Pyrethroid insecticides are synthetic analogues of the natural pyrethrins in flowers of the genus Chrysanthemum. They form, together with chlorinated hydrocarbons (DDT, dieldrin and lindane), organophosphorous compounds (Parathion, melathion, and diazinon) and methyl carbamate esters (Aldicarb, carbofuran and carbaryl), the four major classes of insecticides (Casida et aI., 1983).
Based on the symptomatology after acute intoxication of the insects, pyrethroids fall into two classes: Type I, non-a-cyano-pyrethroids, such as permethrin, which affect generally the peripheral body activities and type 1\ pyrethroids -characterized chemically by a cyano substituent-, such as cyhalothrin, which induce central actions (Wouters and Van den Bercken, 1978; Lund and Narahashi, 1983; Narahashi, 1985).