Abstract:  One of the hot topics in entomology today is the symbiotic microorganism-host interactions in insects. Symbiotic microorganisms in insects can not only provide nutrients for the growth and development of the hosts, but also synthesize many active substances that regulate the ecological adaptability, stress resistance, diversity formation, reproduction, and mating behavior of hosts. Despite of their widespread use, microbial insecticides have led to varying degrees of resistance in pests due to the toxicity of their metabolites and their long-term application in the field. The development of this resistance is closely related to the symbiotic microorganisms in pests. In this article, we reviewed the role of insect symbiotic microorganisms in the development of host resistance to adversity stresses, with focuses on the role of insect symbiotic microorganisms in assisting hosts to resist infestation by different species of biocontrol fungi/bacteria, such as Beauveria, Metarhizium, Pandora neoaphidis and Bacillus thuringiensis. In addition, we deeply clarified the defense mechanisms of symbiotic microorganisms against exogenous pathogen infestation. The present studies showed that the symbiotic microorganisms against exogenous pathogens are mainly distributed in the epidermis, antennal glands and gut of insects, and the symbiotic microorganisms such as Pantoea, Pseudomonas, Wolbachia, Citrobacter freundii and Arsenophonus are more prominent in assisting the hosts to fight the pathogen infestation, having a protective effect on various insect hosts. In addition, these symbiotic microorganisms improve the resistance of insects to pathogens mainly through three pathways: Competition for nutrients with exogenous microorganisms, secretion of antimicrobial substances and modulation of the immune system. This article provides new ideas for a comprehensive elucidation of the mutualistic interactions among pathogenic microorganisms, insects and symbiotic microorganisms, and can also serve as a valuable reference for the development of pest biological control strategies based on the regulation of symbiotic relationships.

Key words:  Insects, symbiotic microorganism, exogenous infestation, defense mechanisms, microbial insecticide development