How discriminating are cabbage butterflies?

The egg‐laying responses of cabbage butterflies Pieris rapae L. to differences in the size, quality and spatial distribution of available oviposition sites, were studied in the laboratory (Australia) and the field (Australia and Canada). Differences on three scales were investigated: between leaves within the same plant, between plants within a patch, and between patches of plants. The butterflies lay most eggs on the larger, older leaves of plants, though the oldest are sometimes avoided. Larger plants also receive more eggs than smaller ones of the same cultivar, but cultivar preferences may override the response to size. The butterflies do not discriminate against plants already bearing eggs or larvae, unless larval feeding damage to the plant is severe. They lay more eggs on young plants than old ones of the same size; plants grown in the lower of two light intensities also received more eggs. Australian butterflies discriminate not only between different species of crucifers, but also between varieties of the same species. This discrimination against less acceptable varieties is just as strong when plants of preferred varieties are not present, as when both varieties occur in the same patch. The observed responses are discussed in relation to the butterflies’ host‐finding behaviour and the adaptive significance of the lack of a response to eggs and larvae is considered.