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.

Prehistoric fox-human interactions in South American caves could have shaped nutrient-rich habitats, favourable to sand fly immatures development. However, surviving on iron-rich and decomposing prey carcases or carnivore feces might be challenging and favour few species. Lutzomyia longipalpis is a cave sandfly adapted to cities, suggesting a pre-adaptation to human-polluted habitats. We tested substrates expected to be found in fox dens or ancient human trash mounds, as female oviposition and immature feeding choices. Further, we described general patterns of oviposition behaviour, to test positive feedback of a favourable microhabitat with previous laid eggs. Females were exposed to six substrate smells: fresh dog feces; rotten dog feces; fresh hairless mice; rotten hairless mice; forest litter; distilled water. Then, to test density-dependence in oviposition, gravid females were left in recipients for five days and changed to clean ones, under different quantities. We also simulated oviposition choices under distinct egg densities, using dodecanoic acid. Immatures were exposed to these substrates to test choice and ability to fully develop on them. The substrates more chosen for oviposition were canid flesh feces, decomposed mouse, water, and flesh mouse respectively. Number of laid eggs per female decreased with female density. On average of 43% eggs were retained and laid later when surviving females were transferred to clean recipients. Early emerged immatures showed no preference for substrates while late instars joined gradually onto decomposed corpses until pupating. The patterns of oviposition choice and immature survival are compatible with the hypothesis of a long-term adaptation of this species.