In‐vivo Synergism of Cypermethrin by Piperonyl Butoxide in Bovicola ovis (L.) (Phthiraptera: Trichodectidae)

Mixtures of existing formulations of piperonyl butoxide (PBO) and cypermethrin were poorly suited for use as sheep dips. Mixture ratios of up to 10:1 PBO:cypermethrin did not offer a significant advantage over cypermethrin alone. A novel formulation of PBO and cypermethrin (5:1) which contained 49% wetting agent provided a much greater degree of control in small scale field trials using sheep infested with highly pyrethroid‐resistant lice. At 10 weeks after treatment, lice numbers on sheep treated with cypermethrin alone had returned to pretreatment levels, whereas a greater than 80% reduction in lice numbers was recorded on sheep dipped in the PBO:cypermethrin mixture. Although impressive, this improvement in efficacy is not sufficient to make PBO:cypermethrin mixtures a realistic proposition for controlling highly pyrethroid‐resistant sheep lice. At present, when highly resistant strains of lice need to be controlled the most cost‐effective option available to Australian woolgrowers is to dip with an organophosphate product.