Isolation and evaluation of bacteria associated with entomopathogenic nematode Heterorhabditis spp. against the spider mite, Tetranychus truncatus Ehara (Acari: Tetranychidae)

Abstract

Abstract Background Entomopathogenic nematodes (EPNs) belonging to the Heterorhabditis spp. harbour symbiotically associated bacteria which are toxic to a wide range of insect pests. Isolation, purification, characterization and mass multiplication of such bacteria will be a promising strategy in the management of the pests. This study was carried out to isolate the EPN from different locations, isolate and purify the bacterial colonies, characterize the bacteria through morphological and molecular strategies and to test the efficacy of different bacteria in the control of polyphagous Tetranychus truncatus Ehara mites. Results EPNs were isolated from soil samples at 11 localities of Kerala State, India, and used to infect the Galleria mellonella L. larvae. Bacteria associated with the haemolymph of the infected larvae were isolated, which on NBTA medium have produced circular to irregular, entire, opaque and smooth colonies. Sequence characterization of the 16S rRNA revealed nine isolates namely: one symbiotic bacterium Photorhabdus luminescens, two Pseudomonas aeruginosa, five Ochrobactrum sp. and one Stenotrophomonas maltophilia. Phylogenetic analysis using the sequences has further confirmed the bacterial identity. Evaluation of the cell suspension (CS) and cell-free supernatant (CFS) of P. luminescens, P. aeruginosa and Ochrobactrum sp. for their adulticidal and ovicidal efficiencies on T. truncatus had identified significant adulticidal effects by P. luminescens, followed by P. aeruginosa. After 96 h of treatment, P. luminescens at 108 cells/ml resulted in a significantly higher mortality rate of adult mites (64.00 and 60.67%, respectively, for CFS and CS), compared to that resulted by P. aeruginosa (38.67 and 33.33%). Conclusions Results of this study showed that P. luminescens associated with the EPN Heterorhabditis spp. is a promising biocontrol agent for T. truncatus.