Insecticides synthetic pyrethroids

Many pesticides are not as novel as they may seem. Some, such as the pyre-throid and neonicotinoid insecticides, are modeled on natural insecticides. Synthetic pyrethroids are related to the natural pyrethrins (see Chapter 12), whereas the neo-nicotinoids share structural features with nicotine. In both cases, the synthetic compounds have the same mode of action as the natural products they resemble. Also, the synthetic pyrethroids are subject to similar mechanisms of metabolic detoxication as natural pyrethrins (Chapter 12). More widely, many detoxication mechanisms are relatively nonspecific, operating against a wide range of compounds that... 

Empiricai C22H18CI2FNO3 Properties YIsh. brn. oil m.w. 434.29 Toxicoiogy LD50 (oral, rat) 900 mg/kg Uses Agricultural insecticide synthetic pyrethroid with good insecticidal activity Reguiatory SARA reportable Trade Name Synonyms Baythroid [Bayer CropScience AG... 

In 1939, the highly efficient contact insecticide DDT (an abbreviation of the technically incorrect name dichlorodiphenyl-trichloroethane) was introduced to the market. Its use, as well as the use of several other organochlorine compounds, spread worldwide in the following years. Important active substances that appeared on the market in the period after World War II include carbamate insecticides and herbicides. Also significant was the discovery of herbicidal phenoxyacetic acids, which represent the first group of the so-caUed hormonally active pesticides. Around the middle of the last century a number of other biologically active substances were discovered, many of which, such as herbicides based on substituted urea, s-triazines (1,3,5-triazines), quaternary ammonium salts or insecticidal synthetic pyrethroids and many others, are still used in many countries around the world. 

These chemorational techniques have generated great interest in, and high expectations for, the acceleration of development of innovative pesticides. However, many purportedly successful appHcations of QSAR procedures have reHed on the quaHtative insights traditionally associated with art-based pesticide development programs. Retrospective QSAR analyses have, however, been helpful in identifying the best compounds for specific uses (17). Chemorational techniques have also found some appHcations in the development of pesticides from natural product lead compounds, the best known examples being the synthetic pyrethroid insecticides (19) modeled on the plant natural product, pyrethmm. 

Synthetic Pyrethroid Insecticides. Elucidation of the chemical stmctures of the naturally occurring pyrethmm esters, their rapid and selective insecticidal action, and their high cost stimulated the search for effective synthetic derivatives (13,17,21). Since the 1940s, stmctural optimisation has produced an array of broad-spectmm insecticides with activity 10- to 20-fold greater than other types of insecticides, and with extended residual action. These synthetic pyrethroids have become one of the most important classes of insecticides with world aimual production estimated at 6000 t (21). 

K. Naumaim, Synthetic Pyrethroid Insecticides Structures and Properties, Sptinger-Verlag, Berlin, 1990. 

Synthetic pyrethroids is one of the group of modern insecticides of cyclopropancai bonic acid derivate. The pyrethroids prepai ation is the racemic mixture of optical isomers or mixture of cis- or tran.s-isomers. 

The compounds featured in Table 1.1 are considered briefly here. Pyrethrins are lipophilic esters that occur in Chrysanthemum spp. Extracts of flower heads of Chrysanthemum spp. contain six different pyrethrins and have been used for insect control (Chapter 12). Pyrethrins act upon sodium channels in a manner similar to p,p -DDT. The highly successful synthetic pyrethroid insecticides were modeled on natural pyrethrins. 

Synthetic pyrethroids are esters of specific acids (chrysantemic acid, halo-substituted chrysantemic acid, 2-[4-chlorophenyl]-3-methylbutyric acid) and alcohols (allethrone, 3-phenoxybenzyl alcohol) (Ray, 1991). They represent a group of insecticides largely used in agriculture and public health because of their relatively low toxicity to humans and mammalian species and their short environmental persistence. 

Pyrethroid insecticides are generally recognized as potent neurotoxicants that interfere with nerve membrane function by interaction with the sodium channel (Elliott and Janes 1978 Vijverberg et al. 1982 Gilbert et al. 1989 Haya 1989). Synthetic pyrethroids are more toxic against insect pests, up to 10 times more potent in some cases, than other insecticides now in general use (Bradbury... 

Detailed information on ecological and toxicological aspects of fenvalerate and other synthetic pyrethroid insecticides is provided in reviews by Elliott (1977), Elliott and Janes (1978), Wouters and Bercken (1978), Glickman and Casida (1982), Vijverberg and Bercken (1982), Gray and Soderlund (1985), Leahey (1985), Smith and Stratton (1986), Coats et al. (1989), Bradbury and Coats (1989a), and Eisler (1992). 

Synthetic pyrethroids now account for at least 30% of the world insecticide market and are rapidly replacing other agricultural chemicals for control of insect pests. Fenvalerate is one of the more widely used synthetic pyrethroid insecticides. It is derived from a combination of a-cyano-3-phenoxybenzyl alcohol and a-isopropyl phenylacetate ester. Technical fenvalerate is a mixture of four optical isomers, each occurring in equal amounts but with different efficacies against insect pests. Fenvalerate does not usually persist in the environment for >10 weeks, and it does not accumulate readily in the biosphere. Time for 50% loss (Tb 1/2) in fenvalerate-exposed amphibians, birds, and mammals was 6 to 14 h for reptiles, terrestrial insects, aquatic snails, and fish it was >14 h to <2 days and for various species of crop plants, it was 2 to 28 days. Fenvalerate degradation in water is due primarily to photoactivity, and in soils to microbial activity. Half-time persistence in nonbiological materials is variable, but may range up to 6 days in freshwater, 34 days in seawater, 6 weeks in estuarine sediments, and 9 weeks in soils. 

Pyrethroids are used primarily for the control of household and agricultural insect pests, and secondarily in industrial, stored product, and veterinary applications. They are especially advantageous for use in northern climates because their toxicity is enhanced at low temperatures (Smith and Stratton 1986). Synthetic pyrethroid insecticides, including fenvalerate, are used as alternatives... 

The sodium channel in the nerve membrane is the major target site for all synthetic pyrethroid insecticides (and many other neurotoxicants)... 

Coats, J.R. and N.L. O Donnell-Jeffery. 1979. Toxicity of four synthetic pyrethroid insecticides to rainbow trout. Bull. Environ. Contam. Toxicol. 23 250-255. 

Flannigan, S.A., S.B. Tucker, M.M. Key, C.E. Ross, E.J. Fairchild II, B.A. Grimes, and R.B. Harrist. 1985. Primary irritant contact dermatitis from synthetic pyrethroid insecticide exposure. Arch. Toxicol. 56 288-294. 

Materna, E.J. 1991. Effects of the Synthetic Pyrethroid Insecticide, Esfenvalerate, on Larval Amphibians. M.S. thesis. Univ. Missouri, Columbia. 96 pp. 

Smith, T.M. and G.W. Stratton. 1986. Effects of synthetic pyrethroid insecticides on nontarget organisms. Residue Rev. 97 93-120. 

Taylor, K.S., G.D. Waller, and L.A. Crowder. 1987. Impairment of a classical conditioned response of the honey bee (Apis mellifera L.) by sublethal doses of synthetic pyrethroid insecticides. Apidologie 18 243-252. Theophilidis, G., M. Benaki, and E. Papadopoulu-Mourkidou. 1997. Neurotoxic action of six pyrethroid insecticides on the isolated sciatic nerve of a frog (Rana ridibunda). Comp. Biochem. Physiol. 118C 97-103. Tippe, A. 1987. Evidence for different mechanisms of action of the three pyrethroids, deltamethrin, cypermethrin, and fenvalerate, on the excitation threshold of myelinated nerve. Pestic. Biochem. Physiol. 28 67-74. 

Webber, E.C., W.G. Deutsch, D.R. Bayne, and W.C. Seesock. 1992. Ecosystem-level testing of a synthetic pyrethroid insecticide in aquatic mesocosms. Environ. Toxicol. Chem. 11 87-105. 

As described in the section on Cross-resistance in this chapter, it was found that some insect species showed extremely low cross-resistance to three ingredients, pyrethrins as well as d-allethrin and prallethrin, although they developed resistance to photostable synthetic pyrethroids. The latter two compounds of d-allethrin and prallethrin have quite similar chemical structures and the same configuration as cinerin I (an ingredient of pyrethrins). It is considered preferable to develop pyrethroids retaining the characteristics of natural pyrethrins and household insecticides containing them in the perspectives of safety and low cross-resistance. 

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. 

Figure 7 shows the course of development of various synthetic pyrethroids developed by retaining chrysanthemic acid as the acid moiety and modifying the alcohol moiety. Numerous useful compounds with favorable characteristics have been derived from the structural modification of natural cinerin I (7). These underlined compounds have been put into practical use as active ingredients, mainly for household insecticides. 

Pyrethroids for agricultural use were developed in the 1970s in Japan, USA, and Europe after research on photostable synthetic pyrethroids. Those compounds were composed of an acid moiety obtained by various modifications and a chemically stable alcohol component, such as benzyl group and m-phenoxybenzylalcohol. According to recent statistics, pyrethroids accounted for approximately 20% in value of agricultural insecticides used annually all over the world in 2009. 

Resistance to insecticides has drawn global attention since the Korean War in 1950 when the mass use of organic synthetic insecticides, such as DDT and BHC, against agricultural pests and sanitary pests became common. Organophosphorus compounds and carbamates were used thereafter, but invited problems of safety concerns and insect resistance. Synthetic pyrethroids were watched with keen interest as alternatives and have become used widely not only for sanitary pests but also agricultural pests. The development of resistance to synthetic pyrethroids is also not a rare phenomenon and has spread all over the world. 

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