Pyrethroids, insecticides

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). 

The horn fly Haematobia irritans an important bloodsucking pest of catde, was found to be completely controlled by ear tags which slowly released permethrin and fenvalerate. These were found to give almost 100% control for 24 weeks. However, following almost universal appHcation of this ideal control measure, control failures of the pyrethroid insecticides were widely reported in North America in 1982 ndash 1983 and the practice is now abandoned. 

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

USE OF SEMI PERMEABLE MEMBRANE DEVICES (SPMDs) TO INDOOR AIR MONITORING OF PYRETHROID INSECTICIDES... 

Weston DP, Holmes RW, You J, Lydy MJ (2005) Aquatic toxicity due to residential use of pyrethroid insecticides. Environ Sci Technol 39(24) 9778-9784... 

You J, Pehkonen S, Weston DP, Lydy MJ (2008) Chemical availability and sediment toxicity of pyrethroid insecticides to Hyalella azteca Application to field sediment with unexpectedly low toxicity. Environ Toxicol Chem 27(10) 2124—2130... 

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. 

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. 

The structures of some pyrethroid insecticides are shown in Figure 12.1. They are all lipophilic esters showing some structural resemblance to the natural pyrethrins. They can all exist in a number of different enantiomeric forms. Permethrin, cypermethrin, and deltamethrin, for example, all have three asymmetric carbon atoms... 

Leahey, J.P. (Ed.) (1985). The Pyrethroid Insecticides—A multiauthor work that covers many aspects of the toxicology and ecotoxicology of the earlier pyrethroids. 

Chapman, R.A. and Harris, C.R. (1981). Persistence of pyrethroid insecticides in a mineral and an organic soil. Journal of Environmental Science and Health B 16, (5) 605-615. 

Kim, I.Y., Han, S.Y., and Kang, T.S. et al. (2005). Pyrethroid insecticides, fenvalerate and permethrin, inhibit progesterone-induced alkaline phosphatase activity in T47D human breast cancer cells. Journal of Toxicology and Environmental Health—Part A—Current Issues 68, 2175-2186. 

Pilling, E.D. (1993). Synergism between EBI Fungicides and a Pyrethroid Insecticide in the Honeybee. Ph.D. Thesis Southampton University, U.K. 

Pilling, E.D., Bromley-ChaUenor, K.A.C., and Walker, C.H. et al. (1995). Mechanism of synergism between the pyrethroid insecticide lambda cyhalothrin and the imidazole fungicide prochloraz in the honeybee. Pesticide Biochemistry and Physiology 51, 1-11. 

Tyler, C.R., Beresford, N., and van der Woning, M. et al. (2000). Metabolism and environmental degradation of pyrethroid insecticides produce compounds with endocrine activities. Environmental Toxicology and Chemistry 19, 801-809. 

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