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To investigate the change in oviposition site of Pieris rapae crucivora with the increase of its parental density, the eggs laid by the 3rd generation were censused in the cabbage field and the net house in which the female adults were released at different densities. The following results were obtained : 1) At a low parental density, the butterflies laid more eggs on the peripheral plants than the inside plants, and the spatial distribution of the eggs was rather heterogeneous in the field or the net house. This heterogeneity of the egg distribution was decreased with the increasing parental density. 2) As the parental density increased, the oviposition site on the plant was extended to the senescent from the mature leaves, and more eggs were laid on the upper surface of the leaves. 3) Such trend to homogenize or extend the oviposition site with the increase of the parental density was considered from the mutual interference among the butterflies in the oviposition behaviour.
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