AbstractThis study evaluates the adaptive significance of host preferences and searching behaviour in Vancouver and Canberra populations of the cabbage butterflyPieris rapae (L.).As a result of a complex of responses to plant age, the butterflies concentrate their eggs on middle‐aged plants. Young larvae develop faster and survive better on young plants than old ones, but larvae on smaller plants are more susceptible to crowding effects. Thus a preference for plants which are well‐grown but not too old is selectively advantageous. By contrast, the butterflies’ host species preferences appear non‐adaptive, and are unrelated to the quality of the host as larval food.Vancouver butterflies change their flight direction often and are very responsive to hosts, thereby generating a very aggregated distribution at a low cost in flight time. Canberra butterflies tend to fly in straight lines and are less responsive to hosts; their egg distribution is consequently more nearly random, but they fly further for each egg they lay. The relative costs of aggregation and increased flight time differ between the populations in a manner consistent with the observed behavioural diff