Insect-microbe symbiosis is a common type of interaction in nature. Insects are the most diverse and abundant group of species with a wide range of distribution. The evolutionary success of insects and their diversification into a wide range of ecological niches are partly due to the beneficial interaction with their associated microbiome. With the development and application of omics, numbers of symbionts have been discovered and novel symbiotic relationships were revealed. Symbionts play important roles on insect hosts in many ways, including provision of nutrients, production of bioactive components, modulation of host immune hemeostasis, protection from predators and pathogens. Therefore, symbionts serve as guardian of the insect health and adaptation Considering the important relationship of insect symbionts and hosts, insect symbionts have become promising in the development of novel tools for the biological control of insect pests, and blocking the transmission of insect-borne diseases. In this paper, we provide an overview on diversity of insect symbionts, the latest advance in the understanding of biological functions of insect symbionts, interactions between insect hosts and symbionts, and the novel strategies for the control of insect pest and vetor-borne diseases. Finally, we will discuss the directions for future study.