Recognition in 1963 in central Queensland of the Ridgelands strain of Boophilus microplus (Can.), resistant in varying degrees in laboratory tests to all acaricides commercially available in Queensland, Australia, led to spraying trials on cattle with these acaricides to assess their effect on the resistant strain. Dosage mortality tests on larvae exposed in acaricide-impregnated filter papers, indicated the following levels of resistance, carbary 13×, carbophenothion 17×, coumaphos 1.6×, diazinon 19×, dioxathion 7.2× and ethion 2.7×.
Stalled cattle infested with either susceptible Yeerongpilly or resistant Ridgelands ticks were sprayed with the recommended concentration of commercially formulated acaricide. Efficiency of treatment was determined by comparing the number of ticks falling from sprayed and unsprayed cattle. Coumaphos (0.025%) and ethion (0.075%) produced efficient control of the resistant strain, but carbophenothion (0.086%), diazinon (0.05%) and dioxathion (0.075%) allowed considerable numbers of the resistant strain of ticks to survive. Carbaryl (0.3%) as a melt-type formulation gave inconsistent results and possible reasons are discussed. Poor control of the resistant strain was largely due to failure to kill the nymph stage.
There was general correspondence between the resisdence level determined in laboratory tests and the degree of control achieved in spraying trials. However, the reletively small difference between resistence levels of 2–3× (coumaphos and ethion) and 7× (dioxathion) was critical in the practical control of resistant ticks on cattle.
The following chemicals gave excellent control of the Ridgelands strain in spraying trials on stalled cattle: bromophos (0.15%), bromophos ethyl (0.1%), chlorfenvinphos (0.06%), chlorxylam (0.5%), Ciodrin (0.1%) Dursban (0.025%), fenchlorphos (0.2%), GS13002 (0.05%), Imidan (0.075%), methidathion (0.05%) and SD8448 (0.025%). Malathion (0.5%), prothidathion (0.075%), macarbam (0.075%) and RD12308 (0.2%) controlled the Yeerongpilly but not the Ridgelands strain. Arsenic (0.2%) produced about equal mortality in both strains, but was clearly a less efficient acaricide than organophosphorus chemicals.