Seasonal Development of Leptotrombidium pallidum (Acari: Trombiculidae) Observed by Experimental Rearing in the Natural Environment

Engorged larvae of Leptotrombidium pallidum Nagayo, Miyagawa, Mitamura & Tamiya, the vector mite of scrub typhus in Japan, were reared by feeding them with fresh eggs of the collembolan Sinella curviseta Brook while confined in small plastic containers under natural conditions in a copse. The larvae were collected from wild rodents (Apodemus speciosus) in autumn 1985 and spring 1986. Adults were kept alive for 2 yr or longer. The larvae obtained in autumn became dormant in the cold winter season, and growth recommenced in the spring. Thus, the development of mites collected in April became synchronized with that of larvae obtained in the autumn. Most larvae developed into protonymphs in May, deutonymphs in June, tritonymphs in July, and adults in August. The females laid eggs in two consecutive summers. Some larvae collected in autumn were kept in a refrigerator until the following summer. They developed into deutonymphs, tritonymphs, or adults and then became dormant in the winter. Development restarted the next spring and all became adult by summer, when the females laid eggs. Under experimental conditions, all larvae are hatched in the autumn, unlike the natural situation in which two peaks of larval occurrence on wild rodents are observed in autumn and spring.