Exploring new acaricidal compounds for environmental control of Amblyomma sculptum (Brazilian spotted fever agent): An in vitro and semi-field comparative study with Rhipicephalus microplus.

Amblyomma sculptum is the main vector of Rickettsia rickettsii, the etiological agent of Brazilian spotted fever (BSF), a relevant zoonosis due to the increasing number of human deaths. This study evaluated the efficacy of seven commercial products against A. sculptum larvae, with comparative assays also conducted for Rhipicephalus microplus. Formulations were first evaluated using the larval packet test (LPT), then in semi-field trials in Urochloa (Syn. Brachiaria) brizantha, Panicum maximum, and Cynodon spp. forages, and last in an ecotoxicity assay. In the LPT, LC99 values for the products ranged from 0.0024 to 1.39 mg/mL for A. sculptum, and from 0.0093 to 0.56 mg/mL for R. microplus. The efficacy rates against A. sculptum in Urochloa were 93.1 %, 96.4 %, and 97.0 % for lambda-cyhalothrin, thiamethoxam + lambda-cyhalothrin, and abamectin, respectively. For R. microplus, the efficacies were 80.8 %, 95.1 % and 75.2 % for lambda-cyhalothrin, 89.8 %, 97.9 % and 88.8 % for thiamethoxam + lambda-cyhalothrin, and 40.7 %, 56.0 % and 6.2 % for abamectin, in Urochloa, Cynodon, and Panicum, respectively. Climatic conditions were similar across forages, favouring larval survival, except for the effect of rainfall on A. sculptum. In ecotoxicity assays, lambda-cyhalothrin and thiamethoxam + lambda-cyhalothrin had mild effects on recovery, growth and fertility of Caenorhabditis elegans. Thiamethoxam + lambda-cyhalothrin offers short-term larval control under semi-field conditions and is another option for managing A. sculptum in areas at risk of BSF. Further studies are required to assess efficacy against nymphs and adult ticks, environmental impacts, effects on non-target organisms, and residual activity, to ensure the sustainable use of these acaricides.