Standardization of larval bioassays for determination of fluralaner susceptibility in Rhipicephalus microplus

Fluralaner is an isoxazoline acaricide with potent activity against Rhipicephalus (Boophilus) microplus, a major tick species affecting cattle in South America. In this study, larval bioassays were performed to evaluate the baseline susceptibility of R. microplus populations from Brazil and Argentina to fluralaner. The larval packet test (LPT) was applied to Brazilian laboratory and field populations, while the larval immersion test (LIT) was used in Argentina. Both tests were carried with technical grade and a commercial formulation of fluralaner. LPT results showed consistent LC₅₀ values across two susceptible laboratory strains, with resistance ratios near unity in all field samples. Similar observations were obtained with the LIT, with resistance ratios also near 1 for all tested populations from Argentina. The LIT produced LC₅₀ values approximately 100 times lower than the LPT, a difference consistent with previous studies using systemic acaricides. Both technical and commercial formulations yielded comparable results in both tests. These findings confirm the high susceptibility of R. microplus to fluralaner and establish reliable bioassay parameters for future resistance monitoring. The choice between LPT and LIT and technical grade or commercial formulation of fluralaner should be guided by laboratory infrastructure and logistical needs, as both methods are valid for surveillance purposes.