Resistance of Rhipicephalus microplus to different acaricides in tropical climates: Are the laboratory and field results related?

Abstract

The aim of this study was therefore to investigate the relationship between laboratory (Larval Packet Test -LPT; Larval Immersion Test – LIT; Adult Immersion Test – AIT) and field studies using active ingredients or commercial formulations to control R. microplus in a tropical region. This comparative study was carried out with four populations of R. microplus from four Brazilian farms. For laboratory assays, technical grade compounds of cypermethrin and chlorpyrifos were used in the LPT, while fipronil and ivermectin were used in the LIT. The AIT was conducted using commercial spray formulations containing pyrethroid and organophosphate: cypermethrin 187.5 ppm + chlorpyrifos 375 ppm + fenthion 187.5 ppm and chlorpyrifos 825 ppm + High-Cis cypermethrin 99 ppm. For the field assays, the same commercial products of the AIT used, a pour-on formulation of fipronil 1 mg/kg, injectable 200 µg/kg ivermectin, injectable 630 µg/kg ivermectin, injectable 200 µg/kg doramectin and injectable 200 µg/kg moxidectin. For field studies, populations of R. microplus with the mean therapeutic efficacy ≤89 %, on days 7 up to 21 post-treatment, were classified as resistant to such compounds. To standardize the comparative analysis between laboratory and field results, this same value of larval efficacy or mortality (≤89 %) was used to classify the population as resistant by laboratory tests (LPT, LIT, and AIT). Of the 16 laboratory tests conducted using R. microplus larvae (LPT and LIT), 66.6 % showed no relation with field study results. Inconsistencies were observed in 100 % of cases for spray formulations, 25 % for fipronil, and 75 % for macrocyclic lactones. Although the efficacy results of the AIT with commercial formulations were slightly higher than the therapeutic efficacy observed in the field, it is important to note that there was a 100 % positive relation in the classification of the status of the four populations, which were all considered susceptible in both laboratory and field analyses. These findings demonstrate that, in situations like this study, it is essential to calibrate laboratory tests using larvae, particularly against field results, for each formulation. This approach will allow for more accurate recommendations regarding the use of a chemical formulation for a specific tick population. Furthermore, it reduces the risk of incorrectly identifying R. microplus population as resistant or susceptible and helps clarify the practical implications of resistance.