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Pyrethrin

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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, (+)-trans-chrysanthemic acid in the case of pyrethrin I.[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.

As an insecticide

Pyrethrins are used in many varieties of insecticide, fogging products and in some pet products and have been used as an insecticide for over 100 years. They affect the flow of sodium out of the nerve cells in insects, resulting in repeated and extended firings of the nerves, causing the insects to die. [5] Piperonyl butoxide, a synergist, is often used in combination with Pyrethrin, making the mixture more effective by not allowing the insect's system to detoxify the Pyrethrin.[6] Although it is used as an insecticide, it also may be used as an insect repellent. Observations in food establishments demonstrate that flies are not immediately killed but are found more often on windowsills or near doorways. This suggests that, due to the low dosage applied, insects are driven to leave the area before dying.[7] Pyrethrin and the synergists are biodegradable and rapidly disintegrate in sunlight and air, thus assuring that there will be no excessive build-up of insecticides dispensed in the area being treated. Pyrethrin is often used by the Vexcon exterminators as portrayed in Dirty Jobs and their television series Billy the Exterminator.

Toxicity

The United States Department of Agriculture has stated that synergized pyrethrum " is probably the safest of all insecticides for use in food plants " and that " a pyrethrum formulation is approved for use around foodstuffs." All pyrethrins are easily hydrolyzed and degraded by stomach acids in mammals, so toxicity following ingestion by pets is very low. However, pyrethrins are dangerous for fish. Toxicity is usually associated with applying much more of the product than directed. Care should be taken to observe direction labels 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.[8] Toxicity symptoms in humans include asthmatic breathing, sneezing, nasal stuffiness, headache, nausea, incoordination, tremors, convulsions, facial flushing and swelling, and burning and itching sensation.[9] Permethrin and other pyrethroids are synthetic versions of pyrethrin which have much greater potential for causing toxicity. The latest information regarding toxicity of piperonyl butoxide has determined that it can pose a distinct health risk when it becomes airborne and pregnant women are exposed during the third trimester. This leads to delayed mental development in young children. A 2011 study found a significant association between piperonyl butoxide (PBO), a common additive in pyrethroid formulations, measured in personal air collected during the third trimester of pregnancy, and delayed mental development at 36 months. Children who were more highly exposed in personal air samples (≥4.34 ng/m3) scored 3.9 points lower on the Mental Developmental Index than those with lower exposures. The lead researcher stated, "This drop in IQ points is similar to that observed in lead exposure. While perhaps not impacting an individual's overall function, it is educationally meaningful and could shift the distribution of children in the society who would be in need of early intervention services."[10]


See also

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.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Wainaina, Job M. G. (1995). "Pyrethrum Flowers -- Production in Africa". In John E. Casida, Gary B. Quistad (ed.). Pyrethrum Flowers. Oxford University Press. ISBN 0-19-508210-9.
  5. ^ http://www.peteducation.com/article.cfm?c=0+1306+1448&aid=1359
  6. ^ http://www.asktheexterminator.com/Pesticide/Pyrethrin.shtml
  7. ^ http://doyourownpestcontrol.com/pyrethrin.htm
  8. ^ Permethrin and Pyrethrin Toxicity in Dogs, Dr. Dawn Ruben
  9. ^ Pyrethrins — Cornell University, Extoxnet
  10. ^ http://www.sciencedaily.com/releases/2011/02/110210103715.htm
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