The evolution of host utilization strategy: At what timing parasitoids attack and consume their host

Parasitoid wasps exhibit remarkable diversity in life-history traits and are categorized into two major groups based on the timing of emergence: those that begin consuming host tissues immediately after hatching ("idiobiont") and those that can delay the emergence depending on host maturation ("koinobiont"). Although delayed emergence allows parasitoids to exploit unparasitized, young hosts, it incurs a mortality cost during the waiting period. While numerous empirical studies have examined the adaptive significance of this trait, how host stage-structure and ecological factors jointly shape the evolution of emergence timing remains poorly understood in a formalized context. Here, we develop a stage-structured mathematical model to analyze the evolutionary dynamics of delayed emergence and its consequences for host exploitation. By explicitly incorporating host developmental stages, our model explores the joint evolution of two traits: the preference for young versus old host, and the timing of emergence. Our results reveal that the optimal strategy is determined by the relative reproductive value of host stages, which depends on the balance between host growth and mortality. We demonstrate that delayed emergence evolves when the future value of a growing host outweighs the immediate cost of mortality, thereby driving the divergence between idiobiont and koinobiont strategies. These findings provide a theoretical framework that unifies the diversity of parasitoid life histories. Significance StatementParasitoids exhibit striking diversity in life-history traits such as body size, fecundity, and clutch size, reflecting adaptations to host exploitation. A key behavioral distinction is whether parasitoids consume the host immediately (idiobionts) or can delay emergence while the host continues to grow (koinobionts). Many empirical studies have suggested and tested some hypothesis. On the other hand, the adaptive significance of delayed emergence have not been formulated yet, so the quantitative relationship between selection pressure and life history traits is yet to be better understood. Here, we develop a mathematical model that integrates host growth with parasitoid emergence timing. Our analysis supports the adaptive significance of delayed emergence and shows how delayed emergence can promote diversification of parasitoid life-history strategies. These results suggest that delayed emergence is a key driver of idiobiont-koinobiont dichotomy, providing new insights into the evolutionary basis of parasitoid-host interactions and aligning directly with the central aims of behavioral ecology.