Pathogenicity of the axenic entomopathogenic nematode Steinernema carpocapsae against Galleria mellonella and Spodoptera litura larvae

Pathogenicity of the entomopathogenic nematode Steinernema carpocapsae depends on its symbiotic bacterium Xenorhabdus nematophila. To understand the pathogenicity of the nematode itself, and its influence on insect tissues, we compared nematode invasion, insect mortality, nematode development in insects, and differences in the effects on the insect tissues between infection by axenic or monoxenic nematodes. Invasion of axenic infective juveniles (IJs) was lower than that of monoxenic IJs. Axenic as well as monoxenic nematodes killed both axenic and xenic Galleria mellonella and Spodoptera litura larvae, although it took a longer time for axenic nematodes to kill the insects, especially axenic ones. Axenic nematodes grew and reproduced in insects; however, their growth and reproduction were delayed as compared to that of monoxenic nematodes. Tissue destruction of axenic IJ-infected insect larvae was delayed as compared with monoxenic IJ-infected insect larvae, and suppression of insect hemolymph melanization was not observed in the axenic nematodes. These results suggest that S. carpocapsae is able to kill insects and partly degrade insect tissues in the absence of any bacteria; however, the pathogenicity of axenic nematodes is weaker than that of monoxenic nematodes.