Field studies have demonstrated mixed success in trap cropping to manage Crocidolomia pavonana (=binotalis) [F.], a major pest of cruciferous crops in Asia. A possible explanation for this is an influence of host plant phenology on oviposition preference. We tested this in simultaneous two-choice oviposition bioassays under laboratory conditions. In cylindrical cardboard experimental arenas, with 5-cm² leaf windows, individual C. pavonana females were offered eight sequential phenological stages of cabbage (Brassica oleracea L., Capitata group, cultivar Gloria) with a constant stage of preflowering Indian mustard (Brassica juncea, variety rugosa, cultivar Green Wave). Results showed a significant effect of cabbage developmental stage on oviposition preference. We continued with two-choice bioassays, using whole leaves, in screen cage experimental arenas. Females were offered the three most preferred phenological stages of cabbage and differing stages of four potential trap crop alternatives: preflowering Indian mustard; preflowering Chinese cabbage (B. rapa L., variety pekinensis [Lour.] Olsson); preflowering, flowering, and “with silique” sawi manis (B. rapa, variety parachinensis [Bailey] Tsen and Lee); and preflowering and flowering sayur pahit (B. rapa, variety parachinensis [Bailey] Tsen and Lee). Results indicated that the phenological stages of both plants had a significant effect on relative oviposition preference. However, patterns of preference changed in the context of the different plant species combinations. In addition, implicit hierarchical preference order was frequently contradicted. These results have implications for the improvement of trap cropping strategies to manage C. pavonana and for the potential success of biological control of Plutella xylostella [L.], where these pests occur in complex.