Preservation of viability and virulence of entomopathogenic fungi, Beauveria bassiana and Metarhizium anisopliae by formulation in an oil invert emulsion

Using Ephestia kuehniella larvae as hosts, invert emulsion (IE) and an aqueous suspension (AS) of Beauveria bassiana (isolate IR34-JS2) and Metarhizium anisopliae (isolate IR41-TT1) formulations were compared when stored for three months at either 3 °C or 25 °C. Initially, IE formulations of isolates JS2 and TT1 were 13- and 14-fold more virulent than AS formulations, based on their LC50 values. JS2 LC50 values increased 1.3- and 5.6-fold for the AS and IE formulations, respectively, after 3 months storage at 3 °C, and 1.6 and 7.3-fold after 3 months at 25 °C, whereas values for TT1 increased 2.1- and 6.8-fold for these formulations at 3 °C, and 3.9- and 15.7-fold at 25 °C. Formulation and storage temperature interacted significantly to affect conidial production post-storage. Vegetative growth, conidiogenesis, and conidial germination of both isolates were better preserved at 3 °C than at 25 °C in vitro. The IE formulation preserved vegetative growth capacity better than the AS formulation at both temperatures, and better preserved conidiogenesis at 3 °C, a temperature that also improved TT1 conidial germination in the IE formulation. At 3 °C, the IE formulation yielded a relative speed of kill that was 3.5- and 1.3-fold faster for the JS2 and TT1 isolates, respectively, than the AS formulation. It can be concluded that IE formulations were superior to AS formulations, and that storage at 3 °C will help preserve the biological activity and pathogenicity of these fungi.