Potencial entomopatogênico de bactérias simbióticas Xenorhabdus associadas a nematoides entomopatogênicos contra Spodoptera frugiperda

Abstract

The fall armyworm, Spodoptera frugiperda, is a polyphagous pest that threatens food security by causing serious crop damages. This pest is traditionally controlled with synthetic insecticides or transgenic plants expressing Bt toxins. However, the evolution of resistance to these control methods has driven the search for more effective, safe, and sustainable alternatives. In this context, entomopathogenic nematodes (EPN) of the genus Steinernema and their symbiotic bacteria of the genus Xenorhabdus have emerged as promising agents for the biological control of insect pests. The aim of this study was to evaluate the entomopathogenic potential of Xenorhabdus symbiotic bacteria associated with entomopathogenic nematodes against S. frugiperda. Initially, the pathogenicity of different EPN species was assessed in larvae of the pest. Subsequently, the symbiotic bacteria were isolated and characterized through phenotypic analyses, biochemical tests, recA gene sequencing, and phylogenetic analysis. In addition, bacterial growth curves were determined both in vitro and in vivo, and the entomopathogenic activity of bacterial cell suspensions and cell-free supernatants was evaluated by injection and oral administration. Molecular analyses identified the bacteria associated with S. carpocapsae and S. brazilense as Xenorhabdus nematophila and X. doucetiae, respectively. Although the strains exhibited distinct growth patterns, both showed high virulence when injected into S. frugiperda larvae. Bacterial cell suspensions caused higher mortality than cell-free supernatants, although the latter also exhibited insecticidal activity. These findings confirm the entomopathogenic potential of Steinernema-associated symbiotic bacteria and highlight their relevance as sources of bioactive metabolites for the development of novel biological control strategies compatible with integrated pest management programs.