D3367: Evaluation of delivery methods for the entomopathogenic fungus Beauveria bassiana in honey bee colonies for the control of Varroa destructor

Varroa destructor is a major stressor contributing to the global decline of honey bee populations. Although synthetic acaricides have been widely used for Varroa mite control, the mites have developed resistance to many active ingredients worldwide. Currently registered chemical pesticides also present challenges due to their toxicity to bees and temperature-dependent performance. In contrast, entomopathogenic fungi have recently emerged as promising biological control alternatives.
In our preliminary cage study, we evaluated the efficacy of entomopathogenic fungus Beauveria bassiana strain HF23 against Varroa mites and its safety to honey bees. Treatment with B. bassiana spores (7 × 10¹² spores/g) achieved an average efficacy of 94% against V. destructor, which was 57% higher than in the untreated control group (p < 0.005). Importantly, no significant difference in bee mortality was observed between treated and control groups, indicating that the formulation was safe for bees.
Based on these results, we further examined spore delivery patterns and persistence in honey bee colonies using B. bassiana stained with fluorescein to optimize its delivery and efficacy.  The fungus infects hosts by attaching to the cuticle, germinating, and penetrating the integument, ultimately causing host death. Among tested application methods, application to the top of the hive provided superior spore distribution across bees, whereas application through the entrance on the bottom resulted in poorer coverage. However, spore persistence declined rapidly with all treatment methods, within 24 hours, likely due to the bees’ intensive grooming behavior.