共生菌在昆虫体内的生长、繁殖过程中起着十分重要的作用，如营养功能、解毒作用、调控生殖及与寄主适应性等等。近些年，一些学者提出了共生菌在寄主抵御天敌等逆境的胁迫时也具有重要的作用。其在寄主防御中的作用可分为3类：(1) 昆虫体内共生菌能产生具有抗真菌作用的代谢物，能够抵御病原真菌的入侵，提高寄主的适应性。(2) 昆虫抵御寄生蜂的作用与其体内的共生细菌密切相关。(3) 共生菌也能产生有毒物质保护寄主免于被捕食。昆虫共生菌在寄主防御中的作用机制除了产生抗菌物质和聚酮类等有毒物质外，还可能通过与水平传播的寄生物（horizontal transmission parasites, HTPs）竞争寄主资源来直接保护寄主。对共生菌的作用的深入研究将有可能进一步明确其功能，并加以利用。

Endosymbionts in insect play an important role in nutritional function, detoxification, manipulating reproduction of their hosts, and providing their hosts with fitness benefits, which influence the growth, development and fecundity of the hosts. The role of endosymbionts in the resistance of their hosts to the adversities, such as natural enemies, was reported recently. Endosymbionts may produce antimicrobial substances that protect their hosts from pathogenic microorganisms. Resistance of insect hosts against the parasitoid wasp was closely related with endosymbionts. Toxic compounds protecting their hosts from predators may also be synthesized by symbionts. They may also directly protect their hosts by outcompeting horizontal transmission parasites (HTPs) for host resources, in addition to producing antimicrobial substances or toxin compounds in hosts. Further research on endosymbionts will make its function clear and utilized.