Comparison of the cold hardiness of Ixodes nipponensis and I. persulcatus (Acari : Ixodidae) in relation to the distribution patterns of both species in the Chichibu Mountains

Abstract

In order to compare the cold hardiness of the two ticks, I. nipponensis and I. persulcatus, the survival of each stage of both species was observed at 25℃ after exposure to low temperatures (4,-2,and -5℃) for 10-50 days. Both in I. nipponensis and in I. persulcatus, the egg hatchability was low (0-62.3%) when the eggs were exposed to 4 or -2℃ for 10-50 days. None of the unfed larvae of either species died with exposure to -2℃ for 10 days. After exposure to -2℃ for 30-50 days or -5℃ for 10-50 days, however, the survival rate of the unfed larvae became lower in I. nipponensis than in I. persulcatus. The molting percentage of engorged larvae of both species was high (76.7-93.3%) when they were exposed to 4℃ for 10-50 days. After exposure to -2℃ for 20-50 days or -5℃ for 20-30 days, however, the molting percentage of engorged I. nipponensis larvae became lower than that of engorged I. persulcatus ones. The survival rate of unfed nymphs of both species was high (72.1-100%) even after exposure to -5℃ for 10-50 days. The engorged nymphs of both species successfully molted (61.9-100%) with exposure to 4 or -2℃ for 10-50 days. After exposure to -5℃ for 10-30 days, however, engorged I. nipponensis nymphs were more seriously impaired than the engorged I. persulcatus nymphs. The high tolerance of I. persulcatus to low temperatures explains why this tick can survive in the upper montane to subalpine zones of the Chichibu Mountains. However, the reasons why the distribution of I. nipponensis is limited in the lower montane zone can not be fully explained by the present results because of the high tolerance of the unfed nymphs to low temperatures.