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 >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.