Fitness gain in a koinobiont parasitoid Diaeretiella rapae (Hymenoptera: Aphidiidae) by parasitising hosts of different ages

► In a host age choice test, Diaeretiella rapae preferred to parasitise older nymphs. ► Development of immature parasitoid occurred faster in older nymphs. ► Higher female/male ratio of offspring emerged from older nymphs. ► Females emerged from older nymphs lived longer and had a high fecundity. ► Therefore, D. rapae females gain fitness by ovipositing in the older nymphs.