Pyrethrum and pyrethroids

Metabolism. The wide acceptance and enormous usefulness of the pyrethrum and pyrethroids are that they pose to be highly toxic particularly to the ectoparasites, whereas they prove to be comparatively much less toxic i.e., nontoxic) to mammals in case absorbed. The magnificent notoxic characteristic feature is associated with the excellent and rapid metabolism of these drug substances either via hydrolysis or oxidation. More specifically, the extent of either hydrolysis or oxidation is exclusively dependent upon the structure of the prevailing pyrethrins or pyrethroids. 

Soderlund DM (1995) Mode of action of pyrethrins and pyrethroids. In Casida JE, Quistad GB (eds) Pyrethrum flowers production, chemistry, toxicology and uses. Oxford University Press, New York, NY, pp 217-233... 

Casida JE (1980) Pyrethrum flowers and pyrethroid insecticides. Environ Health Perspect 34 189-202... 

Pyrethrins and pyrethroids are probably the best known and safest classes of natural or synthetic insecticides, widely used in domestic and agricultural applications (1-7). Pyrethrins are natural insecticides derived from the Chrysanthemum cineraria flowers the plant extract, called pyrethrum, is a mixture of six isomers (pyrethrin I and II, cinerin I and II, jasmolin I and II) which was first used in China in the century AD, during the Chou Dinasty. The world pyrethrum market is worth half a billion US dollars [main producers are East Africa highlands (Kenia, Tanzania and Rwanda) and Australia] however, its availability is subject to cyclical trends, due to rains and relations with farmers, who face high harvest costs also due to the fact that the flowers have to be... 

Pyrethroids consist of pyrethrum and its synthetic pyrethrum analogs called synthetic pyrethroids. Pyrethrum is a solvent extract of dried flowers of Chrysanthemum cinerariaefo-lium. The active ingredients of pyrethrum are called pyrethrins. Pyrethrins consist of four esters, namely, pyrethrins I and II and cinerins I and II, which comprise the combination of two different alcohols (pyrethrolone and cinerolone) and two different acids (chrysan-themic acid and pyrethric acid), as follows ... 

Pyrethrum and pyrethrin products are used mainly for indoor pest control. They are not sufficiently stable in light and heat to remain as active residues on crops. The synthetic insecticides known as pyrethroids (chemically similar to pyrethrins) do have the stability needed for agricultural application. 

Common names pyrethrins, firmotox, pyrethrins and pyrethroids, chrysanthemates, pyrethrum, etc. Chemically, the field pyrethroids consist of hydroxy and non-hydroxy fatty acids, alkanes, carotenoids sterols and triterphenols, flavonoids, etc. 

Functionally, pyrethroids are synthetically made and have similar properties as natural pyrethrum and pyrethrins. They have the same CAS registry number as natural pyrethrum and pyrethins. 

Even if the phytoalexins so far isolated have little commercial utility, it is still possible that useful substances may yet appear. But the more exciting possibility is that consideration of the chemical structures of natural phytoalexins and of their modes of action against fungal infections may provide clues for the development of synthetic pesticides. The complexity of the chemical structures of the natural phytoalexins may make them uneconomical to manufacture but a comparison should be made with the synthetic pyrethroid insecticides. The natural pyrethrums have complex chemical structures but simpler compounds, economical to manufacture, have been developed on the basis of the structures of natural pyrethrums and many of these have much more desirable properties for use in agriculture than the natural substances. There would seem to be no reason why simpler compounds based on the structures of natural phytoalexins should not provide synthetic fungicides as important and useful as the synthetic pyrethroids. This is a future challenge for the synthetic organic chemists in this area. 

The above compounds have been used as synergists of pyrethrum and synthetic pyrethroids, such as allethrin, cyclethrin and others. Later investigations showed that these compounds can also synergise other insecticides, primarily carbamates and DDT. Piperonylbutoxide and sesamex proved to be the most efficient in this respect however, the latter is unsuitable for practical use because of its sensitivity to light and humidity. 

R. Winney, Pyrethrins and pyrethroids in coils—h review, Pyrethrum Post, 13, 17,1975. 

A review of the chemistry and biochemistry of pyre-thrum is presented based mainly on work done subsequent to 1945. The greatest advances in the chemistry of pyrethrum have occurred since the disclosure of the heterogeneity of pyrethroids. On the biological side, the most recent work has been made possible by the use of chromatographic column separation and gas chromatography separation of the four active components of pyrethrins. 

Godin et al. (10), working on the effect of insecticidal activity of pyrethrum flowers from fresh and dried flowers, showed that extracts from fresh flowers had little or no greater effect on pyrethroid content and insecticidal activity than extracts from dried flowers with drying temperatures up to 80 °C. However, some of all the pyrethrin was lost at 120°C. The principal loss was in pyrethin I and not pyrethin II. 

Adsorption column chromatography has been employed to separate the constituents of pyrethrum. Florisil and aluminum oxide have been used as adsorption columns to retain much of the pigmented materials. The pyrethroids may be caused to elute with several solvents. In our experience mixtures of hexane with ethyl acetate, methanol, ethyl ether, dichloromethane, or acetone have provided different elution patterns. 

Pyrethrum became the main source of household insecticides in sprays in the USA (1919) and mosquito coils (1895) as well as oil-based preparations (1924) in Japan. Thereafter, the insecticidal ingredients shifted from pyrethrins to various synthetic pyrethroids, but mosquito coils have been used worldwide for more than 110 years without changing in shape. 

After World War II, the production of pyrethrum in Japan fell markedly and declined to only 1,000 tons in terms of dried flowers in 1965. At present, pyrethrum is not cultivated in Japan and the main producers are Kenya, Tanzania, Tasmania, and China, with worldwide production in 2010 amounting to around 10,000 tons of dried flowers. Dried flowers are extracted and purified at pyrethrum-extracting factories on the spot, producing 25-50% pyrethrin extracts. While pyrethrum extracts have been replaced with various synthetic pyrethroids, they are still used in houses, food factories, gardens, and organic farms, all of which emphasize the importance of safety. Katsuda [1] reported that natural pyrethrins showed a low development of resistance by flies and mosquitoes compared with many synthetic pyrethroids, against which a high development of cross-resistance was observed. 

Natural pyrethrins are a neurotoxin and repel, knock down, and kill by contact with insects at a low concentration. On the other hand, they have ideal features for household insecticides because of their quite low dermal and oral toxicides to warm-blooded animals. Neither plants other than pyrethrum nor synthetic insecticides have been reported to have such properties. Numerous synthetic pyrethroids have been developed by chemists since the complicated chemical structure of natural pyrethrins was elucidated in the middle of the twentieth century. Allethrin was the first synthetic pyrethroid put into practical use. 

Pyrethroids are a class of synthetic insecticides designed and optimized based on the structure of the pyrethrins found in natural pyrethrum extracted from chrysanthemum flowers [1, 2], Pyrethroids are widely used to control insect pests in agriculture and public health because of their relative safety for humans and high insecticidal potency [3]. 

Natural pyrethrins, insecticidal ingredients occurring in the flowers of Tanacetum cinerariaefolium (also known as Chrysanthemum cinerariaefolium or Pyrethrum cinerariaefolium), have been modified for higher stability in the environment as well as better biological performance for more than 40 years, and consequently more than 30 synthetic pyrethroids have been marketed and used worldwide for controlling indoor and agricultural pest insects [1,2]. 

Pyrethrum has been used as an insecticide for around 150 years, and there has been no other insecticide which has so successfully contributed to the control of sanitary pests. Numerous analogs have been developed by chemists worldwide since the elucidation of the chemical structure of pyrethrins, which are the insecticidal ingredients of pyrethrum. As a result, their application has expanded extensively to various fields. To date, many eminent books have been published by scientists in this field and have contributed to advancing pyrethroid science. 

Pyrethroids are a collective term for compounds that are obtained by modifying the structure of natural insecticidal ingredients, pyrethrins, contained in pyrethrum while maintaining safety, to improve efficacy and provide different characteristics from pyrethrins that show high selective toxicity comparable to pyrethrins. 

The most economically important group of natural plant insecticides are the pyrethrins, a group of six closely related esters extracted from pyrethrum (Chrysanthemum clnerariaefollum) flower heads (Figure 1). Pyrethrum has been used as an insecticide since at least the early 1800 s in Persia and Yugoslavia. By 1828 pyrethrum was being processed for commercial Insect control, and by 1939 imports of pyrethrum into the United States reached a peak of 13.5 million pounds. Use of the natural product declined in the early 1950 s because of the advent of synthetic pyrethroid analogs (for example, allethrins), which were both more stable and more effective in the field. The present worldwide demand for pyrethrum flowers remains in excess of 25,000 tons annually and is satisfied by the estimated 150 million flowers still hand-harvested daily, predominantly in natural stands and cultivated fields in Kenya, Tanzania, and Ecuador (9 ). 

The newest class of insecticide, pyrethroids, is loosely based upon the naturally occurring pyrethrum derived from chrysanthemum flowers. Synthetic pyrethroids were developed in the 1980s but the naturally occurring pyrethrum was first commercially used in the 1800s. The chemical structure of pyrethroids is quite different from organochlorines and organophosphates, but the primary site of... 

The most promising alternatives were synthetic pyrethroids. The pyrethroids, developed as derivatives of naturally occurring pyrethrum, cause hyperexcitation, aggressiveness, incoordination, whole-body tremor, and seizures. Acute exposure in humans, usually resulting from skin exposure due to poor handling procedures, usually resolves within 24 hours. While not particularly toxic to mammals, they can cause an allergic skin response in humans. Some pyrethroids may cause cancer, reproductive or developmental effects, or endocrine system effects. 

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