1987 Volume 38 Issue 4 Pages 323-332
Soil samples were taken monthly at 7 stations, 4 in Matsumoto and 3 in Miasa, Nagano Prefecture, and examined for the presence of trombiculid mites by the improved Tullgren's method during the period from June 1985 to May 1986. Additional examinations were continued until October 1986 at 1 each station in the 2 areas. Trombiculid fauna and density of each species varied according to the differences in vegetation and soil nature of the stations surveyed, indicating definite dependency of trombiculids on these environmental factors. Stations in grasslands of Miscanthus and Artemisia, which were newly formed on abandoned fields and abounded with tunnel networks made by field rodents, as well as stations in the similar grasslands on gravelly lands gave the richer yields of trombiculids of the genus Leptotrombidium, particularly L. pallidum and L. intermedium, than stations in the stabilized Miscanthus grasslands. Banks of paddy fields seemed to be the more suitable habitat for L. pallidum than the Miscanthus grasslands formed on abandoned paddies, where the bed soil is chiefly composed of clayish matter. Seasonally, L. pallidum larvae in the soil first appeared in August, increased in number and reached the peak of abundance in September, then declined but maintained a fairly good population through autumn and winter, and became depopulated by June next year. This seasonal pattern of fluctuation differed remarkably from that observed on wild rodents trapped in the same areas, which showed the clear bimodality in autumn and spring. The trombiculid densities in the soil observed at 2 of the 7 stations were unusually high in 1985,comprising L. intermedium and L. pallidum as the main species; but the densities were significantly decreased in 1986. In these stations, replacement of the dominant species by other one was also observed, giving support to the movability of endemic foci of tsutsugamushi disease.
