Increased control of the sheep biting louse Bovicola (Damalinia) ovis with deltamethrin formulated in a fractionated wool grease carrier

Abstract

The synthetic pyrethroid deltamethrin (DM) containing a trace of [ 14C]-DM was formulated with non-oxidised sterol and wax ester fractions (F1) of wool grease and as the commercial preparation ‘Clout-S’. These were applied as a ‘backline’ strip to sheep immediately after shearing and the concentration of [ 14C]-DM at meridians adjacent to the application strip and at 1/4 and 3/4 of the dorsal–ventral distance was determined. The F1 formulation resulted in significantly greater lateral spread of DM with less remaining at the application site (66±8% of dose) 98 days after treatment compared to ‘Clout-S’ (94±3% dose). Autoradiographic examination of treated wool demonstrated that there was more DM in the lower half of the wool staple when formulated in F1 compared to ‘Clout-S’. Greater mortality occurred when sheep biting lice Bovicola (Damalinia) ovis were exposed in vitro to wool containing DM from F1 compared to ‘Clout-S’ treated sheep. In field trials there was increased efficacy against synthetic pyrethroid resistant B. ovis with F1 formulation than with ‘Clout-S’. The study has demonstrated that synthetic pyrethroid availability, and therefore efficacy, can be significantly increased when the insecticide is formulated in a ‘carrier’ with the physicochemical characteristics of wool grease.