Susceptibility of the sheep body louse [Bovicola ovis (Schrank), Phthiraptera: Trichodectidae] to temephos and concentration stability during sheep dipping with a commercial temephos product

Laboratory bioassays were used to measure the susceptibility of sheep lice [Bovicola ovis (Schrank)] strains that were resistant to pyrethroid or benzoylphenyl urea insect growth regulator insecticides to the organophosphate temephos, the most commonly used wet dipping insecticide. Compared with the reference susceptible strain, only the pyrethroid-resistant ‘Rowena’ strain displayed low level (2.2×) tolerance of temephos. Lice exposed to the minimum lethal concentration of temephos began to be affected within 6–7 h and all had responded to the insecticide within 24 h. In a dipping trial with sheep, the working concentration of temephos in a 5000-L plunge dip operating according to constant replenishment principles remained close to the nominal application rate of 350 mg/L. The ease with which temephos dip concentration was maintained simplified the dipping process. If typical of similar field populations, the absence, or low level of cross-resistance to temephos reported here for insect growth regulator-resistant and pyrethroid-resistant lice, respectively, should make wool producers confident that thorough dipping with temephos according to product label directions should be effective in controlling lice on their sheep.