Untitled

Abstract

A bioassay suitable for the determination of dose responses of the sheep body louse, Damuliniu ovis, to avermectins is described. The inclusion of wool/skin substrate resulted in approximately 90% survival of controls over the 48-h test period. Ivermectin and abamectin were highly effective against D . ovis, and similar responses of pyrethroid-susceptible and resistant strains indicated that there was no cross-resistance to ivermectin. IntroductionThe sheep body louse, Damalinia ovis (Schrank), an important ectoparasite of sheep (Arundel and Sutherland 1988), has developed resistance to synthetic pyrethroid insecticides used as pour-ons for louse control (Boray et al. 1988). Ivermectin is highly toxic to larvae of the Australian sheep blowfly, Lucilia cuprina (Wiedemann) , and when applied to sheep as a jetting fluid for fly control shows promise for reducing infestations of D. ovis (unpublished data). However, the laboratory responses of pyrethroid resistant and susceptible lice to ivermectin have not yet been evaluated.The resistance status of D. ovis to pyrethroids can be determined in a laboratory bioassay suitable for any fast-acting contact insecticide ). This assay is not suitable for testing avermectins since these compounds are generally slow acting (Strong and Brown 1987) and more effective through ingestion than by contact action (Lasota and Dybas 1991). Hopkins (1970) successfully maintained an in vitro colony of D. ovis for more than 10 months on a wool/skin substrate held at 37°C and 68% RH. Here, we describe a bioassay modified by the inclusion of wool/skin substrate which is suitable for testing slow-acting compounds such as avermectins, present data on the laboratory efficacy of ivermectin and abamectin against D. ovis, and examine the toxicity of ivermectin to a pyrethroid-sensitive and a resistant population of D. ovis.