Characterization and insecticidal potential of bacteria symbiotic with native Heterorhabditis sp. (Nematoda: Heterorhabditidae) against the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae)

The current study aimed to identify symbiotic bacteria associated with previously identified native entomopathogenic nematodes from southern Benin and evaluate their biocontrol potential on fifth instar larvae of Spodoptera frugiperda. Colony characteristics, bioluminescence capacity, and a molecular approach using the 16S rRNA gene were used to identify the symbiotic bacteria. Pathogenicity tests against fifth instar larvae of S. frugiperda were also conducted. The results indicated that all the eighteen bacterial isolates isolated from strains of Heterorhabditis sonorensis are identical (99% sequence similarity) to Photorhabdus isolate R-66822 (accession number MF353507). The symbiotic bacteria associated with the single strain of H. indica are identical (98% sequence similarity) to the Photorhabdus bacterial isolate R-52361 (accession number MF353497). With regard to pathogenicity tests, nine bacterial isolates were pathogenic to S. frugiperda larvae at 35°C, with insect mortalities varying between 0.24 and 78.47%. Better yet, seven of them could still kill the insects at an exposure temperature of 40°C, inducing insect mortalities ranging from 4.86 to 63.65%. Moreover, bacterial densities of 0.450 x 106 CFU/mL and 0.483 x 106 CFU/mL were sufficient for the bacterial isolates BSKassehlo and BSZoundomey, respectively, to kill 50% of the fifth instar larvae of S. frugiperda, whereas BSKissamey, BSAzohoue2, and BSZe1 needed 3.173 x 106 CFU/Ml and 2.900 x 106 CFU/mL, respectively. These findings are sufficiently encouraging to initiate further investigations into the potential of these native bacterial isolates for sustainable management strategies against S. frugiperda in agricultural areas.