In vitro efficacy of amitraz and cypermethrin on Boophilus microplus from dairy farms in Lara State, Venezuela.

In Venezuela, the acaricides, amitraz and cypermethrin, are widely used to control tick infestations in cattle infested with Boophilus microplus. This work aims to determine the in vitro efficacy of amitraz and cypermethrin through the adult immersion test. A sample of 480 engorged females was collected from four dairy farms (180 ticks/farm) and divided in groups of 10 ticks averaging 220 mg/tick. Commercial preparation of amitraz and cypermethrin were used to prepare the working concentration of amitraz (52, 104, 208, 416 and 832 ppm), and cypermethrin (37.5, 75, 150, 300, 600 ppm). The control groups were submerged only in water. Ticks were placed in a glass lamina at 27 degrees C and 85% relative humidity (RH) and incubated for 18 days. The eggs masses were collected, weighed, separated in vials, and allowed to hatch at 85% RH and 27 degrees C for 21 days. Both the emerged larvae and eggs were quantified in each tube in order to determine the efficacy percentage and the resistance factor. The efficacy average to amitraz was 30.89%, 50.75%, 50.62%, 59.72%, and 60.53% at concentrations of 52, 104, 208, 416, and 832 ppm, respectively. The efficacy of cypermethrin was 14.3%, 11.82%, 17.25%, 20.23%, and 43.23% at concentrations of 37.5, 75, 150, 300, and 600 ppm, respectively. The amitraz resistance factor to each farm was 3.14, 1.19, 1.88, and 1.89, whereas with cypermethrin it was 8.89, 6.03, 7.12, and 18.45. Statistically significant differences between the effectiveness of different concentrations of amitraz or cypermethrin were not observed. Higher concentrations do not increase the efficacy of the products.