Resistance status of synthetic pyrethroids in Rhipicephalus microplus of Indore district, Madhya Pradesh, India.

An investigation was carried out to record the status of resistance against most commonly used synthetic pyrethroids, viz. deltamethrin and cypermethrin in Rhipicephalus microplus of Indore district (Madhya Pradesh). Fully engorged female R. microplus were collected from cattle sheds of four blocks of Indore district, viz. Indore, Hatod, Sanwer and Mhow. Ticks were kept individually for oviposition and eggs laid were allowed to hatch and larval packet test (LPT) was performed on 12 to 14 day-old-larvae with deltamethrin and cypermethrin. The slope of mortality, lethal concentration for 50% (LC50), 95% (LC95) mortality and resistance ratios (RR) were determined from the regression graphs of probit mortality of ticks plotted against log concentrations of deltamethrin and cypermethrin. Values of the coefficient of determination (R2) for LPT ranged from 0.94 to 0.96 and 0.93 to 0.99 for cypermethrin and deltamethrin, respectively indicating the statistical model a good fit. The LC50 and LC95 values for cypermethrin ranged from 49.31 to 117.21 and 193.23 to 683.7 ppm, respectively in various field isolates whereas, for deltamethrin were 10.71–23.80 and 45.26–194.98 ppm. The values of RR50 and RR95 for cypermethrin ranged from 0.2 to 0.48 and 0.55 to 1.94, respectively, showing variable resistance status in field populations. On the basis of RR50 values, susceptible status was recorded in all isolates whereas, level I resistance status was recorded in two field isolates (Sanwer and Mhow) on the basis of RR95. Similarly, for deltamethrin RR50 and RR95 ranged from 0.91 to 2.01 and 1.27 to 5.49, respectively, showing comparatively higher resistance status. Resistance status against deltamethrin was recorded in Mhow isolate (on RR50 basis) and Mhow, Sanwer and Hatod isolate (on RR95 basis). High level of resistance along with more number of resistant tick isolates were recorded in case of deltamethrin than that of cypermethrin from the study area. [ABSTRACT FROM AUTHOR]