SUSCEPTIBILITY OF LABORATORY-SELECTED AND FIELD STRAINS OF THE LUCILIA-CUPRINA (DIPTERA, CALLIPHORIDAE) TO IVERMECTIN

Selection of a composite field strain of Lucilia cuprina (Wiedemann) w ith ivermectin for more than 60 generations resulted in a significant, though slight, increase in resistance (<10-fold). This resistance app eared to increase uniformly over the first 30 generations, then remain ed roughly constant. The resistance was not stable, and the strain rap idly reverted toward susceptibility in the absence of selection pressu re. The selected strain showed some cross-resistance to moxidectin (up to 5-fold). Larval monooxygenase activity (aldrin epoxidation) was el evated 2.9-fold in the selected strain compared with the unselected co ntrol. However, the significance of this elevation is unclear because higher monooxygenase activities were measured in field-collected strai ns showing no resistance to ivermectin. Pretreatment with piperonyl bu toxide 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 th e field-collected strains were similar to a laboratory-susceptible str ain even though the field strains all showed resistance to diazinon (u p to 28-fold). The low level of resistance in the selected strain sugg ests that there is no specific ivermectin resistance mechanism present in the L. cuprina field strains tested. Current mechanisms of insecti cide resistance in L. cuprina do not appear to confer any tolerance to ivermectin, despite the use of the compound for several years as an a nthelmintic for sheep.