Determination of discriminating dose and evaluation of amitraz resistance status in different field isolates of Rhipicephalus (Boophilus) microplus in India.

Field tick isolates of Rhipicephalus (Boophilus) microplus were collected from eleven districts located in the northern and eastern states of India to access the resistance status to "Amitraz". Adult immersion test was optimized using laboratory reared acaricide susceptible IVRI-I line and minimum effective concentration was determined as 487.7 ppm with 95 % confidence interval of 455.8-521.8. The discriminating concentration was determined as 975.4 ppm and was tested on female ticks collected by two stage stratified sampling from organized dairy farms and villages. Based on three variables, viz.,mortality, egg masses and reproductive index, the resistance level was categorized.Resistance to amitraz was detected at level I in 3 isolates (RF = 1.56-5.0), at level II in 6 isolates (RF = 9.3-23.3) and at level III in 1 isolate (RF = 27.3) whereas one isolate was found susceptible. The highest resistance was found in the SKR isolate (RF = 27.3) and minimal resistance was detected in the N-24P isolate (RF = 1.56). These experimental data will help in designing tick control strategy which is suffering from acaricide failure and to overcome development of resistance in ticks.