ENTOMOPATHOGENIC FUNGI – Metarhizium anisopliae

ENTOMOPATHOGENIC FUNGI – Metarhizium anisopliae

General Characteristics of Metarhizium anisopliae

• Metarhizium sp. (Order: Hypocreales, Family: Clavicipitaceae), also known as Green Muscardine Fungi, have long been recognized for their biological control potential against Arthropods.
• As early as 1879, Metarhizium were being evaluated for control of Wheat Chafer Beetles, Anisoplia austriaca, and Sugar Beet Curculio, Cleonus punctiventris in Ukraine. The species level name of one of the more widely researched Metarhizium species (M. anisopliae) was derived from the Anisoplia austriaca beetle.
• Metarhizium anisopliae was also known as Entomophthora anisopliae in the early 1900s.
• The disease caused by the Metarhizium anisopliae is called Green Muscardine Disease because of the green colour of its spores.
• Strains of Metarhizium anisopliae have lost the capability of reproducing sexually.
• The Metarhizium genus comprises mostly Entomopathogenic fungi, some are generalists while some are specialists. The Metarhizium anisopliae is a generalist and is known to infect insects from more than seven orders, while Metarhizium acridum is a specialist as it only infects insects from the Acrididae family.

Habitat of Metarhizium anisopliae 

• Metarhizium species are commonly thought of as soil saprophytes and are most frequently found in disturbed habitats like agricultural fields as compared to forest ecosystems.
• Metarhizium anisopliae is a fungus that grows naturally in soils throughout the world and causes disease in various insects by acting as a parasite.
• Metarhizium anisopliae colonize in soils, plant roots as rhizosphere colonizers, other plant parts as endophytes and insect pests.
• Recent findings suggest that these fungi form associations with plant roots in the rhizosphere and survive better in that environment than in surrounding potting soil over extended periods of time.

Species of Metarhizium

• The Metarhizium genus was originally comprised of four varieties, which were Metarhizium anisopliae, Metarhizium taii, Metarhizium pingshaense, and Metarhizium guizhouense.
• In 2009, Bischoff, Rehner and Humber reclassified the Metarhizium genus into nine species, which are Metarhizium anisopliae, Metarhizium acridum, Metarhizium guizhouense, Metarhizium pingshaense, Metarhizium lepidiotae and Metarhizium majus, Metarhizium robertsii, Metarhizium brunneum and Metarhizium globosum.

Target Insects of Metarhizium anisopliae

• Metarhizium species are known to attack a wide range of arthropods: greater than 200 species in over 50 families.
• In particular, Metarhizium anisopliae is distributed worldwide and is a natural pathogen to coleopteran insects.
• Metarhizium anisopliae is being used as a biological insecticide to control a number of pests such as Grasshoppers, Termites, Thrips, Catterpillers and Aphids and its use in the control of Malaria-transmitting mosquitoes is under investigation.
• Additionally, Metarhizium species have been developed in other countries for use against Cockchafers, Spittlebugs, Grubs, Borers, and for control of Mosquitoes that vector Malaria. 

Mode of action of Metarhizium anisopliae

• Asexual conidia (singular - conidium) of Metarhizium anisopliae come into contact with the arthropod integument as the arthropod travels through the environment.
• The conidia stick to the insect’s exoskeleton, germinate and grow a germ tube, which eventually ends in an appressorium, the flattened and thickened tip of a germ tube.
• A penetration peg grows under the appressorium, pierces the integument and enters the hemocoel. The penetration of the fungus is achieved by the production of a cocktail of Hydrolytic enzymes (Proteases, Lipases and Chitinases), Insecticidal Cyclic Peptides (Destruxins) and Mechanical pressure.
• Single cells of the fungus, Blastospores, Bud off of the penetration structure, circulate in the insect hemocoel and multiply, thereby depleting host nutrients.
• Metarhizium sp. are also known to produce compounds that are toxic to Arthropod insects and presumably aid in killing the host, suppressing host immune defenses and fending off potential microbial competitors.
• Finally, after the host dies due to Mycosis, the fungus will penetrate out of the integument and grow conidiophores, on which environmentally stable aerial conidia are produced. These conidia are passively disseminated into the environment and eventually infect new hosts.

Figure – 1: Insects affected with Metarhizium anisopliae

Cultural characteristics of Metarhizium anisopliae

• In fungal culture medium, Metarhizium anisopliae grows as a Green mould.
• On most common fungal cultural media, Metarhizium anisopliae produces many dry, powdery conidia, initially produce white spore and later turns green colour.

Figure – 2: A) LPCB Staining of Metarhizium anisopliae; B) Colony morphology of Metarhizium anisopliae

Advantages of Metarhizium anisopliae 

• Non-toxic to beneficial insects, humans and animals.
• Used for Integrated Pest Management (IPM) by many organic farmers.
• Reduce the use of chemical insecticide sprays, which are extremely toxic and expensive.