Factors influencing the selection of Rhipicephalus microplus resistance to acaricides

Rhipicephalus microplus is a hematophagous ecto-parasite of cattle that has a cosmopolitan distribution in tropical and subtropical regions of the world. This tick species causes economic losses by consuming blood and reducing meat and milk production, as well as facilitating the transmission of pathogens. Different classes of acaricides, including organophosphates, synthetic pyrethroids, insect growth regulators, formamidines, phenylpyrazoles, and macrocyclic lactones, are currently used sequentially or simultaneously to control R. microplus infestations. The frequent use of these acaricides has led to the development of resistant ticks worldwide. This is a result of tick exposure to chemical acaricides and the subsequent survival and further spread of ticks that are less affected by acaricides. Although acaricide resistance is difficult to prevent, the main factor that influences its appearance and increase in a farm is the high frequency of acaricide use, especially with products with the same mode of action. Other factors that favor the selection of R. microplus for resistance to the main acaricides are the biology of ticks, genetics of ticks, characteristics of acaricides, operational practices, and environmental effects. Despite numerous reports on factors influencing the selection of acaricide-resistant R. microplus, these have not been comprehensively addressed. This review discusses and summarizes the main factors that favor the selection of resistant ticks, aiming to understand the genetic selection process for resistant individuals and implement measures to mitigate this phenomenon for the sustainable control of this tick species, which affects cattle worldwide.