The nematode-bacterium complex kills insects so rapidly that the nematodes do not form the intimate, highly adapted, host-parasite relationship characteristic of other insect-nematode associations, e.g., mermithids. This rapid mortality permits the nematodes to exploit a range of hosts that spans nearly all insect orders a spectrum of activity well beyond that of any other microbial control agent. In laboratory tests, S. carpocapsae alone infected more than 250 species of insects from over 75 families in 11 orders. The nematodes attack a far wider spectrum of insects in the lab where host contact is assured, environmental conditions are optimal, and no ecological or behavioural barriers to infection exist. For example, foliage feeding lepidopteran larvae are highly susceptible to infection in petridishes, but are seldom impacted in the field, where nematodes tend to be quickly inactivated by the environmental extremes (i.e., desiccation, radiation, temperature) characteristic of exposed foliage. Although EPNs possess an extremely broad laboratory host range, ecological and behavioural barriers restrict their natural host range. Temporal and spatial factors, such as synchronous life cycles and differing habitat preferences, also appear to determine the host rang . For example New Zealand populations of S. feltiae occur at the basis of tussock grass, where they have become adapted to parasitize lepidopteran larvae feeding on the roots of these grasses. These strains of S. feltiae are ineffective parasites of native scarabaeid larvae living in the same habitat. Behavioural barriers also restrict nematode efficacy to a few selected hosts or host groups. Some species are sit-and -wait strategists or ambushers that tend to stay near the soil surface where their specialized foraging behaviours (nictation, jumping) allow them to efficiently infect mobile host species (e.g., S. carpocapsae and S. scapterisci). Other species are widely searching foragers or cruisers that distribute themselves actively throughout the soil profile and are well adapted to infecting sessile or less mobile hosts (e.g., H. bacteriophora and S. glaseri). Most known species appear to be situated somewhere along a continuum between these two extremes (e.g; S. riobrave, S. feltiae). Cruisers, which are highly mobile, respond strongly to long-range host chemical cues and are therefore best adapted to find sedentary hosts.

Host recognition is a vital step in the life cycle of most parasites. In plant and animal parasitic nematodes, host recognition consists of a sequence of behavioural responses to an array of stimuli associated with host or host-related materials. The strategy that EPN employ to find hosts varies between species. Nematodes rely to some extent upon environmental cues for orientation toward, recognition of, or assessment of potential hosts. Ambushers and cruisers differ in their responses to host associated cues. Contact cues (host faeces or cuticle) are short range and provide specific information about the host. Volatile host cues are long range and usually provide general, directional information for host location. Cruisers have strong attraction to volatile cues containing CO2 whereas ambushers rely on host movement for successful parasitization and be relatively unresponsive to chemical host cues until after the host is located. Location of sedentary host in soil matrix necessitates the use of chemical cues, thereby exerting considerable selection pressure against individuals unable to follow chemical gradients to find hosts.