Toxicity of Pyrethroids

Pyrethroids, such as p,p -DDT, are toxic because they interact with Na+ channels of the axonal membrane, thereby disturbing the transmission of nerve action potential (Eldefrawi and Eldefrawi 1990, and Chapter 5, Section 5.2.4 of this book). In both cases, marked hydrophobicity leads to bioconcentration of the insecticides in the axonal membrane and reversible association with the Na+ channel. Consequently, both DDT and pyrethroids show negative temperature coefficients in arthropods increasing temperature brings decreasing toxicity because it favors desorption of insecticide from the site of action. 

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. 

Note Mean values given for birds the number of species tested is given in brackets. 

Source Data from Environmental Health Criteria 82, Environmental Health Criteria 94, Environmental Health Criteria 95, and Environmental Health Criteria 97. 

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Amweg EL, Weston DP, Ureda NM (2005) Use and toxicity of pyrethroid pesticides in the Central Valley, California, USA. Environ Toxicol Chem 24(4) 966-972... 

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... 

Because of the high toxicity of pyrethroids to aquatic invertebrates, these organisms are likely to be adversely affected by contamination of surface waters. Such contamination might be expected to have effects at the population level and above, at least in the short term. In one study of a farm pond, cypermethrin was applied aerially, adjacent to the water body (Kedwards et al. 1999a). Changes were observed in the composition of the macroinvertebrate community of the pond that were related to levels of the pyrethroid in the hydrosoil. Diptera were most affected, showing a decline in abundance with increasing cypermethrin concentration. Chironimid larvae first declined and later recovered. 

Clark, J.R., L.R. Goodman P.W. Borthwick, J.M. Patrick, Jr., G.M. Cripe, P.M. Moody, J.C. Moore, and E.M. Lores. 1989. Toxicity of pyrethroids to marine invertebrates and fish a literature review and test results with sediment-sorbed chemicals. Environ. Toxicol. Chem. 8 393-401. 

Haya, K. 1989. Toxicity of pyrethroid insecticides to fish. Environ, Toxicol. Chem. 8 381-391. 

Table 2 Fish toxicity of pyrethroids for agricultural uses in Japan...

Table 6 Oral and contact acute toxicity of pyrethroids to honeybees...

Smart LE, Stevenson JH (1982) Laboratory estimation of toxicity of pyrethroid insecticides to honeybees relevance to hazard in the field. Bee World 63 150-152... 

Hintzen EP, Lydy MJ, Belden JB (2009) Occurrence and potential toxicity of pyrethroids and other insecticides in bed sediments of urban streams in central Texas. Environ Pollut 157 110-116... 

Yang, W., Spurlock. F.. Liu, W., and Gan. J. Inhibition of aquatic toxicity of pyrethroid insecticides by suspended sediment, Environ. Toxicol. Chem., 25(7) 1913-1919, 2006. 

There are some striking examples of the parabolic relationship, for example the relationship between relative toxicity of pyrethroid insecticides and mobility on reverse-phaseTLC plates shown by Briggs et aJ (12). However, there... 

This assumption is reasonable because the toxicity of pyrethroids to benthic organisms is predictable from the equilibrium partitioning-derived pore water concentration (8), and the pyrethroids in this study have values comparable to those of cypermethrin (10). 

Examples of these include sesamin and sesamolin, both active principles of sesame oil, and piperonyl butoxide (PBO) and sesamex, both synthetic products. Although initially developed for use with pyrethrins, they have since been observed to synergize some, but not all, carbamates, organophosphates, pyrethroids, and organochlorines. In addition to the MDP compounds, another type of synergist called MGK 264 was developed to synergize the toxicity of pyrethroids and rotenone. The structure of piperonyl butoxide and MGK 264 are as follows ... 

Pyrethroids. Pyrethroids, such as natural pyrethrins and synthetic analogs, allethrin, permethrin, and others, are well known for their neurotoxicity (48-59). However, as a major class of insecticide, they have a remarkable safety margin for mammals, principally because of the rapid metabolic degradation of pyrethroids in mammalian species (48-50). The acute toxicity of pyrethroids involves two distinct syndromes in rats and mice (49-51). The first one, T syndrome or tremor (Type I), is characterized by a rapid onset of tremor, initially in the limbs and gradually extending over the whole body. Death is associated with clonic seizures. The second... 

The high acute toxicity of DDT analogs and most pyrethroids to fish is one of the most significant hazards associated with the widespread use of these compounds. The high toxicity of pyrethroids to fish appears to result in part from high intrinsic sensitivity of the CNS to these compounds (JJ3,J7) Fish are also highly sensitive to some... 

Structure-activity correlation studies have shown that polarity is not important for the toxicity of pyrethroids, while receptor-substrate interactions due to molecular size, shape and electronic effects are important and gave good correlation with bioactivity values (Briggs et al., 1976 Lee, 1976). 

The rapid hydrolytic and oxidative transformation is a decisive factor in the low toxicity of pyrethroids to mammals. 

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