Response surface methodology-based optimization of culture conditions of Cordyceps javanica with effects on virulence and protective enzyme activity of Spodoptera litura

Abstract:

[Objective] To optimize the soliD-state fermentation conditions of Cordyceps javanica Bd01, determine the virulence of the conidia to the 3rd-instar larvae of Spodoptera litura, and study the changes of protective enzyme activities in S. litura after infection by C. javanica. [Methods] The optimum soliD-state medium and culture conditions of C. javanica Bd01 were first determined by single factor experiments and then optimized by Box-Behnken response surface method. The virulence of the conidia to the 3rd-instar larvae of S. litura was determined by the immersion method, and the changes of enzyme activities in the larvae were determined by a spectrophotometer. [Results] With the spore yield as an indicator, the sporulation conditions of C. javanica were optimized as medium nutrient content of 30.24 g/L, pH 7.55, and light duration of 12.06 h. Under these conditions, the spore yield reached 2.78×10⁸ spores/mL. The conidial suspension of C. javanica at the concentration of 1×10⁸ spores/mL was virulent to the 3rd-instar larvae of S. litura. After seven days of treatment, the LT₅₀, LC₅₀, and corrected mortality rate were 3.11 d, 4.68×10⁵ spores/mL, and 88.68%, respectively. The activities of protective enzymes in the survived larvae of S. litura after treatment were significantly different from those in the control group. [Conclusion] The optimized culture medium can significantly increase the spore production of C. javanica. The lethal rate and lethal efficiency of C. javanica to S. litura larvae were dependent on the concentration. The protective enzymes in S. litura larvae may play a key role in the defense against the infection of C. javanica.