BEAUVERIA BASSIANA: CHARACTERISTICS, MORPHOLOGY, LIFE CYCLE - BIOLOGY - 2025

Beauveria bassiana is an imperfect fungus belonging to the Clavicipitaceae family, it reproduces by conidiospores and is widely used as an entomopathogen. It is the causal agent of the white muscardina disease, which parasitizes a variety of pests that affect various commercial crops.

B. bassiana is cottony in appearance, white in color, widely distributed and located on the ground, which is its natural habitat. As a biological control agent it is highly effective thanks to its saprophytic and pathogenic phase once installed in the host.

Biological Control of Beauveria bassiana. Source: elfram.com

Indeed, the conidia of B. bassiana adhere to the surface of the hosts, penetrate, secrete toxins and cause death. Under favorable environmental conditions, the fungus continues to reproduce conidia on pest insects to infect new individuals.

This species adapts to different agroecosystems and elevational floors, with temperatures ranging between 10 and 40 ºC. In fact, once the host is infested, depending on the phase of the insect and the degree of virulence, the pest dies in four to six days.

Applications of biopesticides such as B. bassiana are appreciated for their low effect on the environment and humans. However, disproportionate applications can have negative consequences on beneficial insects, such as pollinators.

At the commercial level, the fungus is mixed with other ingredients such as powders, micro talcs or substrates. In liquid formulations, adjuvants are added, in order that the conidia remain viable, that it is easy to manipulate and maintains a high pathogenic effectiveness.

characteristics

The fungus B. bassiana is one of the main pathogens of pests that affect commercial crops. Due to its ability to grow in artificial media and on various hosts, it is classified as a facultative parasite.

It is common to locate it in the soil or in the remains of insects that have been colonized and invaded by the pathogen. Once the fungus has detected its prey, it proceeds to enter through the integument, the mouth or the anus.

As conidia develop, by mechanical action and the intervention of enzymes, the germ tube penetrates the host. The fungus grows and invades the insect, feeding on the hemolymph and generating toxins that destroy the prey.

The life cycle of the fungus depends on the environmental conditions and the species to be infected. Under normal conditions, the fungus develops enough mycelium, conidiophores, and conidia to affect large numbers of individuals.

The Beauveria bassiana species is associated with the entomopathogenic disease called white muscardina, characterized in that the mycelium of the fungus with white cottony hyphae completely covers the surface of the host.

It is used in the biological control of various commercial crops, such as coffee, musaceae, vegetables, cruciferous, forage, fruit, ornamental and floriculture. Attacking coleopteran pests, moths, flies, locusts, aphids, bed bugs, weevils, thrips, lepidopteran larvae, ants, termites, mealybugs and spider mites.

Taxonomy

Beauveria bassiana, an entomopathogenic fungus that causes white muscardina disease, was initially identified by Balsamo Crivell as Botritys bassiana (1835). In later studies based on the morphology of the fungus, Vuillemin (1912) determined the genus Beauveria and bassiana as the species.

Subsequent descriptions of the fungus have allowed the identification of up to 14 different species, including bassiana, effusa, densa and globulifera (Beauveri, 1914).

In 1954, McLeod unified the species into bassiana and tenella, and later Samson and Evans (1993) included amorpha and velata as specific entomopathogens.

The genus Beauveria is an imperfect fungus belonging to the Cordycipitaceae family, of the Hypocreales order, Hypocreomycetidae subclass, Sordariomycetes class, Pezizomycotina subphylum, Ascomycota phylum, of the Fungi kingdom.

Phylogenetically B. bassiana is linked to the genus Cordyceps. B. bassiana represents the asexual phase and Cordyceps bassiana the sexual phase (Rehner and Buckley, 2005).

Morphology

The entomopathogenic B. bassiana, classified as superior fungi or imperfect fungi, reproduces through conidia. These conidial cells of globose or subglobose form of 2 to 3 x 2.0 to 2.5 microns have a short neck.

Beauveria bassiana morphology. Source: emlab.com

The conidia have a smooth surface and hyaline appearance, globose ellipsoidal in shape with a wavy rachis. The conidiophores are grouped in compact form constituting the sinemas where the conidia originate.

Indeed, these structures appear as a white powder on the host when it completely covers the prey. In addition, in laboratory cultures it also appears as a white powder on the surface, with a yellowish color on the back of the plates.

Lifecycle

The entomopathogen Beauveria bassiana has a high degree of adaptability to live in saprophytic and parasitic conditions. This condition allows it to live freely in the soil and to maintain itself in the absence of guests for a long time.

Indeed, as a free-living organism and in the presence of organic matter, conidia generate a filamentous micellar network. However, once the host is colonized, the conidia germinate forming a network of hyphae, destroying the host and forming blastospores.

The life cycle of the Beauveria bassiana fungus on the host takes place in four phases: adherence, germination, differentiation and penetration.

Phases

Adherence

Adhesion occurs when the conidium of the entomopathogenic fungus adheres to the cuticle of the host insect. In this regard, the recognition and compatibility between the conidium membrane and the epithelial cells of the cuticle must occur.

This process is framed in two actions: one passive and one active. In the passive, hydrophobic and electrostatic forces intervene that facilitate adherence to the cuticular surface. In the active, chemical substances intervene that favor the development of the conidia on the insect's integument.

Germination

Once established, the enzymatic process between the conidium of the fungus and the cuticular membrane of the host begins germination. This process depends on the environmental conditions: humidity, temperature and nutrients; and the availability of the insect.

Differentiation

The differentiation process begins with the growth and introduction of the apprehensive through the cuticular membrane. This germ tube allows the exchange of protease enzymes, lipases, chitinases and estreases from the fungus to the host.

In addition to creating a mechanical pressure between the pathogen and host organisms. Migration towards the epidermis and hypodermis of the insect is accelerated.

Penetration

Once the pathogen is established within the digestive tract of the insect, the hyphae that produce the antibiotic oosporin multiply. This substance acts on the host's bacterial flora, causing death from toxicity, malnutrition, physical damage and subsequent mummification.

Control b

The entomopathogen Beauveria bassiana has a wide pathogenic potential to infest various insects, generating a high mortality rate.

The fungus has the ability to colonize insects of the orders Coleoptera, Hymenoptera, Homoptera and Lepidoptera, which are important agricultural pests.

Action mode

The conidia are located on the surface of the host adhering to the cuticle. Under favorable conditions, the appressorium or germ tube develops that penetrates the host, facilitating the infection of the fungus.

Within the digestive tract of the insect, it is dispersed by the hemolymph, and produces toxins that affect the physiological activity of the host. In a period of 4 to 6 days, the host is paralyzed and its subsequent destruction occurs.

Subsequently, the fungus completely invades the host, covering its entire surface with the characteristic white mycelium. Finally, this structure proceeds to release new infective conidia into the environment in order to contaminate new pest organisms.

Application

Products formulated based on Beauveria bassiana are marketed as a powdered suspension of spores of the fungus. With this bioinsecticide, a spray broth is made at the foliar level or dissolved in a substrate to be applied to the soil.

Generally, the entomopathogen formulation is obtained in the form of a dry powder (100% pure conidia). Similarly, it is available dispersed on substrates (rice or clay) on a wet basis or on a dry basis (25 to 40%).

The mode of application depends on the pest to be controlled, the development of the crop and the environmental conditions. For the preparation of the suspension it is recommended to use clean water, equipment in good condition, recommended dose and apply at the end of the afternoon.

In the case of controlling foliage pests, a suspension that covers the host insects should be applied. For soil pests, it can be incorporated into the substrate or compost, or use a suspension that penetrates until it reaches the larvae or worms.

When it is necessary to attract the insect to be parasitized, traps with baits impregnated with the fungus are used to contaminate the pest insect. Regardless of control method, it is important to follow the manufacturer's instructions regarding dosage and mode of application.

Of the wide variety of pests controlled by the Beauveria bassiana fungus, the following can be mentioned:

• Cane weevil (Metamasius hemipterus)
• Cabbage moth (Plutella xyloatella)
• Solanaceae ladybug (Leptinotarsa ​​decemlineata)
• Giant borer (Castnia licus)
• Moth (Cydia pomonella)
• Flying lobster (Schistocerca piceifrons)
• Blind Hen (Phyllophaga spp)
• Fall Armyworm (Spodoptera frugiperda)
• False meter (Mocis latipes)
• Chili weevil (Anthonomus grandis)
• Black weevil in musaceae (Cosmopolites sordidus)
• Coffee borer (Hypothenemus hampei)
• Palm weevil (Rhynchophorus palmarum)
• Corn borer (Ostrinia furnacalis)
• Stem borer (Diatraea saccharalis)
• Chapulín (Brachystola magna)

Biological control of CBB

The coffee borer (Hypothenemus hampei) is the main pest of coffee beans in most commercial plantations. The entomopathogen B. bassiana is currently the main natural enemy of this tiny beetle.

Coffee borer (Hypothenemus hampei). Source: Wikimedia Commons

The drill penetrates the coffee bean, perforating it and decreasing the productivity of the plantations and the quality of the bean. Once the pest is installed in the plantation, they reproduce exponentially, reaching up to eight generations in a year.

For effective control of the pest, it is necessary to use virulent strains and make applications when flying insects are observed. In this regard, the fungus cannot attack the insect inside the grain, since the conidia cannot penetrate inside the fruit.

Indeed, it is necessary for the conidia to adhere to the body of the CBB, so that they grow and penetrate the insects' integument. Then the reproduction of the mycelium begins, the fungus feeds on the host, generates toxins that weaken it and finally eliminates it.

Field studies have shown that the effectiveness of the applications with B. bassiana report better results attacking the attack foci. It is recommended to spray the productive branches and the tree plate.

Biological control of cutter ants

Cutter ants, belonging to the genera Atta and Acromyrmex, are the causal agents of damage in horticultural, fruit and forestry productions. At present, the application of baits impregnated with chemical pesticides in or around the anthills is common.

Acromyrmex cutter ant. Source: Wikimedia Commons

The main damage caused by cutter ants is the defoliation of the plant, reducing the yield and generating economic losses. The use of chemical products produces high environmental pollution, so the use of B. bassiana constitutes a viable alternative.

The applications with spores of the entomopathogen are carried out directly on the ants that circulate around the affected plantations. Likewise, impregnated baits are used that the workers will take to the interior of the anthill so that the fungus reproduces.

When the conidia adhere to the ants, they develop and produce toxins that kill the pest. In the same way, B. Bassiana attacks the ant food source, the fungus Attamyces sp., Fulfilling two types of control.

Biological control in stored grains

The protection and preservation of stored grains is essential to maintain post-harvest control of different crops, especially cereals and legumes.

The corn weevil (Sitophilus zeamais) is a high commercial value pest of corn kernels stored in silos and granaries.

Corn weevil (Sitophilus zeamais). Source: ozanimals.com

Research works have shown that B. bassiana applied in different forms and dosages has allowed to control this pest by 100%. Pellet applications report excellent results seven days after the pest was in contact with the entomopathogen.

The corn weevil (S. zeamais) is highly susceptible when exposed to high concentrations of the entomopathogen B. bassiana. Studies suggest that the use of these microorganisms are an alternative for the integral management of pests in stored grains.

References

1. Bravo García Saul and Donado Alexandra P. (2018) The most effective and efficient ant bioinsecticide on the market. Recovered in: reddicolombia.com
2. Castillo Carmen Elena et al. (2012) Morphological characterization of Beauveria bassiana, isolated from different insects in Trujillo - Venezuela. Recovered at: researchgate.net
3. Echeverría Beirute Fabián (2006) Biological and molecular characterization of isolates of the entomopathogenic fungus Beauveria bassiana (Balsam) Vuillemin. (Graduate Thesis) Technological Institute of Costa Rica.
4. Gomez, HDS (2009). Pathogenicity of Beauveria bassiana (Deuteromycotina: hyphomycetes) on Sitophilus zeamais motschulsky (Coleoptera: curculionidae) pest of stored corn. Intropica: Journal of the Tropical Research Institute, 4 (1), 5.
5. Jaramillo Jorge L. et al. (2015) Beauveria bassiana and Metarhizium anisopliae for the control of coffee berry borer in soil fruits. Colombian Journal of Entomology 41 (1): 95-104.
6. Taxonomy Beauveria bassiana (2018) UniProt. Recovered at: uniprot.org/taxonomy.