Bioinsecticide potential of the fungus Metarhizium rileyi

Metarhizium rileyi fungus, which has excellent natural control action on caterpillars in soybean, corn and cotton, has potential for use as a bioinsecticide expanded based on genetic sequencing

29.05.2020 | 20:59 (UTC -3)
Daniel R. Sosa-Gómez, Eliseu Binneck, Claudia Lopez Lastra

Like humans, caterpillar populations can be affected by natural control agents in several economically important crops. Among these agents, possibly one of those with the best effect is the fungus Metarhizium rileyi. This species causes epizootics (the infection and mortality of caterpillar populations with high prevalence is called epizootics, analogous to epidemics in human populations), commonly known as white disease. Its name is due to the fact that, once the insect is killed by the fungus, it presents a white appearance, considering that the entire surface of the caterpillar is covered by mycelial growth (Figure 1). Subsequently, in the presence of humidity greater than 80%, the fungus forms spores giving the caterpillar the light green color, characteristic of this fungus. These spores are the “seeds” that begin a new infection process in other caterpillars. The fungus can remain in the soil in vegetative form or forming small sclerotia (which are survival structures), thus closing its life cycle.

Figure 1 - Fungus formed on the surface of the caterpillar integument, white vegetative phase
Figure 1 - Fungus formed on the surface of the caterpillar integument, white vegetative phase

Figure 2 - Microphotograph of the reproductive structure of Metarhizium rileyi, which resembles a bowling stick, arranged in a whorled shape around the hyphae, used to identify the species
Figure 2 - Microphotograph of the reproductive structure of Metarhizium rileyi, which resembles a bowling stick, arranged in a whorled shape around the hyphae, used to identify the species

Until 2014, this species was known by its scientific name Nomuraea rileyi, when it was then transferred to the genus Metarhizium by Kepler and other authors.

In Brazil, this pathogen infects the soybean caterpillar (anticarsia gemmatalis), the false caterpillars (Chrysodeixis includens, Rachiplusia naked e Trichopluisa ni), the cotton leafhopper (Alabama argillacea), among other species, causing high mortality, which can be close to 100% in individuals in a given area. Cases with lower prevalence have been observed in the fall armyworm, Spodoptera frugiperda, is at Helicoverpa armigera

The incidence of M. rileyi decreased from the 2000s onwards, probably due to the increased widespread use of fungicides in soybean crops to control Asian rust, a disease that was detected in Brazil in 2001.

Metarhizium rileyi It has a worldwide distribution, therefore, despite the absence of morphological variability, it is possible to detect high genetic variability between isolates from different locations. This variability can result in the expression of different characteristics, such as their aggressiveness, their ability to multiply and/or their ability to survive different environmental conditions. Therefore, the differentiation of these strains is of fundamental importance to develop bioinsecticides with greater chances of success in pest control.

Since the differentiation of strains based on micro and macromorphological characters is not possible, the remaining alternative is the use of molecular tools obtained from knowledge of their genome.

This fungus has the potential to be used as a microbial bioinsecticide. Studies carried out under field conditions indicate that it can be used with variable efficiencies, between 40% and 100% control, depending on the quality of the material produced, environmental conditions, susceptibility and behavior of the target pest.

Knowledge of the genomic sequence will allow us to differentiate breeds and carry out comparative studies between them, determining genetic variability and locating regions in the genome that allow differentiation between them. Therefore, sequencing the genome of M. rileyi will allow the development of tools that will facilitate the characterization and identification of different strains of the fungus.

It will also be possible to compare genomes with other already known species to, by homology, recognize genes associated with characteristics of interest, such as virulence factors, sporulation capacity, tolerance to fungicides, growth rate and other parameters of interest, in addition to contributing in genetic transformation studies.

One perspective to improve the efficiency of insect pathogenic fungi as biological control agents consists of the use of gene editing or genetic transformation. Thus, for example, the genetic transformation of other species of fungi that cause diseases in insects has helped to improve the control of some pests, such as the fungus Metarhizium pingshaense, genetically improved to control mosquitoes that transmit malaria in Burkina Faso, Africa, a country where the disease is endemic. This genetically modified fungus has greater virulence, that is, it kills more quickly, and infected female mosquitoes significantly reduce the number of eggs laid.

Daniel R. Sosa-Gómez and
Elisha Binneck,
Embrapa Soy
Claudia Lopez Lastra,
CEPAVE

Cultivating Great Crops May 2020

With each new edition, Cultivar Grandes Culturas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on the plant health of Major Crops, the Magazine helps farmers in the search for management solutions that increase their profitability. In the May edition you can also see: 

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