Isolation and Identification of Entomopathogenic Fungus GC23620 and Its Virulence and Control Efficacy Against Gynaephora qinghaiensis Larvae

In June 2023, a larva of grassland caterpillar Gynaephora qinghaiensis naturally infected by an entomopathogenic fungus was collected from an alpine rangeland in Gangcha County, Haibei Tibetan Autonomous Prefecture, Qinghai Province. After laboratory isolation and cultivation, the pathogen was identified as Beauveria bassiana (designated as GC23620) based on morphological characteristics and ITS-rDNA sequence similarity analysis. The larvicidal efficacy of B. bassiana GC23620 against fourth-instar larvae of G. qinghaiensis were assessed using two inoculation methods in laboratory conditions. The infection process and pathogenicity were analyzed by simulation and parameter estimation using the Time–Dose–Mortality (TDM) model. The estimated parameters for the concentration effect of strain GC23620 (β) were 0.56 (leaf dipping method) and 0.30 (insect immersion method), respectively. After treatment with conidial suspensions (1.05 × 105 to 1.05 × 109 conidia/mL), the cumulative corrected mortalities were 72.73–100.00% (leaf dipping method) and 42.42–90.91% (insect immersion method) at 8 days after inoculation (DAI), and the median lethal doses (LD50) decreased to 1.74 × 103 conidia/mL (leaf dipping method) and 1.85 × 104 conidia/mL (insect immersion method), respectively, during the same post-inoculation period. After inoculation with conidial suspension under a concentration of 1.05 × 106 conidia/mL, the median lethal times (LT50) were 2.40 (leaf dipping method) and 4.51 days (insect immersion method). A control efficacy of 84.27% was obtained for G. qinghaiensis larvae on grassland at 21 days post-treatment after spraying the fermentation solution with a low dose of 1.05 × 105 conidia/mL. In conclusion, B. bassiana strain GC23620 exhibited high pathogenic activity against G. qinghaiensis larvae and has strong potential for the green control of grassland pests.