Adult emergence of the gall maker Aiolomorphus rhopaloides Walker and its inquiline, Diomorus aiolomorphi Kamijo, was examined in three stands of the bamboo, Phyllostachys heterocycla Matsumura. A. rhopaloides emerged from the galls from mid-April to early May, coinciding with the bamboo bud elongation period. This synchronization suggests that eggs laid in the new shoots have a nutritional advantage for resulting larvae, and that a slight advance or delay in emergence may reduce suitable oviposition sites, causing population fluctuations. D. aiolomorphi emerged from late April to early June during the bamboo shoot elongation period and females could lay eggs in elongated shoots providing relatively higher nutrition for larvae. The difference in the gall density on P. heterocycla and P. bambusoides Siebold and Zuccarini is explained on the basis of synchronization with host plant phenology.