Insect pathogen interaction with special reference to Bacillus thuringiensis and Entomopathogenic fungi

Abstract

Insects are attacked by several pathogens (entomopathogens) such as bacteria, fungi, viruses, protozoa and nematodes. Entomopathogens are eco-friendly and target-specific thus holding ecological, biological and economic significance. These pathogens can sicken or kill the insects. Cessation of feeding and reproduction are generally observed in the surviving insects. Hence these pathogens have been exploited for biological control of insect pests through inundative applications. Many different types of bacteria are known to acutely or chronically infect insects. A naturally occurring insect-pathogenic bacterium Bacillus thuringiensis (Bt), is the world’s most extensively used biological control agent and is specific to certain insect orders. Bt produces several toxins like Cry, Cyt and Vip proteins that are stomach poisons and must be ingested by the plant-feeding insects for exerting toxicity. Cry toxin action is a multistage process leading to paralysis of the mid-gut resulting in feeding cessation followed by starvation and death. Initially, the activated toxins bind to receptors located on the apical microvillus membrane of epithelial midgut cells. Two important insect proteins aminopeptidase-N (APN) and Cadherins have been identified as receptors for Cry toxins. More than 240 holotype of Cry toxins have been reported under 68 major classes (Cry1 to Cry68) that are specific to various insect orders - lepidoptera (Cry1, Cry2, Cry9 and Cry15), diptera (Cry4, Cry10, Cry11, Cry16, Cry17, Cry19 and Cry20) and coleoptera (Cry3, Cry7 and Cry8). Toxins of the Cry6 class are nematicidal in nature. More than 700 species of fungi are known to infect insects out of which less than 20 have been exploited for insect pest management. These entomopathogenic fungi (EPF) have an extensive host range but need cool, humid environments to be effective. Individual strains of the same fungal species often have different host ranges or pathogenicities. Infection is by attachment of the spore to the cuticle followed by germination and secretion of chitinolytic enzymes by the germ tube. These enzymes digest the cuticle enabling invasion of the body cavity by the germ tube followed by blastospore production, mycosis and finally death. Under congenial environmental conditions, mycelial growth followed by sporulation occurs on the surface of the cadavers. Majority of these fungi secrete toxins eg. destruxins, bassianolide, beauvericin, tenellin, bassiacridin etc. that weaken their host’s immune system and could be the virulence factors involved in pathogenesis. Bt can be effectively used in combination with EPFs taking advantage of the dual mode of action for enhancing the speed of kill as well as mitigating the problem of resistance development.