Diaeretiella rapae is all important biological control agent of cabbage aphid. Host searching, handling and oviposition behaviour were investigated in relation to host age. The parasitoid spent 61% of her foraging time searching for hosts. Host handling time of the parasitoid decreased with increased number of host encounters. The females were more successful in finding older hosts (7 days old) and spent more time (94.9 [plus or minus] 20.5 s/encounter) and did more stings (9.9 [plus or minus] 1.4/encounter) on them than the younger hosts. They preferred to sting the abdomen rather than the thorax, head or legs of the host. The average number of eggs laid per host was highest (1.4 [plus or minus] 0.2 eggs) in 7-day-old hosts. When attacking 7-day-old hosts, they gained 42% success in ovipositing the host compared to 10, 18 and 30% success in 1-, 3- and 5-day-old hosts, respectively. Results Suggest that D. rapae may have adaptive preference for larger hosts and mass produ! ction could be more efficient by using 7-day-old aphids. Brevicoryne brassicae (Aphidoidea): Host; Diaeretiella rapae (Aphidiidae): Parasite. Brevicoryne brassicae: Age, host parasite relationships, host seeking, Hymenopteran parasites, Diaeretiella rapae; Diaeretiella rapae: Rearing techniques, Mass rearing implications, host parasite relationships, host seeking, host age effects, Egg laying, Oviposition, Hemipteran hosts, Brevicoryne brassicae. Arthropods; Hymenopterans; Insects; Invertebrates; True Bugs. none