The bamboo borer, Omphisa fuscidentalis, is a moth found in northern Thailand, Lao and Myanmar and the larvae feed on the inner pulp of bamboo shoots. In a tropical highland (about 500 m sea level) forest at 19^o near Chiang Mai, Thailand, adults appear in August and lay a cluster of eggs on a newly grown shoot of bamboo. Newly hatched larvae hore a hole to enter the internode to feed on the inner pulp. After maturation in September, the larvae enter diapause and remain inside the internode of bamboo culm until next June when they pupate in the same internode. Since it has been established that high JH titer is responsible for maintaining larval diapause in several lepidopteran larvae, we asked if JH is involved in maintaining larval diapause in O. fuscidentalis. In order to study this issue, we assayed the reponsiveness of the diapause larvae to juvenile hormone analog (JHA : S-methoprene). In addition, we examined the effects of 20-hydroxyecdysone (20 E) to estimate the time of onset of diapause break since responsiveness of diapause larvae to 20E might increase at the early phase of the break of diapause. Surprisingly, topical application of JHA induced pupation in a dose-dependent manner. JHA also induced pupation of the larvae whose brains were removed before JHA application. In those larvae, the hemolymph ecdysteroid concentration initiated to increase 12 days after JHA application while their prothoracic alands became active and competent to respond to brain extracts in 7 days after JHA treatment. Accordingly, high hemolymph JH is not involved in maintaining the larval diapause in contrast with the previously accepted view. Current results indicate that JHA may directly stimulated prothoracic glands and such slightly activated prothoracic glands may be activated with positive feedback to produce sufficient ecdysone for pupation. 20E injections indicated that diapause became deepest in December and initiate to be broken in March, two to three months before pupation in the field.