Abstract: 【Objective】 To screen highly effective microbial biocontrol strains against Plutella xylostella.【Methods】 The strains of Serratia nematodiphila（FQ268,LJ232,YC310,and YC278） were subjected to physiological and biochemical characterization. High-activity strains were screened using the leaf-dip method,followed by antibiotic susceptibility testing and insecticidal virulence determination against P. xylostella.【Results】 All four S. nematodiphila strains were Gram-negative. The methyl red test,indole test,cellulose degradation test,and lipase activity test were negative,while the catalase test and citrate utilization test were positive. All four strains could utilize glucose and mannitol to produce acid but could not utilize fructose and inositol. The high-activity strain identified was S. nematodiphila FQ268,which was susceptible to ceftriaxone,gentamicin sulfate,and norfloxacin,but resistant to rifampicin,cefradine,penicillin,erythromycin,and tetracycline. The fermentation broth of S. nematodiphila FQ268（5.58×10⁹ CFU/mL） exhibited LC₅₀ values of 1.742×10⁹ and 2.566×10⁷ CFU/mL against second-instar larvae of P. xylostella at 48 and 72 h,respectively.【Conclusion】 S. nematodiphila FQ268 shows high insecticidal activity against second-instar larvae of P. xylostella and represents a relatively efficient microbial biocontrol strain.

Key words: Serratia nematodiphila, Plutella xylostella, Strain screening, Virulence determination