Enhanced virulence of Beauveria bassiana against Thrips tabaci by the addition of bacterial metabolites derived from Xenorhabdus hominickii

Abstract

• Immunosuppressive agents are supposed to enhance the virulence of microbial pathogens against insect pests. • The culture broth of Xenorhabdus hominickii inhibited immune responses of Thrips tabaci . • The addition of the bacterial broth to Beauveria bassiana enhanced the fungal insecticidal activity. Xenorhabdus and Photorhabdus are two bacterial genera specifically symbiotic to Steinernema and Heterorhabditis , which are the entomopathogenic nematode genera, respectively. These bacteria are well known to produce potent secondary metabolites suppressing insect immune responses. This study aimed to develop a potent microbial insecticide against the onion thrips, Thrips tabaci , using the bacterial metabolites. Among the chemical insecticides that have been used to control the thrips, spinosad was highly effective against both larvae and adults of T. tabaci . Three different entomopathogenic fungi were also effective to kill the thrips. However, the fungal virulence was much less than the control efficacy of the chemical insecticide, spinosad. To enhance the fungal virulence of Beauveria bassiana (Bb), the bacterial culture broth of Xenorhabdus / Photorhabdus was added to suppress the thrips immune defense. Among six different bacterial species, X. hominickii (Xh) produced highly potent metabolites to enhance the fungal virulence. Indeed, four different bacterial metabolites (GameXPeptide, benzylideneacetone, oxindole, and 3-ethoxy-4-methoxyphenol) of the bacteria suppressed the gene expressions of an antimicrobial peptide, lysozyme , which was highly inducible to the fungal infection. To optimize the mixture ratio of fungal and bacterial pathogens, the fungal conidia and bacterial culture broth were freeze-dried and mixed in different ratios. Laboratory and field assays showed that a mixture spray of freeze-dried Xh culture broth (3 g) and Bb conidia (1.17 × 10 9 conidia) in a liter was effective to control T. tabaci infesting welsh onion.