Toxicology and metabolism of some synthetic pyrethroids in larvae of susceptible and resistant strains of the cattle tick boophilus microplus (Can.)

Larvae of the Malchi and DDT‐R pyrethroid‐resistant strains of ticks, compared with larvae of the Yeerongpilly (susceptible) strain, exhibited knock‐down resistance equally to (1 RS )‐cis‐ and (1 RS )‐trans‐permethrin but little resistance to either (1 RS )‐cis‐ or (1 RS )‐trans‐cypermethrin. Kill resistance was higher in Malchi than in DDT‐R ticks and more pronounced against the permethrin isomers. The cis‐isomers were more potent overall than the trans‐isomers. Cypermethrin isomers penetrated more slowly than permethrin isomers, especially into Malchi larvae. Metabolic detoxication, mainly esteratic, was fastest in Malchi larvae, trans‐isomers consistently being attacked faster than cis‐isomers. The greatei resistance of Malchi ticks is therefore probably due to a combination of these two differences. Dual mechanisms of the toxic action of pyrethroids are discussed.