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Pyrethrin I, R = CH3
Pyrethrin II, R = CO2CH3

The pyrethrins are a pair of natural organic compounds that have potent insecticidal activity. Pyrethrin I and pyrethrin II are structurally related esters with a cyclopropane core.[1][2] They differ by the oxidation state of one carbon. They are viscous liquids that oxidize to become inactivated. They are non-persistent, being biodegradable, and break down on exposure to light or oxygen. The chemical structure of pyrethrins is the basis for a variety of synthetic insecticides called pyrethroids such as bifenthrin, permethrin and cypermethrin.

The pyrethrins are contained in the seed cases of the perennial plant pyrethrum (Chrysanthemum cinerariaefolium), which is grown commercially to supply the insecticide. Although extracts of the plant were already used as insecticide, the structure was first published by Hermann Staudinger and Lavoslav Ružička in 1924.[3] Pyrethrins are neurotoxins that attack the nervous systems of all insects. When present in amounts not fatal to insects, they still appear to have an insect repellent effect. Pyrethrins are gradually replacing organophosphates and organochlorides as the pesticide of first choice.

The majority of the world's supply of pyrethrin and Chrysanthemum cinerariaefolium comes from Kenya. The flower was first introduced into Kenya and the highlands of Eastern Africa during the late 1920s. Now, Kenya produces as much as 70% of the world's supply of pyrethrum.[4] A substantial amount of the flowers are cultivated by small scale farmers who depend on it as a source of income. It is a major source of export income for Kenya and source of over 3,500 additional jobs.

Toxicity

Pyrethrins are used in many varieties of insecticide, fogging products and in some pet products. Care should be taken when using this substance around humans and animals. Overdose and toxicity can result in a variety of symptoms, especially in pets, including drooling, lethargy, muscle tremors, vomiting, seizures and death.[5] Toxicity symptoms in humans include asthmatic breathing, sneezing, nasal stuffiness, headache, nausea, incoordination, tremors, convulsions, facial flushing and swelling, and burning and itching sensation.[6].

Pyrethrin is extremely toxic to aquatic life, such as bluegill and lake trout while it is slightly toxic to bird species, such as mallards. Toxicity increases with higher water temperatures and acidity. Natural pyrethrins are highly fat soluble, but are easily degraded and thus do not accumulate in the body. These compounds are toxic to bees also.[7]

References

  1. ^ Merck Index, Eleventh Edition, 7978.
  2. ^ McGraw-Hill Ryerson Chemistry 12 p.99 Michael Townsend
  3. ^ Staudinger, H. und Ruzicka, L. (1924). "Über die wirksamen Bestandteile des dalmatinischen Insektenpulvers". Helv chim acta. 7: 177. doi:10.1002/hlca.19240070124.  
  4. ^ Wainaina, Job M. G. (1995). "Pyrethrum Flowers -- Production in Africa". in John E. Casida, Gary B. Quistad. Pyrethrum Flowers. Oxford University Press. ISBN 0-19-508210-9.  
  5. ^ Permethrin and Pyrethrin Toxicity in Dogs, Dr. Dawn Ruben, http://www.petplace.com/dogs/permethrin-and-pyrethrin-toxicity-in-dogs/page1.aspx
  6. ^ Cornell University, Extoxnet, http://pmep.cce.cornell.edu/profiles/extoxnet/pyrethrins-ziram/pyrethrins-ext.html
  7. ^ Cornell University, Extoxnet, http://pmep.cce.cornell.edu/profiles/extoxnet/pyrethrins-ziram/pyrethrins-ext.html

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