Entomopathogenic fungus

Entomopathogenic fungi are parasitic unicellular or multicellular microorganisms belonging to the kingdom of Fungi, that can infect and seriously disable or kill insects.

Pathogenicity for insects is widely distributed in the kingdom of fungi and occur in six fungal phyla (Ascomycota, Oomycetes, Basidiomycota, Chytridiomycota, Zygomycota, and Microsporidia). It plays a vital ecological role in controlling insect populations by impacting 19 out of 30 known insect orders. Some fungal entomopathogens are opportunistic whereas some have evolved into highly specific pathogens of insects.

Unlike many other insect pathogens (entomopathogenic viruses, nematodes, or bacteria), most entomopathogenic fungi do not require entry through ingestion or oral intake and instead directly attack the insect cuticle and penetrate the insect body through the exoskeleton. These fungi use a broad spectrum of virulence factors such as adhesins (to attach to insect cuticles), lytic enzymes (to hydrolyze insect cuticles), and secondary metabolites.

Entomopathogenic fungi show a diversity of lifecycles, with differences across lineages, as well as within lineages, between species, and even between isolates within species. Some fungal entomopathogens are opportunistic whereas some have evolved into highly specific pathogens of insects.

Entomopathogenic fungi all typically disperse through the environment through the use of microscopic spores (usually asexual or Conidia) that commonly use hydrophobins and adhesins to attach to and recognize the host cuticle. Germination is environmentally triggered under specific temperature and humidity conditions. Upon germination, many filamentous fungal pathogens begin growing hyphae and colonize the insect's cuticle. Some also produce structures called appressoria. These appressoria (also produced by plant pathogenic fungi) apply mechanical force onto the insect cuticle to support entry of the pathogen into the insect body.

Fungi penetrate the insect exoskeleton by boring through it using enzymatic hydrolysis. When the infection reaches the insects' body cavity (hemocoel), the fungal cells proliferate in the host body cavity, usually as walled hyphae or in the form of wall-less protoplasts (depending on the fungus involved).

Regardless of whether the infecting fungus is a host generalist or a specialist, the infection usually leads to the death of the insect, upon which the fungus emerges and sporulates on the dead insect. Sporulation usually takes place on the external surface of the cadaver. However, it can also occur on the internal surfaces of the cadaver depending on the environmental humidity. Some fungi keep the insect cadavers attached to foliage using rhizoids to ensure that they remain in the same environment where it is more likely to encounter suitable new hosts.

Entomopathogenic fungi differ in virulence. Some (usually those with a broad spectrum of hosts - host generalists) immediately kill the insect using various toxins (e.g. Metarhizium robertsii) and grow on the dead insect body, digesting it for nutrients. Others (usually fungi that have evolved host-specificity) keep the host alive longer and progress onto systematically invading and infecting host tissues (e.g. Ophiocordyceps spp.). Some host specialist entomopathogenic fungi have even evolved mechanisms of behavioral manipulation (e.g. Ophiocordyceps unilateralis - Zombie ant fungus) of their hosts. These fungi hijack the insect nervous systems using various secondary metabolites and manipulate insect behavior to move the infected insect to a place which is appropriate for the fungus to grow, sporulate and acquire new hosts.