Development of Biphasic Culture System for an Entomopathogenic Fungus Beauveria bassiana PfBb Strain and Its Virulence on a Defoliating Moth Phauda flammans (Walker)

<i>Beauveria bassiana</i> PfBb is a new strain with high host specificity to the target pest <i>Phauda flammans</i>. We conducted a series of experiments to optimize the biphasic fermentation system of <i>B. bassiana</i> PfBb by screening the medium compositions and fermentation environmental conditions in both liquid and solid fermentations. In the liquid fermentation, glucose and yeast extract with a C:N ratio of 17:1 were the optimal carbon and nitrogen sources, respectively, for <i>B. bassiana</i> PfBb mycelium growth and blastospore production, and liquid fermentation with an inoculation concentration of 1 × 10⁸/mL and an inoculum content of 50 mL conidial suspension, at 180 rpm/min rotation speed, pH 7 and 26 °C, favored mycelium growth. However, additional trace elements did not significantly improve liquid fermentation. In the solid fermentation, wheat bran and chaff at a ratio of 8:2 were identified as the best substrates that facilitated <i>B. bassiana</i> PfBb sporulation and conidial germination, and optimal substrates with 20% inoculum content, 50% water content, and 3-day fermentation in darkness had the highest conidia yield. The resulting conidia, stored at -20, 4, and 20 °C for one year, did not significantly change the water content, and with prolonged storage duration, conidial germination was significantly higher at -20 and 4 °C. Moreover, conidia stored at 4 °C for one year maintained its validity and virulence, which were toxic to all instar larvae of <i>P. flammans</i>. Our results provide essential support for the commercial production of <i>B. bassiana</i> PfBb-based biopesticides.