Permethrin selective toxicity

Carboxylesterases are responsible for the selective toxicity of malathion that favors mammals over insects. Carboxylesterase hydrolyzing trans-permethrin has been found in numerous insect species, including the fall armyworm, velvetbean caterpillar (Anticar-sia gemmatalis), cabbage looper (Trichoplnsia ni), tobacco budworm (Heliothis virescens), corn earworm (Helicoverpa zea), and spined soldier bug (Podisus maculwentris) (Yu, 1990). 

Pyrethrins and synthetic pyrethroids are among the safest of the topically applied ectoparasiticides, because of their selective toxicity for insects (mam-malian-to-insect toxic dose ratio is greater than 1000, compared with 33 for organophosphates and 16 for carbamate insecticides). In contrast to the very wide margin of safety for mammalian species, pyrethroids are toxic to fish. The synergistic action of pyrethrins and piperonyl butoxide (in combination preparations) is due to the inhibition by piperonyl butoxide of the microsomal enzyme system of some arthropods. Preparations of synthetic pyrethroids (permethrin, cypermethrin) often contain a mixture of drug isomers in varying proportions. 

Sibley, P.K. and Kaushik, N.K. Toxicity of rtricroencapsulated permethrin to selected nontarget aquatic invertebrates, Arch. Environ. Contam. Toxicol, 29(2) 168-176, 1991. 

In laboratory microcosms, ira 5-permethrin was selectively degraded compared to the other diastereomer, cw-permethrin, by six bacterial strains [19]. These strains also preferentially biotransformed 15-cw-bifenthrin over their antipodal l/ -cw-enantiomers, which were more toxic to daphnids [19]. Enantioselectivity was more pronounced for cw-permethrin than for cw-bifenthrin, and was strain-dependent. The (—)-enantiomer of both pyrethroids was preferentially depleted in sediments adjacent to a plant nursery, suggesting that in situ microbial biotransformation was enantioselective [20]. Although all enantiomers of permethrin were hydrolyzed quickly in C-labeled experiments in soils and sediments, the degradates of both cis- and irara-permethrin s -enantiomers were mineralized more quickly than those of the 5-enantiomer, while degradation products of cA-permethrin were more persistent than those of the trans-isomex [185]. Enantioslective degradation of fenvalerate in soil slurries has also been reported [83]. These smdies underscore how enantiomer-specific biotransformation can affect pyrethroid environmental residues, the toxicity of which is also enantiomer-dependent [18-20]. 

Liu W, Gan J, Lee S, Werner I (2005d) Isomer selectivity in aquatic toxicity and biodegradation of bifcmthrin and permethrin. Envirrai Toxicol Chem 24(8) 1861-1866 Lu AYH, West SB (1980) Multiplicity of mammalian microsomal cytochrrane P450. Pharmacol Rev 31 277-291... 

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. 

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