Onstad et al. (2006) define a pathogen as a microorganism capable of producing disease under normal conditions of host resistance and rarely living in close association with the host without producing disease.” In mycology, the term “entomopathogenic fungus” (from Greek, entomon = insect, pathos = suffering, and gennaein = to produce) is used to describe a fungus that as part of its life cycle causes disease of a host insect. Consequently, if death of the infected host occurs, it does as a direct result of the disease caused by the pathogen (Fig. 5). This is contrary to infection by a parasite. Entomopathogenic fungi typically kill their host and go on to produce infective fungal spores on the host itself— these new spores then subsequently infect a new host. In this way the pathogen ensures its own propagation and dissemination. There are exceptions: fungi in the genus Strongwellsea represent an interesting outlier among entomopathogenic fungi. These are fly specialists and do not need to kill their host prior to spore dispersal. Strongwellsea fungi instead form an abdominal hole in the living fly host through which spores are actively discharged (Eilenberg, 2002; Eilenberg et al., 2020, 2022).
Death or survival of an insect host following infection by a pathogen is strongly correlated with pathogen features – host specificity, degree of virulence, life strategy – in combination with the host’s immune response to infection. Knowledge related to host specificity (i.e., generalist versus specialist) and life strategy (i.e., obligatory versus facultative) of entomopathogenic fungi is of utmost interest for ecological studies. Host specificity of entomopathogenic fungi ranges from specialists with extreme to moderate host specialization to generalists encompassing a broad host range. Life strategy is intrinsically linked to host specialization. Obligate pathogens are strictly dependent on their host for nutrients and for survival; this entails adaptations and counteradaptations from pathogen and host. Therefore, obligate pathogens have high host specificity. An extreme example is Entomophthora muscae sensu stricto of which certain isolates infect only a single fly species (Jensen et al., 2021). On the other end of the spectrum are facultative pathogens, those that do not need the host for survival and can obtain nutrients as saprotrophs from other sources. Beauveria bassiana is an example of a facultative pathogen. It infects hosts in multiple different orders and is able to persist in living plant tissues and soil (Meyling and Eilenberg, 2006; Meyling et al., 2009).

Epilogue

Scientific breakthroughs have challenged our understanding of parasites and pathogens throughout history, repeatedly changing their definitions accordingly. We acknowledge that the line between the definitions of parasites and pathogens is not always clear. Microbial pathogens that are biotrophic may be similar to parasites in lifestyle—deriving a benefit from their host without killing it. In this case, one might wonder about the difference between a multicellular parasite and a multicellular microbial pathogen. Rapid developments in the field of cell biology, molecular biology, and bioinformatics will continue to increase our knowledge about parasite–host and pathogen–host interactions. We would not be surprised if the definitions of parasites and pathogens continue to change into the future, including new interactions and excluding old ones.

Acknowledgements

P.K. and M.L. contributed equally to this book chapter. The Research Foundation – Flanders supported this work in part through a Fundamental Research PhD Fellowship (1142722N to M.D.d.G.) and a Junior Postdoctoral Fellowship (1206620N to D.H.). Dr. James Bradeen (University of Minnesota), Dr. Tommy L.F. Leung (University of New England), and Jeffery K. Stallman (Purdue University) are thanked for providing comments to earlier manuscript drafts of this chapter.

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