Resistance to all chemical groups of acaricides in a single isolate of Rhipicephalus microplus in Mato Grosso do Sul, Brazil

The aim of the present study was to evaluate acaricide resistance in Rhipicephalus microplus on a ranch in Brazil. Adult immersion tests were performed using commercial formulations containing organophosphorus compounds (OPs), amidines, synthetic pyrethroids (SPs), OP+SP combinations, and fluazuron. The larval immersion test was used to evaluate susceptibility to ivermectin and fipronil. Resistance was detected to all chemical classes with efficacies of 3.2% (Amitraz 12.5 g), 34.2% (Dichlorvos 45 g + Cypermethrin 5 g), 14.6% (Chlorpyrifos 5 g + Cypermethrin 2 g), 72.9% (Deltamethrin 2.5 g), 60.3% (Cypermethrin 15 g), 70.2% and 51.5% (Cypermethrin 15 g + Chlorpyrifos 25 g + Citronella 1 g), 20.5% (Dichlorvos 60 g + Chlorpyrifos 20 g), 0% (Cypermethrin 15 g), 46.2% (Ethion 2 g + Chlorpyrifos 10.6 g + Cypermethrin 6.2 g). Only two associations, one containing two OPs + one SP and another with a high concentration of OP + SP exhibited efficacy of 100% and 96.3%, respectively. Resistance to fluazuron, ivermectin (Resistance ratio [RR] = 3.08), and fipronil (RR = 3.18) was also detected. The present results indicate resistance to all classes of an existing contact or systemic acaricides and combinations of pyrethroids and organophosphates in an isolate of R. microplus.