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).<ref name=resistance>{{cite journal |doi=10.1016/j.ibmb.2012.03.008 |pmid=22504519 |title=Identification of mutations associated with pyrethroid resistance in the voltage-gated sodium channel of the tomato leaf miner (Tuta absoluta) |journal=[[Insect Biochemistry and Molecular Biology]]|volume=42 |issue=7 |pages=506–13 |year=2012 |last1=Haddi |first1=Khalid |last2=Berger |first2=Madeleine |last3=Bielza |first3=Pablo |last4=Cifuentes |first4=Dina |last5=Field |first5=Linda M |last6=Gorman |first6=Kevin |last7=Rapisarda |first7=Carmelo |last8=Williamson |first8=Martin S |last9=Bass |first9=Chris |doi-access=free |bibcode=2012IBMB...42..506H |url=https://repository.rothamsted.ac.uk/download/d740e025fe45d0f2cca5d0f582176c8641c90afcf8dc497d6e7ed7fb7407c889/4615152/1-s2.0-S0965174812000471-main.pdf }}</ref>

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,<ref>{{cite web|url=http://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:31991L0414|title=EUR-Lex - 31991L0414 - EN - EUR-Lex|work=europa.eu|date=15 July 1991 }}</ref> 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).