Influence of Subculture of Beauveria bassiana on Virulence and Spore Production of Spodoptera litura Larvae and its Strain Rejuvenation

Abstract

【Objective】The study aimed to investigate the impact of continuous subculture of Beauveria bassiana on the virulence and spore production of second-instar larvae of Spodoptera litura. Additionally, a study on strain rejuvenation was conducted to offer references for the biological control of B. bassiana against S. litura.【Method】The primary strain, designated as F0 (SCAUJH19), was inoculated on PDA medium, and subsequent generations (F1~F10) were obtained through ten consecutive subcultures. The second-instar larvae of S. litura were exposed to 1×108 CFU/mL B. bassiana treatment by insect dipping method. The virulence of different sub-cultured strains against second-instar larvae was determined, and spore production for each generation was quantified. A model was constructed to analyze the relationship between virulence, spore production and successive subcultures in B. bassiana strains and laws of virulence degeneration. Live insect rejuvenation method was implemented on strains from generations F4, F6, F8 and F10, followed by a comprehensive comparison and analysis of the virulence effects on second-instar larvae before and after rejuvenation, along with measuring their spore productions.【Result】No significant difference in virulence was observed among F1~F5 subcultures of B. bassiana strains, with mortality rates ranging from 42% to 45% when treat for 7 d. However, the mortality rate of F6, F8 and F10 strains against S. litura decreased to 37%, 22.94% and 12.17%. Spore production analysis indicated that F1~F5 strains maintained a spore quantity of above 9.65~10.43×108 CFU/mL, whereas spore quantities of F6, F8 and F10-generation strains decreased to 7.62×108 CFU/mL, 5.14×108 CFU/mL and 4.69×108 CFU/mL. The constructed model revealed an exponential decrease in spore production with subcultures from the F5 generation, with a positive correlation between spore production and virulence. Strains with fewer subculture generations exhibited higher spore production and greater pathogenicity to second-instar larvae of S. litura. After the rejuvenation of F4, F6, F8 and F10 strains with live insect rejuvenation method, the corrected mortality of the treated strains of S. litura second-instar larvae remained above 50%, surpassing the highest mortality of the original strains, and the spore production of the rejuvenated strains exceeded that of the pre-rejuvenated strains.【Conclusion】After 4 to 5 generations of subculture, B. bassiana demonstrates a gradual decrease in spore production and virulence against second-instar larvae of S. litura and strain rejuvenation is needed. The sub-generation of B. bassiana could be effectively restored or improved to its original characteristics through the live insect rejuvenation method. Spore production of the same strain can be served as a valuable reference index for evaluating its virulence.

Keywords

• beauveria bassiana
• second-instar larvae of spodoptera litura
• rejuvenation
• virulence
• spore production quantity
• corrected mortality rate