Abstract:  Insect immunity has gradually attracted broad attention in recent years because of its important physiological functions in the process of insect resistance to harmful exogenous stimuli. Although both internal and external immunity are parts of the insect immune defense system, it is generally believed that the internal immunity is an immediate and primary immune response of insects while the external immune defense is only considered as a kind of secondary immune response. Nevertheless, the external immunity using defensive secretions as the core component constitutes the first barrier to foreign substances which invade insects. Accordingly, external immunity and internal immunity are two kinds of immune defense strategies for insects. When insects suffer exotic organism infection or adverse environmental factors, the external immunity is often the priority to be employed to fight against the invasion of natural enemies or adversity. However, the internal immunity and external immunity are both synchronized in the whole process of defense, which will cause a dilemma on energy allocation between the two defense strategies. Moreover, it is essential for insects to employ a series of immune reactions to prevent potential threats, which will seriously impair the effectiveness of pest biological control. This review focuses on insect external immunity and summarizes the current understandings on defensive secretions in insects, as well as the two immune defense strategies and their trade-offs, aiming to provide a synthesis of important updates, potential applications, and novel ideas and technologies for further research in this area. Also, the related research will not only contribute to further analysis about the immune interaction and diversified immunization strategies between insects and exogenous factors to improve the control effect in the field, but also pave the way for the development of novel insecticides targeting the insect immune system.

Key words: Defensive secretion, immune defense strategy, external immunity, internal immunity, trade-off