Differences in Acaricide Susceptibility Between Parasitic Stages of Rhipicephalus microplus.

Acaricide resistance is a major challenge for the control of Rhipicephalus microplus, and differences between laboratory and field efficacy may complicate the assessment of resistance. However, the influence of parasitic developmental stages on acaricide susceptibility remains poorly understood. Metanymphs were historically considered an acaricide-resistant stage, although this assumption was later questioned. To re-evaluate this issue, three experiments were conducted. First, replicated field spray treatments were performed to evaluate mortality over time. Second, feeding chambers attached to cattle were used to expose ticks at different stages to acaricide. Third, partially engorged females were sprayed and surviving engorged females were evaluated for reproductive performance. Field mortality varied according to the developmental stages corresponding to each assessment day. In feeding chambers, mortality reached 83% in non-engorged nymphs and 90% in young adults, compared with 29% in engorged nymphs and 8% in controls. Females surviving treatment while partially engorged maintained high reproductive performance, whereas subsequent acaricide immersion markedly reduced hatchability and egg production. These findings demonstrate stage-dependent differences in acaricide susceptibility and indicate that developmental and engorgement status may contribute to discrepancies between laboratory and field efficacy tests.