Integrated Biological Control Using a Mixture of Two Entomopathogenic Bacteria, Bacillus thuringiensis and Xenorhabdus hominickii, against Spodoptera exigua and Other Congeners

SIMPLE SUMMARY: An entomopathogen, Bacillus thuringiensis (Bt), has been used to control insect pests. On the other hand, insect immune responses defend the bacterial pathogenicity. An idea to enhance the Bt virulence was to inhibit insect immune defense using immunosuppressant. Secondary metabolites from another entomopathogen, Xenorhabdus hominickii, are known to inhibit the insect immune responses, but they alone give little virulence against insects. The addition of the bacterial culture broth containing the secondary metabolites to Bt spores significantly enhanced the Bt virulence in laboratory and field trials. This study demonstrates an integrated biological control by ideally combining two entomopathogenic bacteria. ABSTRACT: Insect immunity defends against the virulence of various entomopathogens, including Bacillus thuringiensis (Bt). This study tested a hypothesis that any suppression of immune responses enhances Bt virulence. In a previous study, the entomopathogenic bacterium, Xenorhabdus hominickii (Xh), was shown to produce secondary metabolites to suppress insect immune responses. Indeed, the addition of Xh culture broth (XhE) significantly enhanced the insecticidal activity of Bt against S. exigua. To analyze the virulence enhanced by the addition of Xh metabolites, four bacterial secondary metabolites were individually added to the Bt treatment. Each metabolite significantly enhanced the Bt insecticidal activity, along with significant suppression of the induced immune responses. A bacterial mixture was prepared by adding freeze-dried XhE to Bt spores, and the optimal mixture ratio to kill the insects was determined. The formulated bacterial mixture was applied to S. exigua larvae infesting Welsh onions in a greenhouse and showed enhanced control efficacy compared to Bt alone. The bacterial mixture was also effective in controlling other Spodopteran species such as S. litura and S. frugiperda but not other insect genera or orders. This suggests that Bt+XhE can effectively control Spodoptera-associated pests by suppressing the immune defenses.