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Entomopathogenic fungi are parasitic unicellular or multicellular microorganisms belonging to the kingdom of Fungi, that can infect and seriously disable or kill insects. Entomopathogenicity is not limited to a particular class of fungi and is found in six divisions in the fungal kingdom (Ascomycota, Oomycetes, Basidiomycota, Chytridiomycota, Zygomycota, and Microsporidia).[1] Some fungal entomopathogens are opportunistic whereas some have evolved into highly specific pathogens of insects.[1]
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.[2] 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.[3] Some entomopathogenic fungi have evolved ability to manipulate insect behavior by hijacking insect nervous systems using various secondary metabolites[4]
Entomopathogenic fungi perform a vital ecological role in natural environments by controlling insect populations. 19 out of 30 insect orders have fungal pathogens.[1][4] Because of this ability, they are used in agriculture as biocontrol agents/bioinsecticides and are considered to a sustainable alternative to chemical pesticides.[5]
Typical life cycle and general biology
Green peach aphid, Myzus persicae, killed by the fungus Pandora neoaphidis (Zygomycota: Entomophthorales) Scale bar = 0.3 mm
Entomopathogenic fungi do not form a monophyletic group. Fungi from different lineages can have different life cycles and differences can be seen even within lineages, between species, or even between isolates within a species.[5] Because of this high diversity and contrasting differences between fungi from different lineages, it is difficult to define a detailed typical lifecycle that is common to all entomopathogenic fungi.
However, use of microscopic spores (usually asexual or Conidia) dispersed throughout the environment to initiate the infection process is common across all known entomopathogenic fungi. These spores commonly use Hydrophobins and Adhesins to attach to and recognize the host cuticle.[3] Under the right conditions of temperature and (usually high) humidity, these spores germinate, grow as hyphae, and colonize the insect's cuticle; which they bore through by way of enzymatic hydrolysis, reaching the insects' body cavity (hemocoel). Then, 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.[5]
Groups
Cordyceps fungi are parasitoids of various arthropod species. Here is a wasp parasitized by the fungus Cordyceps.
Since they are considered natural mortality agents and environmentally safe, entomopathogenic fungi for biological control of insects have been studied for more than 100 years. In particular, the asexual phases of Ascomycota (Beauveria spp., Isaria spp., Lecanicillium spp., Metarhizium spp., Purpureocillium spp., and others) are under scrutiny due to traits favouring their use as biopesticides.[6] The development of entomopathogens as pesticides depends on research into their host specificity, stability, formulation, and methods of application.
Production
Most entomopathogenic fungi can be grown on artificial media. Some require complex media, while others, like Beauveria bassiana and exploitable species in the genus Metarhizium, can be grown on starch-rich substrates such as rice or wheat grains.
Virulence
Entomophthorales are often reported as causing epizootics (outbreaks with many deaths) in nature. These fungi are virulent. The anamorphic Ascomycota (Metarhizium, Beauveria etc.) are reported as causing epizootics less frequently in nature.
Host relationship chemical cues
Entomopathogenic fungi such as Beauveria bassiana and Metarhizium spp. successfully infect susceptible host populations through conidia.[7][8] The signaling cues between these fungi and their host targets are under investigation. The ability to sense these parasites can increase fitness for the host targets. Evidence suggests that signal recognition occurs within some hosts, but not others. For example, the ectoparasiteCephalonomia tarsalis is susceptible to B. bassiana but it cannot detect the presence of free conidia of this fungus or infected hosts.[9] Because they cannot detect these parasites, either the host or the host's offspring become infected and/or die. In contrast, termites detect and avoid some lethal conidia strains.[10] Other soil-dwelling insects have evolved the ability to detect and avoid certain entomopathogenic fungi.[citation needed]
^Strasser H, Vey A, Butt TM (28 June 2010). "Are There any Risks in Using Entomopathogenic Fungi for Pest Control, with Particular Reference to the Bioactive Metabolites of Metarhizium, Tolypocladium and Beauveria species?". Biocontrol Science and Technology. 10 (6): 717–735. doi:10.1080/09583150020011690. S2CID83558063.
^Vandenburg, J.D., Ramos, M. & Altre, J.A. (1998) Dose –response and age- and temperature-related susceptibility of the diamondback moth (Lepidoptera: Plutellidae) to two isolates of Beauveria bassiana (Hyphomycetes: Moniliaceae). Environmental Entomology, 27, 1017–1021.
^Hughes, W.O.H., Petersen, K.S., Ugelvig, L.V., Pedersen, D., Thomsen, L., Poulsen, M. et al. (2004) Density-dependence and within-host competition in a semelparous parasite of leafcutting ants. BMC Evolutionary Biology, 4, 45.
^Lord, J.C. (2001) Response of the wasp Cephalonomia tarsalis (Hymenoptera: Bethylidae) to Beauveria bassiana (Hyphomycetes: Moniliales) as free conidia or infection in its host, the sawtoothed grain beetle, Oryzaephilus surinamensis (Coleoptera: Silvanidae). Biological Control, 21, 300–304.
^Myles, T.G. (2002) Alarm, aggregation, and defense by Reticulitermes flavipes in response to a naturally occurring isolate of Metarhizium anisopliae. Sociobiology, 40, 243–255.
