Comparative phenotypic characterization of acaricidal resistance in Rhipicephalus microplus and Hyalomma anatolicum field populations from Marathwada region of Maharashtra state in India.

The Marathwada region, comprising eight districts of Maharashtra state in India, was surveyed to explore the current phenotypic resistance picture of Rhipicephalus microplus and Hyalomma anatolicum ticks against commonly applied synthetic acaricides (deltamethrin and cypermethrin) and organophosphate (coumaphos). The phenotypic resistance was identified in the larvae of ticks through larval packet test (LPT). The increasing resistance prevalence against synthetic pyrethroid chemicals in tick populations provides a major challenge to the livestock sector. Results indicate a high resistance level to deltamethrin in R. microplus, with resistance factors ranging from level II to IV across various isolates. In contrast, H. anatolicum exhibited type I resistance in select districts. In case of R. microplus, deltamethrin exhibited the highest LC50 value in the Jalna isolate (2254.27 ppm) and the lowest in Hingoli (206.34 ppm) isolate persisting six isolates under level IV resistance. Cypermethrin showed comparatively lower level resistance levels, with LC50 values from 488.12 to 1010.62 ppm. All the isolates were found susceptible to coumaphos at the baseline concentration against both tick species. It is important that the resistance profiles determined in the laboratory study are indicative and should be validated with further field-based studies. Biochemical assays revealed elevated α- and β-esterase activity in Dharashiv and Parbhani isolates of R. microplus, whereas GST activity was variable in different populations. However, there was no significant correlation between enzyme activities and resistance ratios (RR50 and RR95) in the two tick species, advocating the presence of alternative resistance mechanisms. These results highlight the passive role in tick management and require immediate action on the development of alternative strategies in the Marathwada region.