Evaluation of the entomopathogenic fungi Beauveria bassiana GHA and Metarhizium anisopliae UAMH 9198 alone or in combination with thymol for the control of Varroa destructor in honey bee ( Apis mellifera ) colonies

The efficacy of two entomopathogenic fungi, Beauveria bassiana GHA and Metarhizium anisopliae UAMH 9198 for controlling Varroa destructor in honey bee colonies was determined during early fall in a northern climate. Fungal treatments were applied in hives as dry conidia mixed with corn flour either alone, or in combination with a thymol treatment. The treatments were applied using two delivery methods, dispenser tray and duster. The combined treatments of fungi and thymol showed the highest miticidal efficacy (>82%) but these treatments were not significantly different to thymol only treatments for mite control. Of the fungal treatments applied alone, M. anisopliae UAMH 9198 delivered by dispenser tray caused significantly higher mite mortality than the control treatments (62%, p < 0.0001). Treatments of B. bassiana GHA applied alone also had a significantly higher rate of miticidal efficacy relative to controls, but the rates of efficacy were relatively low (41–53%). There were significant interactions between fungal treatment and delivery method. M. anisopliae UAMH 9198 delivered by tray was significantly more effective at controlling mites than when applied with a duster, whereas the opposite occurred for B. bassiana GHA.