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 Rhipicephalus microplus (Canestrini, 1888) (Acari: Ixodidae) resistance to chemical acaricides at intra-farm level under different environmental (favourable and unfavourable areas for tick development) and management (different schemes of acaricides applications) conditions using ivermectin as a model. The in vitro larval immersion test (LIT) was used to determine quantitatively the levels of resistance to ivermectin in the different populations and subpopulations of R. microplus analysed. In the first case study, differences in resistance levels among tick samples within the same paddock across time and among tick samples from different paddocks were recorded. These results stress the importance of taking successive samples so that they represent the spatial and temporal variabilities in the levels of resistance that can occur within the same farm. In a second case study, the evolution of resistance in R. microplus subpopulations subjected to strategic and threshold control methods based on the application of three annual treatments with alternation of chemical groups was compared. No changes in resistance/susceptibility status were observed in both tick subpopulations. These results show that the application of a particular drug once a year within a scheme of alternation with other chemical groups could be an appropriate strategy to delay the development of resistance. Data of the third case study showed that environmental constraints is not only a key modulator of R. microplus abundance but could also affect the evolution of resistance in the tick populations. The decreasing trend of LC₅₀ values in the tick subpopulation not exposed to chemical treatments but also in that subpopulation exposed to three annual chemical treatments under unfavourable environmental conditions allow us to hypothesise that environmental constraints can modify the levels of resistance in a tick population because it can constitute a stronger selection factor than the treatments themselves.