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

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.

Microbial degradation of plant toxins

“…One of the current MPC approaches relies on employing bacterial species as natural enemies against pests. The spores of an entomopathogen Bacillus thuringiensis ( Bt ) enhanced with bacterial culture broth suppressed the immune response of beet armyworm Spodoptera exigua (Hrithik et al, 2022). Further, Photorhadbys luminescens EGAP3 was found an effective biocontrol agent against the African migratory locust Locusta migratoria migratorioides (Muhammad et al, 2022).…”

Enhancement of an entomopathogenic fungal virulence against the seedcorn maggot, Delia platura, by suppressing immune responses with a bacterial culture broth of Photorhabdus temperata subsp. temperata

“…Entomopathogenic fungi and nematodes have been considered for use as biological controls through single or mixture treatment (Davidson and Chandler, 2005; Filgueiras et al, 2023). However, these traditional biological controls are not favored much by farmers due to their slow and unstable control efficacy along with their resistance to pathogens (Hrithik et al, 2022; Zhou et al, 2019).…”

Manipulation of GameXPeptide synthetase gene expression by a promoter exchange alters the virulence of an entomopathogenic bacterium, Photorhabdus temperata temperata, by modulating insect immune responses

“… Shi et al (2022) suggested that GXP-A has inhibitory activity against insect immunity because it inhibits hemocyte-spreading behavior and a cellular immune response measured by nodule formation. This immunosuppressive activity of GXP-A was applied to enhance the insecticidal activity of a commercial biopesticide, Bacillus thuringiensis , against a lepidopteran insect, Spodoptera exigua because insect immunity is one of the infection barriers to bacterial pathogenicity ( Hrithik et al, 2022 ). GXP is produced by the catalytic activity of an NRPS called GXP synthetase ( gxpS ) because the heterologous expression of gxpS in Escherichia coli produces GXPs ( Nollmann et al, 2015 ).…”