Around the cabbage field the author settled an artificial nesting place, into which the Polistes nest was introduced and observed the effectiveness of the wasps against the larvae of Pieris rapae Linne. It is the best time to introduce the nest into the artificial nesting place after the cocooning of the larvae and before the emergence of the worker caste. If the introduction of the nest is made earlier than this period, the settlement of the wasp colony seems to be somewhat insufficient. If the introduction is made after the emergence of the worker caste, the settling ability of the colony increases but the introduction-technique becomes more difficult owing to the increasing tendency of the wasp's attacking attitude. The population of Pieris larvae which were liberated artificially in the cabbage field by the author decreased rapidly owing to the hunting activity of the Polistes wasps. In this case about 1.3 to 8 larvae are captured by one founding female per day. After the emergence of the worker caste, the decrease of Pieris larva population is extremely high and almost all the larvae are killed or carried away by the wasps before pupation. Even if the density of the Pieris larva population is low in the cabbage field, the wasps can catch the larvae during a short period. The result shows that far more effective biological control of Pieris larvae may be achieved by the introduction of Polistes colonies into the artificial nesting places around the cabbage field.