Spatiotemporal heterogeneity of Rhipicephalus microplus resistance to chemical acaricides at intra‐farm level: A case study using ivermectin

The aim of this work was to analyse the spatiotemporal heterogeneity of Rhipicephalusmicroplus (Canestrini, 1888) (Acari: Ixodidae) resistance to chemical acaricides atintra-farm level under different environmental (favourable and unfavourable areasfor tick development) and management (different schemes of acaricides applications)conditions using ivermectin as a model. The in vitro larval immersion test (LIT) wasused to determine quantitatively the levels of resistance to ivermectin in the differentpopulations and subpopulations of R. microplus analysed. In the first case study,differences in resistance levels among tick samples within the same paddock acrosstime and among tick samples from different paddocks were recorded. These resultsstress the importance of taking successive samples so that they represent the spatialand temporal variabilities in the levels of resistance that can occur within the samefarm. In a second case study, the evolution of resistance in R. microplus subpopulationssubjected to strategic and threshold control methods based on the applicationof three annual treatments with alternation of chemical groups was compared. Nochanges in resistance/susceptibility status were observed in both tick subpopulations.These results show that the application of a particular drug once a year withina scheme of alternation with other chemical groups could be an appropriate strategyto delay the development of resistance. Data of the third case study showed thatenvironmental constraints is not only a key modulator of R. microplus abundance butcould also affect the evolution of resistance in the tick populations. The decreasingtrend of LC50 values in the tick subpopulation not exposed to chemical treatmentsbut also in that subpopulation exposed to three annual chemical treatments underunfavourable environmental conditions allow us to hypothesise that environmentalconstraints can modify the levels of resistance in a tick population because it canconstitute a stronger selection factor than the treatments themselves.