Evaluation of fifteen adjuvants for compatibility with Beauveria bassiana and their synergistic efficacy against Rhipicephalus microplus Ticks

The compatibility of adjuvants with entomopathogenic fungi (EPFs) is a critical determinant of biopesticide efficacy. This study evaluated the effects of fifteen commercial adjuvants on the viability and virulence of Beauveria bassiana isolate Bb-41, as a potential biological control agent against the cattle tick Rhipicephalus microplus. Using three concentrations (0.025 %, 0.05 %, and 0.1 %), colony-forming unit (CFU) counts were used to assess conidial viability at two inoculum levels (10³ and 10⁴ conidia·mL⁻¹). In addition larval mortality was measured at 10⁶ conidia·mL⁻¹. The results demonstrated that adjuvant effects were both concentration- and dose-dependent. Adjuvants such as Designer®, Bond®, Summit Super®, Aquawet®, and Tronic® significantly inhibited fungal viability across all concentrations, likely due to the presence of fungitoxic components. In contrast, non-ionic surfactants like Tween® 80, Ballista®, Break-thru®, Silwet l-77®, and Tween® 20 maintained or enhanced conidial viability, particularly at 0.025 %. Virulence assays revealed that only a subset of adjuvants that promoted viability also enhanced pathogenicity. It was suggested that, Break-thru®, Tween® 20, and Ballista® enhanced conidial adhesion and penetration, as they improved larval mortality at 0.025 %. These findings highlight the importance of comprehensive compatibility screening in EPFs formulation development and identify several promising adjuvants for field deployment against R. microplus.