Resistance to acaricides in the cattle tick Rhipicephalus microplus (Acari: Ixodidae) from dairy farms in the Triângulo Mineiro region, Minas Gerais, Brazil.

Rhipicephalus microplus is a major ectoparasite of cattle in tropical and subtropical regions and poses a serious threat to animal welfare and productivity. Chemical acaricides remain the main method of control in Brazil. Nevertheless, resistance to these compounds has become increasingly widespread. Minas Gerais is the leading milk-producing state in Brazil, yet data on acaricide resistance in one of its key dairy-producing region, the Triângulo Mineiro, remain scarce. This study evaluated the resistance of R. microplus populations from 15 dairy farms from nine municipalities in the Triângulo Mineiro using the larval packet test (LPT) against five commonly used acaricide compounds: cypermethrin, chlorpyrifos, fipronil, ivermectin, and amitraz. All tick populations showed high resistance to cypermethrin. High resistance to chlorpyrifos and fipronil was observed in 80 % and 67 % of farms, respectively. Resistance to ivermectin and amitraz was more variable but still present in most populations. Resistance to at least two compounds was detected on all farms. The most frequently used products on surveyed farms were based on pyrethroids and organophosphates, including unapproved or off-label compounds. These findings indicate a critical need for strategic, evidence-based tick control programs in the region and regular resistance monitoring to guide the selection of effective acaricides. These results provide an updated overview of acaricide resistance an important dairy region of Brazil and highlight the importance of coordinated efforts to slow down resistance development.