Evaluation of a deltamethrin formulation as a back-line treatment of sheep for the control of the sheep body louse (Damalinia ovis)

Abstract

Abstract A formulation containing 1% deltamethrin was tested for efficacy against the sheep body louse. It was applied as a back-line treatment at a rate of 1 m1/5 kg bodyweight (2 mg/kg) to sheep either immediately after shearing or 3 weeks later. The formulation killed all observable lice within a day when applied off-shears to sheep which were subsequently confined overnight. When sheep were treated 3 weeks after shearing, and not subsequently confined, all lice were killed within 2–7 days. About 10 weeks protection from reinfestation was provided by off-shears treatment, and about 15 weeks by treatment 3 weeks after shearing. Breakdown in protection occurred when deltamethrin levels in the wool fell to about 1 μg/g greasy wool. Although the results suggest high efficacy, residue data indicate that the spread of insecticide was much less even than with plunge dipping in deltamethrin. A method for determining deltamethrin levels in wool is recorded.