Hepialus xiaojinensis Tu., the main host insect of Cordyceps sinensis in Xiaojin County, has one generation every four years at 3 850 m altitude under natural conditions and hibernates in the soil as larvae during winter. Pupae occurred from early May to early July each year. The adults occurred from middle June to late July. Adults usually emerged from 16: 00 to 20: 30. Adults mate on the same, or the next, day after emergence, and have two mating peaks at 18:30 and 20:00. Oviposition begins 20 minutes after mating. The average number of eggs laid by each female is 428. Eggs appeared from midJune to late August. The larval stage has six instars. Larvae are generally distributed in the soil at a depth of from 5 to 40 cm, and feed on the tender parts of the roots and stems of plants such as Kobresia spp. The optimum temperature for larval growth is 8—12℃, and the optimum soil moisture is 45%—50%. Under natural conditions, developmental duration was 45—60 days for the egg stage, 1 370—1 385 days for the larval stage (including 580—695 days of dormancy) 50—65 days for the pupal stage and 4—6 days for the adult stage.