Crocidolomia pavonana (=binotalis) [F.] demonstrates oviposition peaks in the field that we believe to be correlated with host plant phenology. In previous two-choice laboratory experiments, we found the highest relative proportion of oviposition on cabbage to correspond either to plant growth stages ≈7–8 wk or ≈9–11 wk old, depending on the alternate host plant with which it was presented. In cabbage-only trials, leaves from 7- to 8-wk-old plants were preferred. Inconsistency in preference led to the question of whether oviposition on either cabbage growth stage would confer adaptive advantages in offspring performance. We simulated oviposition on four phenological stages of cabbage in two ways. In a study of complete immature development, growth rate, pupal weight, and survivorship were measured. We also compared food utilization efficiency during the fourth larval instar by analyzing growth rate, efficiency of biomass accumulation, and frass production by analysis of covariance (ANCOVA). For both experiments, cabbage plants of defined phenological stages were designated at the time of oviposition, and larvae were fed from these as plants continued to grow throughout larval development. Our data indicate adaptive advantages in larval growth rate and food conversion efficiency to oviposition on cabbage at ≈7–8 wk from planting. Oviposition on later cabbage growth stages resulted in comparatively poor larval performance. Possible explanations for C. pavonana oviposition behavior in light of these results are discussed.