Entomopathogenic Fungi: How Beauveria and Metarhizium Control Insect Pests Naturally

The use of entomopathogenic fungi has been linked to the control of insect pest in agriculture using non-chemical methods.Entomopathogenic fungi are beginning to play an increasingly important role in sustainable agriculture, as they present a biological solution to insect pests, complementing the use of chemical insecticides. Stages in the bio-infection process of these natural fungal species against insects include attachment to the insect’s cuticle, penetration, colonization inside the insect and eventual death of the insect. Two Beauveria spp., B. bassiana and B. tenella, as well as two Metarhizium spp., M. anisopliae and M. hof állt are, among the most studied, that are applied as biological agents in agricultural and biological pest management programs.

In contrast to traditional insecticides, which mostly target specific biochemical and physiological pathways, the first point of contact for infection to occur is the insect host's external surface with an entomopathogenic fungus. This process is significant when it comes to grower, agronomist, biological control distributor, agricultural research, IPM and student/people interested in microbial biocontrol applications.

Entomopathogenic Fungi are a group of fungi that affects insects

The meanings 

Entomopathogenic fungi are fungi which naturally infect insects/arthropods and kill them. They are found in agricultural soils, forests, plant surfaces and natural ecosystems; and are thought to be involved in the control of insect pests. They play an ecological role and are especially beneficial in biological control of insect pests since they are a critical component of a natural interaction among microorganisms and insects.

These fungi can be used in the biological control integrated pest management programs as microbial insecticides as they are both biological pest control agents. They can be delivered to the insects by direct contact - so need not ingest the treated plant matter.

Why Beauvaria bassiana and Metarhizium anisopliae Matter

How does Beauveria bassiana kill insects? Two significant fungi species, which were investigated and developed for biological control integrated pest management are Beauveria bassiana and Metarhizium anisopliae infection process. They are known to have a high spectrum of activity to study against a wide variety of agricultural pests and can be used in biological control IPM programs.

Beauveria bassiana is known as a biological control agent for above-ground insect pests and Metarhizium anisopliae is known as a biological control agent particularly important for control of soil associated insects. However, because of their different ecological characteristics they are effective Beauveria bassiana infection mechanism in designing biological control strategies for specific combinations of crop and pest.

What are the steps taken in the process of entomopathogenic fungi infecting insects?

The best explanation of the action of these fungi is the infection process as the explanation of the process involved in the life cycle of fungi. When fungal spores invade the susceptible insect there are a number of biological and mechanical stages involved in the process.

Step 1: Adhesion site of spores to insect cuticle identified

The first stage occurs when an insect's outer protective body layer (the cuticle) is exposed to a spore from the fungus. Adhesion is essential as without it the fungus would not germinate and establish an infection until it detached itself from its host.

Spores use surface molecules as well as adhesion related proteins to select and adhere to the appropriate surface on an insect. The attached spore may or may not germinate depending upon environmental factors (relative humidity and temperature).

One Beauveria bassiana infection mechanism to impact insects without ingesting the fungus is this contact-dependent one. It is critical to comprehend this first stage of Beauveria bassiana infection in order to get a grasp of the Beauveria bassiana mechanism of infection.

Step 2: Building a hydraulic penetration tool was to create the Appressorium

Some of the most unique families of fungal infection are the development of unique structures which aid the fungal penetration of the insect cuticle. Once germinated, the germ tube may develop into an appressorium, a specialized infection structure of the fungal origin that is related to the penetration.

As the appressorium develops, it is filled with substances (glycerol) raising the internal turgor pressure. This pressure enables the fungus to generate a strong mechanical force in the point of contact to the surface of the insects. This can then give rise to a penetration peg, which can push through the cuticle.

This process shows that the switching on of a toxic chemical to an insect is not the only way that fungi can control that insect. It is likely that the mechanical force produced by the body of the fungus responsible for the infection is important in overcoming the physical barrier of the insect.

Step 3: Break-Insect Cutsicle

The insect cuticle is made up of multiple structures, such as chitin and proteins. The fungus does this by mechanical pressure and also by weakening and destroying the cuticular material with enzymes it possesses to break down the cuticle.

Chitinases are enzymes which are able to break down the chitin and proteases are able to break down proteins present in the cuticle. These enzymes play a role in conjunction with the mechanical force of the penetration structure in helping the fungus enter into the host.

In conclusion, the biochemical and enzymatic activity plus mechanical activity is a characteristic of fungal infection. It also suggests that this infection need not be obligate ingestion based and accounts for entomopathogenic fungi being able to infect pests by external contact.

Step 4: Colonizing the Insect Body

After penetrating through the cuticle the fungus penetrates into the insect's internal body cavity (the hemocoel). The fungus will need to get past or circumvent the insect's immune system.
The fungus forms special structures like hyphal bodies which are transported and multiplied in the insect, as per the species and host. With the increase in fungal biomass, their normal life processes of the host are disrupted.

Additionally, the fungus can generate metabolite(s) with host stress/destabilizing and killing effects. Most of the time, however, the overall infection is more appropriately viewed as a consequence of penetration, enzymatic activity, interaction with the immune system and internal fungal growth, but not as a consequence of a single toxin.

Once the spores are formed, and a new infection cycle starts, they complete the same processes.These spores go through the same processes as a new infection cycle begins.

The fungus can grow in the body of the host after it is dead (depending on the environmental conditions). New spores appear on the surface of the dead host from which more spores can be disseminated and may come in contact with other insects that are susceptible to infection.

This gives the opportunity for additional infections to occur. The influence of the other factors, environmental conditions, host density, fungal strain and survival of spores outside the host, however, are very strong in influencing natural spread.

Mechanical penetration is so effective, why?

The entomopathogenic fungi have a significant biological advantage over insecticides, which are based primarily on ingestion or the action of a biochemical target, by being able to penetrate the insect tissues mechanically. As the insects are infected at the surface, fungi are able to impair the performance of a pest whether it is a chewing insect, a plant sap sucker or an insect that develops in the soil.

This is especially true for piercing-sucking insects like aphids and whiteflies that may eat a lesser amount of treated plant tissue than chewing insects. An infection can, however, be established by direct contact with a fungal pathogen to the insect's cuticle.

The progression of the infection process also may offer a beneficial resistance biological pest management benefit. Traditional pesticides often function on specific mechanisms and when traditionally used on successive generations, populations can evolve resistance to the pesticides. While resistant to microbial agents is a possibility, resistance should be a part of responsible biological pest management which must include several biological stages that fungal infection can undergo.

Beauveria bassiana vs. Metarhizium anisopliae: What's the Difference?

This doesn't imply that each species will only thrive in a certain environment. Both fungi have been shown to be active against a number of different groups of insects; their suitability will be dependent on fungal strain, formulation, target insect, crop, environment, and mode of action.

Insect microbiomes with Fungal infection

There has been recent research that has enhanced understanding of the interactions between entomopathogenic fungi and insect microbiomes. A wide variety of microorganisms are associated with insects in the internal organs and on the bodies. These microorganisms can alter the immunoregulatory system of the host, and may modify their susceptibility to fungal infection.

Surface microbiota can provide some form of colonization resistance via competition with fungal spores, or alteration of the environment near the insect cuticle surface. Gut microorganisms in the insect can affect its nutrition, immunity and physiological responses which can in turn affect the development of pathogens.

To this end, the effect of disrupting the insect microbiome on the extent of fungal infection will be studied, as well as the role of the microbiome in biological control which could hold promise to improve the situation. Knowing more about the engineering of antimicrobial peptides or other microbial technologies may eventually provide aid in developing new more targeted biocontrol strategies.

The successes of these developments underscore how important the interactions between the microbial agent, the host insect, the plant environment, and the plant microbial community are for successful logical pest control agents.

Entomopathogenic fungi for use in sustainable agriculture

Entomopathogenic fungi have the potential to be included in various agriculture production systems. In logical pest control agents, they can be used along with cultural, mechanical, biological, and chemical management, but without them.

Fungal biological control agents can be used in IPM programs for more than just adding another mode of action, and can help provide diversity in pest-management strategies. They are also applicable to organic farming systems that include choices of acceptable biological materials in their pest biological pest management program.

Another application that might fit locally is greenhouses as growers are able to reasonably control greenhouse environments for temperature, humidity, crop and pest status. Fungal pathogens are able to contribute to the management of particular pest populations if conditions are favourable in both the environment and the way the crop is managed, e.g. in field crops and forestry.

Beauveria bassiana or Metarhizium anisopliae: the biological insecticide solutions from Novobac

Beauveria bassiana and Metarhizium anisopliae infection process will need to be compared, and for this the initial decision should be for the pest and its habitat. Beauveria bassiana could be a suitable natural control agent for many above-ground insect pests including aphids, whiteflies and other susceptible species. Soil-inhabiting insect pests (such as grubs) and other root zone insects might be better suited to use Metarhizium anisopliae.

When utilizing for greenhouse crops, it should be determined by the location of the pests, humidity, temperature, crop canopy and coverage. Soil properties, pest biology, rain and timing of applications are some key aspects in field crops.

Novobac's biological insecticide solutions, using microbial technology, can fit into these various pest-management schemes. Not always the best solution is the use of any particular species of fungus all over, but rather to select the biological control species and method in response to the specific pest, crop, environment, and IPM program.

Conclusion

Biological pest control is capable of functioning via an advanced infection mechanism—instead of chemical toxicity—provided by the entomopathogenic fungi. Most fungal pathogens employ a combination of biological processes to infect susceptible insects such as spore attachment, development of appressoria, mechanical penetration, enzymatic degradation, evading flagellar sensing and/or immune system, internal colonization, and sporulation.

The Beauveria bassiana infection process and the Metarhizium anisopliae infection mechanism aid in better decision making for growers and agricultural specialists on when to apply microbial pest management. The use of these fungi as alternatives to conventional insecticide products is not a universal solution, but they could be important biological tools if used as appropriate for the biological nature of the insect pests and environmental condition.

With ongoing research on fungal biology, insect microbiomes, microbial metabolites and microbial formulation technology, these biological pesticides will be more important in sustainable agriculture. They are most useful when combined with other IPM tools including monitoring, cultural activities, resistance management, and other pest management resources for a more comprehensive and effective pest-management program.

Reference:

• Mudrončeková, Silvia, Marián Mazáň, and Marek Nemčovič. "Entomopathogenic fungus species Beauveria bassiana (Bals.) and Metarhizium anisopliae (Metsch.) used as mycoinsecticide effective in biological control of Ips typographus (L.)." Journal of Microbiology, Biotechnology and Food Sciences 2.6 (2013): 2469-2472.
• Gul, Hafiza Tahira, Shafqat Saeed, and FZ Ahmad Khan. "Entomopathogenic fungi as effective insect pest management tactic: A review." Appl. Sci. Bus. Econ 1.1 (2014): 10-18.