ENTOMOPATHOGENIC FUNGI – Beauveria bassiana

ENTOMOPATHOGENIC FUNGI – Beauveria bassiana

General Characteristics of Beauveria bassiana

• Beauveria bassiana (also called White Muscadine Fungi) is an entomopathogenic fungi that causes White Muscadine Disease in a range of insects.
• Beauveria bassiana belongs to fungal Sub-division: Deuteromycotina and Order: Hyphomycetes. It has a simple life cycle with no known sexual stage; the asexual spores are called conidia.
• The genus Beauveria contains at least 49 species of which approximately 22 are considered pathogenic.
• Species within the genus Beauveria are typically differentiated from other fungi by morphological characteristics. They are filamentous fungi that produce colorless (hyaline) aerial conidia from conidiogenous cells freely on the mycelia.
• Beauveria bassiana are able to reproduce either sexually or asexually. The production of multiple spore types increases the chances of survival during adverse environmental conditions. These spore types can be produced in response to environmental conditions, as well as at different times in the life cycle and can have different dispersal mechanisms.
• Beauveria bassiana is a naturally occurring fungus in soils and has been researched for control of soil borne insects.
• Many soil insects, however, may have a natural tolerance to this pathogen, which is not exhibited in many foliar pests. Therefore, commercial development of this fungus for biological control has primarily been targeted against foliar feeding pest.
• Beauveria sp. produces a number of metabolites some of which have Cytotoxic effects. These metabolites include Beauvericin, Bassianolide, Beauveriolides, Bassianin, Tenellin and Oosporein.

History of Beauveria bassiana

• The species is named after the Italian Entomologist Agostino Bassi, who discovered it in 1815 as the cause of the Muscardine disease which then led to carriers transmitting it by airborne means.
• Beauveria bassiana was formerly also known as Tritirachium shiotae.

Target Insects

• Mole cricket, Chiggers, White grubs, Fire ants, Ants, Flea beetle, Boll weevil, Whiteflies, Plant bug, Grasshoppers, Thrips, Aphids, Mites and many others.
• Its use in the control of Bedbugs and Malaria-transmitting Mosquitoes is under investigation

Mode of action of Beauveria bassiana

• When spores of this fungus come in contact with the cuticle (skin) of susceptible insects, they germinate and grow directly through the cuticle to the inner body of their host.
• The fungus proliferates throughout the insect's body, producing toxins (Beauvericin) and draining the insect of nutrients, eventually killing it.
• Unlike bacterial and viral pathogens of insects, Beauveria and other fungal pathogens infect the insect with contact and do not need to be consumed by their host to cause infection.
• Once the fungus has killed its host, it grows back out through the softer portions of the cuticle, covering the insect with a layer of white mold (hence the name white muscadine disease). This downy mold produces millions of new infective spores that are released to the environment.

Figure – 1: Insect affected with Beauveria bassiana

Cultural characteristics of Beauveria bassiana

• Beauveria is ubiquitous in plant debris and soil and may be isolated from foodstuffs, infected insects and indoor air environment. It has a wide host range of insects and is common in nature.
• Beauveria bassiana colonies grow relatively slowly and can appear powdery or wooly, with colors ranging from white to yellow and occasionally pinkish.
• Aerial hyphae are septate, smooth, hyaline and about 2 μm wide. Submerged hyphae are similarly structured, but larger (1.5 – 3 μm).
• On most common cultural media, it produces many dry, powdery conidia in distinctive white spore balls.
• Each spore ball is composed of a cluster of Conidiogenous cells.
• The Conidiogenous cells of Beauveria bassiana are short and ovoid to spherical, and terminate in a narrow apical extension called a Rachis.
• The Rachis elongates after each conidium is produced, resulting in a long zig-zag extension. The conidia are single-celled, haploid, and hydrophobic.

Figure – 2: Colony morphology of Beauveria bassiana in Sabouraud’s Dextrose Agar

Advantages of Beauveria bassiana

        Beauveria bassiana is considered to be one of the most effective entomopathogenic fungi for various reasons including

• Cosmopolitan distribution.
• Ability to infect any life stage of its host.
• Wider host range than the other Deuteromycetes.
• Infect almost all orders of insects and can infect certain plant tissues.
• Easily isolated from insect cadavers or from soil in forested areas by using simple media, as well as by baiting soil with insects.
• Cultured on simple media.
• 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.