Biodiversity and pathogenicity of bacteria associated with the gut microbiota of beet armyworm, Spodoptera exigua Hubner (Lepidoptera: Noctuidae)

In order to find an effective and environmentally friendly biocontrol agent against Spodoptera exigua, we isolated and identified a total of 15 different bacterial species belonging to phyla Firmicutes, Proteobacteria and Bacteroidetes. According to the phenotypic, genotypic and phylogenetic properties, bacterial isolates were identified as Bacillus cereus (Sel), Lysinibacillus macroides (Se2), Pseudomonas geniculata (Se3), Paenibacillus tylopili (Se4), Staphylococcus succinus (Se5), Acinetobacter soli (Se6), Chryseobacterium indologenes (Se7), Bacillus toyonensis (Se8), Serratia marcescens (Se9), Paenibacillus amylolyticus (Se10), Paenibacillus xylanexedens (Sell), Enterobacter ludwigii (Se12), Bacillus thuringiensis (Se13), Bacillus thuringiensis (Se14) and Lysinibacillus fusiformis (Se15). Screening of bacterial isolates for insecticidal potential was conducted at 10(9) cfu ml(-1) bacterial concentration. The highest larvacidal effect was obtained with Bacillus thuringiensis Se13 with 100% mortality. In the dose response experiments performed with this bacterium, the median lethal concentration (LC50) was estimated as 7.5 x 10(4) cfu ml(-1) against 3rd instar larvae of the pest at 10 days post treatment. The median lethal time (LT50) value of 10(9) cfu ml(-1) bacterial concentration was also determined as 1.59 days. Phase-contrast and scanning electron microscope studies exhibited that B. thuringiensis Se13 produced different shape and size crystals (bipyramidal, cubic and spherical). Phylogenetic analysis of cryl and cry2 gene content of this isolate displayed that B. thuringiensis Se13 had 99% homology with crylAc and cry2Aa, respectively. Finding from this study indicated that B. thuringiensis Se13 appears to be a promising microbial control agent for use against S. exigua.