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More than 750 species of entomopathogenic fungi (EPF) belonging to 85 genera are reported to date infecting more than 1000 species of insect pests. The typical EPF mode of action by direct penetration through the insect cuticle and establishment in host haemocoel makes them successful biocontrol agents. However, this process requires a biochemical artillery like the production of enzymes, toxins and other metabolites that facilitates host infection and invasion. Enzymes like chitinase, proteinase and lipase are directly involved in degradation of the host cuticle, the first and foremost barrier towards EPF infection. Secondary metabolites such as destruxins of Metarhizium, beauvericins of Beauveria, hirsutellides of Hirsutella, isarolides of Isaria, cordyols of Cordyceps, vertihemipterins of Verticillium etc., directly and indirectly disable the defence mechanism of insect hosts and accelerate the EPF infection process. The chemical nature of these secondary metabolites range from simple non-peptide pigments like oosporine to highly complex piperazine derivatives, like vertihemiptellides. These structural distinctions imply multiple modes of action which are yet to be deciphered along with their synthesis and regulatory mechanisms. In this chapter we focus on a few important issues related to the utilization of metabolites by EPF for insect host invasion. The major focus is given to enzymes, toxins and other metabolites synthesised by a few important EPF species, and their mode of action to counteract the host cellular and humoral defence mechanisms. Some strategies to enhance the infection efficiency of EPF, their regulatory mechanism and genetic basis behind production are detailed. |
