Resistance and the control of sheep ectoparasites

For about 100 years Australian woolgrowers have used a variety of chemicals to control blowflies, lice and other ectoparasites of sheep. While the chemicals have changed considerably the application technology has not changed very much at all. Chemicals registered for use on sheep have paralleled the evolution of synthetic insecticides with the unfortunate consequence of the development of resistance in the Australian sheep blowfly, Lucilia cuprina, following closely behind. Organochlorine (dieldrin) resistance peaked at about 70% in 1958 when unacceptable residues in meat and wool forced their withdrawal. Organophosphate (OP) resistance appeared in 1965. With no alternative insecticide classes until 1979, OP resistance reached near fixation levels by the early 1970s and has remained unchanged. OP resistance has reduced the protection period from over 16 weeks to about 6 weeks. Moreover, resistance has decreased the effectiveness of many flystrike dressings to unacceptably low levels. OPs are still very effective against sheep body lice, Bovicola ovis but control is hampered by inadequate application via plunge or shower dipping. Synthetic pyrethroid (SP) pour-on products were released in 1981 but resistance developed by 1985 and many woolgrowers were unable to eradicate lice with pour-on products. Highest Resistance Factors at this time were only about 26 x but this was sufficient to prevent pour-ons working efficiently. By 1991 a population from Hartley in NSW was found to be 642 x resistant to cypermethrin with side-resistance conferred to the other SPs. SP resistance was partially suppresible by piperonyl butoxide but field trials suggested that the resulting improvement in efficacy was not sufficient to be commercially attractive. OPs remain very effective if applied correctly and the release of ivermectin and 2 benzoylphenyl urea products significantly improves the prospects for resistance management. However the increasing environmental concern about the persistence of chemical residues in wool has stimulated interest in biological control of sheep lice by Bacillus thuringiensis.