Screening of High-Virulence and High-Yield Beauveria bassiana Strains for the Biocontrol of Cylas formicarius (Fabricius)

Cylas formicarius (Fabricius) is a major pest of sweet potato in Taiwan, where no microbial control agents are currently available. This study aimed to identify Beauveria bassiana strains with both high virulence and high spore productivity using a rice-based solid-state fermentation (SSF) system. Eleven field isolates were characterized using morphological traits and multilocus sequence analyses (Bloc, RPB1, RPB2, and TEF). Laboratory assays compared conidial production on 1/2 potato dextrose agar (PDA) and rice substrates and evaluated pathogenicity against adult C. formicarius by estimating median lethal time (LT50) and the onset of mortality (t0) based on cumulative mortality data. Substantial variation was observed among isolates in growth performance, sporulation, and virulence. Strain TyEf0054 consistently exhibited high pathogenicity, causing more than 80% mortality with conidia produced from both 1/2 PDA and rice, and showed rapid killing activity, with LT50 values of approximately 5.5–5.9 days and t0 values ranging from 3.55 to 3.99 days. In contrast, some strains exhibited high sporulation and virulence on 1/2 PDA but failed to produce conidia or showed severe clumping during rice-based solid-state fermentation, indicating poor suitability for large-scale production. These results demonstrate that high laboratory virulence alone is insufficient for strain selection and highlight the importance of integrating killing speed, final mortality, and SSF compatibility as joint selection criteria. Overall, TyEf0054 represents a promising locally adapted candidate for the development of commercial mycoinsecticides for sustainable management of sweet potato weevil in Taiwan.