Fungi That Infect Insects: Altering Host Behavior and Beyond

Approximately 1,000 species of the fungal phyla Microsporidia, Chytridiomycota, Entomophthoromycota (order: Entomophthorales), Basidiomycota, and Ascomycota are known to infect and kill insects [1]. Of these, species such as Beauveria bassiana and Metarhizium robertsii (Ascomycota: Hypocreales) are well-studied models for exploring the mechanisms of fungus–insect physiological interactions, and they are used as biological controls for insect pests [2]. Entomopathogenic Hypocreales are phylogenetically closely related to plant pathogens and endophytes [3], and their sexual stages belong to Cordyceps sensu lato [4]. However, these species are taxonomically diverse and differ from each other considerably in their genomic features [5–8] and, consequently, their host ranges, infection cycles, and life strategies. Fungal infection normally, but not always, results in the death of an insect in situ. The parasitic fungi such as the host-specific pathogen Ophiocordyceps unilateralis sensu lato can control insect brains and manipulate their behavior to reach death locations that are optimal for spore dispersal, the so-called fungal extended phenotype [9]. On the other hand, on top of physiological immune surveillance, insects (especially social insects such as ants and termites) can smell and avoid fungal pathogens, groom each other to clear pathogenic spores, generate a fever response, or die well away from their nestmates, which is termed behavioral or social immunity [10,11]. Either side of these interactions will benefit from behavioral changes in insects that maximize their adaptive fitness. In this paper, both types of insect behavior alternations are reviewed, and the underlying mechanisms are discussed.