none The female D. rapae walks randomly on the aphid infested plant and discovers its host by macro-orientation. The higher host instars and the adult aphid show stronger defensive reactions in comparison to the younger ones. Contacts of the female parasitoid with aphid were recorded into three categories, 1. mere encounters; 2. encounters followed by ovipositional attempts; 3. encounters followed by actual oviposition. The alate stage of the host is found less parasitized while the half grown nymphal stages are more parasitized. Behaviour; Agonistic behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships Diaeretiella rapae (Braconidae); Hemipteran hosts; Lipaphis erysimi; Host selection & discrimination behaviour; Host preference; Lipaphis erysimi (Hemiptera) host stages; Host seeking; Lipaphis erysimi (Hemiptera) host; Reaction of parasite to host; Lipaphis erysimi (Hemiptera) host defensive reactions Lipaphis erysimi (Aphidoidea); Defensive behaviour; Reaction to hymenopteran parasite; Hymenopteran parasites; Diaeretiella rapae; Host parasite relationships behaviour; Host preference; Developmental stage; Host seeking; Diaeretiella rapae (Hymenoptera) parasite; Reaction of parasite to host; Defensive reactions against Diaeretiella rapae (Hymenoptera) parasite