The emergence of resistance to multiple acaricides in the cattle tick Rhipicephalus microplus (Acari: Ixodidae) in the northwestern Himalayan region of India: a study

Rhipicephalus microplus (Acari: Ixodidae) is a tick species of economic significance that afflicts cattle populations throughout the tropical and subtropical regions. The use of synthetic acaricides is the most widely used approach for tick control, but its indiscriminate and extended usage has led to selection of acaricide-resistant ticks and development of acaricide resistance. The present study aims to evaluate the acaricide resistance status against regularly used acaricide classes, viz., synthetic pyrethroids, macrocyclic lactones, and formamidines in R. microplus ticks from the northwest Himalayan region. Ticks collected from 12 districts of North Indian state of Himachal Pradesh were used for the conduction of the larval packet test (for deltamethrin and amitraz) and larval immersion test (ivermectin) for the assessment of resistance status. The resistance ratios (RR50 and RR95) values against deltamethrin, amitraz, and ivermectin showed that the field isolates had variable resistance status. On the basis of RR50 values, 11 isolates (all except Spiti) were resistant to deltamethrin, five field isolates (Bilaspur, Chamba, Kangra, Sirmaur, and Una) were resistant to amitraz, and all 12 isolates were resistant to ivermectin. Furthermore, three tick isolates (Bilaspur, Chamba, and Una) showed resistance to all three acaricides. The findings of this study suggest the emergence of multi-acaricidal resistance and it is the first study to document this phenomenon in R. microplus from the state of Himachal Pradesh, India. This research will provide new insights and the data obtained will contribute to the formulation of effective tick management strategies in the region. Acaricidal efficacies evaluated against synthetic pyrethroids (SPs), viz., deltamethrin and cypermethrin; macrocyclic actones (Ivermectin); and formamidine (amitraz) in R. (B.) microplus ticks infesting cattle of 12 districts located in the northwestern Himalayan region of indiaLC50, LC95, and resistance ratios along with the H value (heterogeneicity) were determined using mortality data.The results obtained through the laboratory bioassays revealed that isolates collected from one district were found to be having level I resistance, whereas isolates collected from other five districts were having level II resistance against deltamethrin.The resistance ratios (RR50 and RR95) values against deltamethrin, amitraz, and ivermectin showed that the field isolates had variable resistance status. On the basis of RR50 values, all isolates (except Spiti) were resistant to deltamethrin, five field isolates (Bilaspur, Chamba, Kangra, Sirmaur, and Una) were resistant to amitraz and all isolates were resistant to ivermectin. Furthermore, three tick isolates (Bilaspur, Chamba, and Una) showed resistance to all three acaricides.The first study conducted on the multi-acaricidal resistance status of cattle ticks in this region of north-western Himalayas Acaricidal efficacies evaluated against synthetic pyrethroids (SPs), viz., deltamethrin and cypermethrin; macrocyclic actones (Ivermectin); and formamidine (amitraz) in R. (B.) microplus ticks infesting cattle of 12 districts located in the northwestern Himalayan region of india LC50, LC95, and resistance ratios along with the H value (heterogeneicity) were determined using mortality data. The results obtained through the laboratory bioassays revealed that isolates collected from one district were found to be having level I resistance, whereas isolates collected from other five districts were having level II resistance against deltamethrin. The resistance ratios (RR50 and RR95) values against deltamethrin, amitraz, and ivermectin showed that the field isolates had variable resistance status. On the basis of RR50 values, all isolates (except Spiti) were resistant to deltamethrin, five field isolates (Bilaspur, Chamba, Kangra, Sirmaur, and Una) were resistant to amitraz and all isolates were resistant to ivermectin. Furthermore, three tick isolates (Bilaspur, Chamba, and Una) showed resistance to all three acaricides. The first study conducted on the multi-acaricidal resistance status of cattle ticks in this region of north-western Himalayas