Laboratory bioassay was used to determine the pathogenicity of nine isolates of fungi of the genera Metarhizium, Beauveria and Clonostachys to the parasitic mite, Varroa destructor. All nine isolates were pathogenic to V. destructor with Metarhizuim anisopliae UAMH 9198, Clonostachys rosea UAMH 9161 and Beauveria bassiana GHA being the most pathogenic within their respective species. Metarhizium anisopliae UAMH 9198 was more lethal to V. destructor than B. bassiana GHA and C. rosea UAMH 9161 with LC50 values of 1.6 x 10(5), 9.6 x 10(6) and 5.4 x 10(6) conidia/mL, respectively. Metarhizium anisopliae and B. bassiana significantly affected brood and adult honey bee survivorship and their immune responses. They were lethal to the bees with LC50s of 3.70 x 10(6) and 2.62 x 10()5 conidia/mL, respectively. The effect of temperature and thymol on conidia germination, production and colony growth of the fungal isolates was determined. Temperature significantly affected conidia germination, production and colony growth (P < 0.05) but thymol did not. Efficacy of the two most promising isolates (M. anisopliae UAMH 9198 and B. bassiana GHA) as potential bio-control agents against V. destructor in hives was evaluated. Fungal inocula were applied as dry formulation, with corn flour as carrier, using dispenser tray and dusting applications. Treatments were either applied alone or in combination with thymol to determine any synergistic effects. All treatments significantly increased mite mortality (P < 0.05), however, the mite control efficacy varied between fungal treatments and application methods. Combined treatments of fungi and thymol caused significantly higher mite mortality than single fungal treatments, which showed control levels of ≤61%. Significant differences in mite mortality were found between the two delivery methods with the differences depending on the fungal isolate. The results suggest that M. anisopliae UAMH 9198 would be a more effective bio-control agent for the management of V. destructor in honey bee colonies than the other isolates tested when dispensed continuously in hives using delivery methods such as a dispenser tray. However, future research is needed to improve delivery methods and investigate the effect of carriers used in the formulation on the efficacy of such entomopathogenic fungi. / Ontario Ministry of Agriculture and Food (OMAF), University of Guelph