Evaluation of delivery methods for the entomopathogenic fungus Beauveria bassiana in honey bee colonies for the control of Varroa destructor

Varroa destructor is a primary stressor contributing to decline of Apis mellifera populations. While V. destructor has been controlled using synthetic acaricides, it has developed resistance to active ingredients. Entomopathogens have emerged as promising alternative. This study assessed the efficacy of the entomopathogen Beauveria bassiana HF23 – a strain not previously reported as a biological control agent against V. destructor - using inert dust formulations with talc, corn starch and powdered sugar. Furthermore, the study evaluated both mite-efficacy and bee safety under laboratory conditions simulating a beehive environment. Conidia alone (7 × 10¹² conidia/g) demonstrated 97.1 ± 12.1 % efficacy against V. destructor, which was 63.6% higher than the untreated group (p = 0.03). Other mixed formulations (2.17 × 10¹¹ conidia/g) showed efficacy ranging from 42.73 to 55.85 ± 12.1%, though no significant differences were observed among them. Additionally, there was a significant difference in one-hour mite drop between conidia-only group and untreated, inert-dust treatment/control groups (p < 0.001). Bee mortality ranged from 14.1 to 29.2 ± 11.8%, with no significant differences between groups. In conclusion, B. bassiana strain HF23 demonstrated potential as a biopesticide against V. destructor under the conditions of this study.