Relationship between cold hardiness and northward invasion in the great mormon butterfly, Papilio memnon L. (Lepidoptera: Papilionidae) in Japan

To investigate factors involved in the recent northward invasion of the great mormon butterfly, Papilio memnon L. (Lepidoptera: Papilionidae), cold hardiness of pupae was compared among 4 populations including those in a subtropical and a northernmost area of Japan. The mean supercooling points (SCP) of diapausing and non-diapausing pupae were lower than −20°C in all populations. The SCP was not affected by an acclimation period nor by pupal weight, although it became higher with inoculation. Diapausing pupae were placed in 4 sites at different altitudes in southern Osaka Prefecture during the winter, then survival rates were compared in the spring. All individuals died at altitudes of 1, 100 and 800 m, while more than half of the individuals survived the winter at 400 and 30 m. There was no difference in the survival rate at each site among populations. Even the subtropical population showed cold hardiness to survive winter in Osaka, which is located near the northern edge of the distribution of this species. These results show that the recent northward invasion has occurred without any substantial changes in the cold hardiness of this species. Based on the results, we inferred that the climatic lethal limits during the winter lie between −3.8 and −7.3°C for the minimum temperature, and between 52 and 68 frost days. We discuss the effect of climatic warming on the northward invasion of this species.