•Steinernema-Xenorhabdus-insect interaction is an ideal model to investigate symbiosis.•Insect virulence and reproductive fitness of these partners is tightly associated and codependent.•The level of specificity and dependence varies from facultative to obligate.•The insect host serves as the environment for mutualism maintenance. Entomopathogenic nematodes in the genus Steinernema (Nematoda: Steinernematidae) have a mutualistic relationship with Xenorhabdus bacteria (Gram-negative Enterobacteriaceae). This partnership however, is pathogenic to a wide range of insect species. Because of their potent insecticidal ability, they have successfully been implemented in biological control and integrated pest management programs worldwide. Steinernema-Xenorhabdus-insect partnerships are extremely diverse and represent a model system in ecology and evolution to investigate symbioses between invertebrates and microbes. The reproductive fitness of the nematode-bacterium partnership is tightly associated, and maintenance of their virulence is critical to the conversion of the insect host as a suitable environment where this partnership can be perpetuated.