Host-specific pathogenicity of Heterorhabditis bacteriophora in Galleria mellonella and Plodia interpunctella larvae

Entomopathogenic nematodes (EPNs) are useful as biological agents for the control of insect pests and disease vectors. They are also important for understanding nematode pathogenicity and host anti-nematode response. The pathogenic properties of EPNs towards insects are attributed to effector molecules, which are produced by both the nematodes and their symbiotic bacteria during infection. Here, we hypothesized that the contribution of the mutualistic bacterium Photorhabdus luminescens to the virulence of Heterorhabditis bacteriophora is host-dependent. To test this hypothesis, we compared the survival responses of two natural lepidopteran hosts, the greater wax moth Galleria mellonella and the Indianmeal moth Plodia interpunctella, following infection with either axenic H. bacteriophora (lacking P. luminescens) or symbiotic nematodes carrying their bacterial partner. We find that the presence of P. luminescens enhances the pathogenicity of H. bacteriophora towards G. mellonella larvae, whereas P. interpunctella larvae show broadly similar survival trajectories following infection with symbiotic or axenic nematodes, despite a shorter median survival time after symbiotic infection. These findings imply that EPNs may exert distinct infection strategies against different but related natural insect hosts, which may in turn activate differential immune mechanisms to deal with the combined threat of the nematodes and their symbiotic bacterial partners.