Pyrethroid insecticides selectivity

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). 

The organophosphorons insecticides dimethoate and diazinon are mnch more toxic to insects (e.g., housefly) than they are to the rat or other mammals. A major factor responsible for this is rapid detoxication of the active oxon forms of these insecticides by A-esterases of mammals. Insects in general appear to have no A-esterase activity or, at best, low A-esterase activity (some earlier stndies confnsed A-esterase activity with B-esterase activity) (Walker 1994b). Diazinon also shows marked selectivity between birds and mammals, which has been explained on the gronnds of rapid detoxication by A-esterase in mammals, an activity that is absent from the blood of most species of birds (see Section 23.23). The related OP insecticides pirimiphos methyl and pirimiphos ethyl show similar selectivity between birds and mammals. Pyrethroid insecticides are highly selective between insects and mammals, and this has been attributed to faster metabolic detoxication by mammals and greater sensitivity of target (Na+ channel) in insects. 

Ortego, L.S. and Benson, W.H. (1992) Effects of dissolved humic material on the toxicity of selected pyrethroid insecticides, Environmental Toxicology and Chemistry 11 (2), 261-265. 

Batrachotoxin at present remains an important, indeed often essential, tool for mechanistic studies of the function of voltage-dependent sodium channels and for the investigation of the role of depolarization and/or influx of sodium ions on physiological functions. Batrachotoxin has been particularly useful in the study of the function of sodium channels, purified and reconstituted into artificial lipid bilayers. A summary and overview of the extensive studies with batrachotoxin appeared in 1986 (5). Since that time more than 100 articles dealing with the activity of batrachotoxin and/ or the radioligand batrachotoxinin A 20a-[ H]benzoate have appeared, and it is beyond the scope of the present review to summarize this extensive recent literature. A few selected developments are as follows allosteric enhancement of the action of batrachotoxins by pyrethroid insecticides... 

When certain cyclodipeptides are used as catalysts for the enantioselective formation of cyanohydrins, an autocatalytic improvement of selectivity is observed in the presence of chiral hydrocyanation products [80]. A commercial process for the manufacture of a pyrethroid insecticide involving asymmetric addition of HCN to an aromatic aldehyde in the presence of a cyclic dipeptide has been described [80]. Besides HCN itself, acetone cyanohydrin is also used (usually in the literature referred to as the Nazarov method), which can be activated cata-lytically by certain lanthanide complexes [81]. Acetylcyanation of aldehydes is described with samarium-based catalysts in the presence of isopropenyl acetate cyclohexanone oxime acetate is hydrocyanated with acetone cyanohydrin as the HCN source in the presence of these catalytic systems [82]. 

Pyrethroid insecticides are also of interest currently, probably because they are selective and are fairly safe to mammals, but mostly because they are a few orders of magnitude more potent than previous categories of insecticides and are, therefore, marketable despite their higher price. 

The selective propionylation of benzodioxole 37 at posihon 5 with propanoic anhydride can be performed in the presence of catalyhc amount of aqueous perchloric acid (20% mol) (Scheme 3.9). The reaction is performed in cyclohexane or decalin for 3 h at room temperature. Compound 38, obtained in 65% yield, represents an intermediate for the industrial production of pyrethroid insecticides. A typical batch reactor for this process is depicted in Figure 3.3. 

Abernathy CO, Casida JE (1973) Pyrethroid insecticides esterase cleavage in relation to selective toxicity. Science 179 1235-1236... 

Symptomatic solutions to counteracting mildew and insect infestation have employed various natural and synthetic insecticides that are selected for their specificity for the targeted species. Application methods for their maximum efficiencies must be specified. Some are noninvasive to humans and provide toxicity only to larvae while others are regulated compounds that could provide environmental risk. The broad range of development in this area includes chemical modification of wool s disulfide crosslinks and side-chain hydroxyl and amine groups. Applied topically or by infusion, certain aromatic or cyclic compounds are available under recognized trademarks. In recent years these approaches have been eclipsed by the use of pyrethroid insecticides such as permethrin. They can be used alone or coadded to conventional formulations. 

Mechanism of action can be an important factor determining selectivity. In the extreme case, one group of organisms has a site of action that is not present in another group. Thus, most of the insecticides that are neurotoxic have very little phytotoxicity indeed, some of them (e.g., the OPs dimethoate, disyston, and demeton-5 -methyl) are good systemic insecticides. Most herbicides that act upon photosynthesis (e.g., triaz-ines and substituted ureas) have very low toxicity to animals (Table 2.7). The resistance of certain strains of insects to insecticides is due to their possessing a mutant form of the site of action, which is insensitive to the pesticide. Examples include certain strains of housefly with knockdown resistance (mutant form of Na+ channel that is insensitive to DDT and pyrethroids) and strains of several species of insects that are resistant to OPs because they have mutant forms of acetylcholinesterase. These... 

Pyrethroids show very marked selective toxicity (Table 12.2). They are highly toxic to terrestrial and aquatic arthropods and to fish, but only moderately toxic to rodents, and less toxic still to birds. The selectivity ratio between bees and rodents is 10,000- to 100,000-fold with topical application of the insecticides. They therefore appear to be environmentally safe so far as terrestrial vertebrates are concerned. There are, inevitably, concerns about their possible side effects in aquatic systems, especially on invertebrates. 

Interestingly, it appears that earlier selective pressure by dichlorodiphenyl trichloroethene (DDT) raised the frequency of kdr genes in the population before pyrethroids came to be used. Thus, some pyrethroid resistance already existed before these insecticides were applied in the field. 

The development of class-selective antibodies is another approach to multi-analyte analysis. The analyst may design haptens that will generate antibodies that recognize an epitope common to several compounds, as explained above for the analysis of pyrethroids by measuring PBA. Other examples of class-selective immunoassays that have been developed are mercapturates," glucuronides, pyrethroids, organophosphate insecticides, and benzoylphenylurea insecticides." ... 

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