Pyrethroid Synergism by Piperonyl Butoxide in Bovicola ovis (Schrank) (Phthiraptera: Trichodectidae)

Piperonyl butoxide (PBO) synergised the synthetic pyrethroids (SPs) cypermethrin, deltamethrin, cyhalothrin and alpha‐cypermethrin in a concentration‐dependent manner in susceptible and both low‐and high‐SP‐resistant strains of the sheep body louse Bovicola ovis. Synergism ratios as high as 23 x were recorded when 200 mg/L PBO was mixed with deltamethrin in bioassays with the highly resistant Hartley strain. High PBO synergism implicates mono‐oxygenase detoxification of SPs as a major cause of SP resistance in B. ovis. the possibility of mixing PBO with existing SP lousicide products to control pyrethroid resistant lice is discussed.