Preservation of viability and virulence of Beauveria bassiana and Metarhizium anisopliae by formulation in a sesame 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.