Entomopathogenic nematodes (EPN) are insect parasites that depend on the limited resources available within the host for development and reproduction. They carry symbiotic bacteria that aid in killing the host and provide nutrition for the nematodes. Several generations of EPN are produced within a single host and when the resources are depleted stress-resistant, free-living infective juveniles (IJs) leave the insect cadaver in search of a new host. This thesis focuses on how EPN of the genera Heterorhabditis and Steinernema defend resources within the host cadaver from interspecific and intraspecific competitors, scavengers, and abiotic factors. Steinernema males fight and kill male competitors in interspecific competition for females. In this thesis, it is shown that S. carpocapsae males that have mated are better killers than unmated males, independent of size or sexual development. IJs of the two genera can co-occur geographically, but species from one genus will usually dominate over the other genus when found within the same host. This thesis demonstrates that this dominance is mainly due to failure of one genus to develop in a host infected by the other genus, and not due to a lack of attraction to infected hosts. The insect cadaver is an attractive source of nutrition for scavengers. Heterorhabditis associates with the bioluminescent bacteria Photorhabdus. Here it is shown that the bioluminescence produced by Photorhabdus helps to deter scavengers from feeding on Heterorhabditis-infected cadavers. The host cadaver protects EPN from abiotic stresses. It is demonstrated here that damage to the host’s cuticle, imitating scavenging, leads to desiccation of the cadaver, and reduces both the quantity and size of IJs that emerge from the host. This effect is more severe for Heterorhabditis than for Steinernema, presumably due to the slower development of Heterorhabditis within the host. Heterorhabditis are better than Steinernema at deterring scavengers from feeding on the cadaver, preventing damage from occurring. This thesis demonstrates the various defensive mechanisms adapted by Steinernema and Heterorhabditis, highlighting their distinct phylogenies and the relative importance of their symbionts.