Susceptibility of Laboratory-Selected and Field Strains of the Lucilia cuprina (Diptera: Calliphoridae) to Ivermectin

Abstract

Selection of a composite field strain of Lucilia cuprina (Wiedemann) with ivermectin for more than 60 generations resulted in a significant, though slight, increase in resistance (<10-fold). This resistance appeared to increase uniformly over the first 30 generations, then remained roughly constant. The resistance was not stable, and the strain rapidly reverted toward susceptibility in the absence of selection pressure. The selected strain showed some cross-resistance to moxidectin (up to 5-fold). Larval monooxygenase activity (aldrin epoxidation) was elevated 2.9-fold in the selected strain compared with the unselected control. However, the significance of this elevation is unclear because higher monooxygenase activities were measured in field-collected strains showing no resistance to ivermectin. Pretreatment with piperonyl butoxide caused only a slight increase in toxicity of ivermectin to the selected strain, whereas triphenyl phosphate and tridiphane showed no significant synergism. The concentration responses to ivermectin in the field-collected strains were similar to a laboratory-susceptible strain even though the field strains all showed resistance to diazinon (up to 28-fold). The low level of resistance in the selected strain suggests that there is no specific ivermectin resistance mechanism present in the L. cuprina field strains tested. Current mechanisms of insecticide resistance in L. cuprina do not appear to confer any tolerance to ivermectin, despite the use of the compound for several years as an anthelmintic for sheep.