In Venezuela, the acaricide amitraz is widely used to control tick infestations in cattle, and therapeutic failures in the control of the Boophilus microplus
species have been detected. The aim of this work was to determine the in vitro efficacy of amitraz on B. microplus
using five different concentrations, including the manufacturer’s recommended dosage (208 ppm), through the adult immersion test (Drummond test). A sample of 240 engorged females was collected from dairy farms in Lara state, (60 ticks/farm), divided in groups of 10 ticks averaging 220 mg/tick. Commercial amitraz formulation (12.5%) was used to prepare the working concentrations of 52, 104, 208, 416 and 832 ppm. The control group was submerged in water. Ticks were placed in glass lamina and incubated at 27ºC and 85% R.H for 18 days. The eggs masses were collected, weighted, separated in vials and allowed to hatch at 85% R.H. and 27º C for 21 days. Both the emerged larvae and eggs were quantified in each tube in order to determine the reproductive efficiency, efficacy percentage and the resistance factor. The four populations of B. microplus
studied were resistant to amitraz. Statistical significant differences between the effectiveness of highest concentration of amitraz and that recommended by the manufacturer were not observed.