Effective medium for Photorhabdus luminescens bioinsecticide production and exploration of optimal mixture with Bacillus thuringiensis subsp. aizawai against Plutella xylostella

Plutella xylostella is a major pest of cruciferous vegetables worldwide. We evaluated a biopesticide using different culture treatments of Photorhabdus luminescens (ATCC 29,999) and Bacillus thuringiensis subsp. aizawai and examined their insecticidal activity against Pl. xylostella. L8 and L9 Taguchi orthogonal designs were adopted to evaluate the optimal culture medium for enhanced insecticidal activity. Feeding bioassays were conducted to determine the potential synergistic effects of combining B. thuringiensis and Ph. luminescens, and the most effective synergistic combination was identified by analyzing the time–mortality response against Pl. xylostella. Analysis of the pH value indicates that Ph. luminescens has enhanced tolerance to acidic conditions and can alkalize the medium. Among the various medium combinations, Ph. luminescens optimized using A3B1C3N2—d10 × concentration, 24-h cultivation time, 1.25 % molasses as the carbon source, and a nitrogen source consisting of yeast extract powder, yeast powder, and amino acids (0.2:0.3:0.5 %)—resulted in the highest insect mortality. Synergistic interactions between B. thuringiensis and Ph. luminescens were observed in the 1:1 mixture, likely due to immune system suppression. These results highlight the potential of combining Ph. luminescens and B. thuringiensis to prepare a potent bioinsecticide against Pl. xylostella and other lepidopteran pests, thereby decreasing the reliance on chemical pesticides.