These include the fungal strain, the host's physiological state, nutrition, defense mechanisms, cuticle and epicuticular microorganisms. In addition, the development of epizootics is controlled by many diverse factors such as the environment as well as the density and dispersion of both host and pathogen populations. Fungi with narrow host ranges are generally obligate parasites, are often responsible for epizootics and are generally highly virulent. Such fungi are prime candidates for use in classical or inoculative biological control. In contrast, fungi with wide host ranges are generally facultative pathogens, ubiquitous soil saprophytes, enzootic and relatively weakly pathogenic and are prime candidates for inundative control.
Most risk assessments rely heavily on laboratory bioassay results.
Such bioassays are useful when determining release of exotic fungi with narrow host ranges. Host ranges of such fungi are already quite well known and it is not difficult to verify specificity under laboratory conditions. However, the value of such host range testing is dubious for fungi with wide host ranges. As facultative parasites, such fungi usually infect any invertebrate, given adequate inoculum levels and laboratory stressed hosts. If detrimental effects on non-target organisms are detected with indigenous fungi, then all one has to do is either stop using the pathogen or alter its use.