Koinobionts are parasitoids that allow hosts to grow even after infection, while they finally kill their host individuals when the hosts reach required size for parasitoid development. Since fatal accidents of host organisms directly result in deaths of parasitizing koinobionts, longer parasitization period in vulnerable hosts is likely to increase the mortality of koinobionts. However, for hosts inhabiting concealed environments in later developmental stages, koinobionts should get started parasitization in early-stage hosts to use hosts grown into the appropriate size. A koinobiont parasitoid, Aneurobracon philippinensis (Braconidae), mainly use a leaf-mining moth, Acrocercops transecta (Gracillariidae) as a host. Due to the three-dimentional structure of mines constructed by later instars of A. transecta, females of A. philippinensis hardly oviposit later instar hosts, whereas feeding on the final instars is essential for successful pupation of A. philippinensis. This implies that oviposition targets in the wild is shifted to early instars, though the final instar is the ideal target to reduce the parasitization period. The observation of parasitoid immatures in the wild host population confirms this expectation, and the dissection of experimentally parasitized hosts demonstrates that the first instar of parasitoid larvae functions as an adjusting period to synchronize parasitoid and host developmental stages.