The adult behavior of an ambrosia gall midge Illiciomyia yukawai (Diptera: Cecidomyiidae) that induces leaf galls on Illicium anisatum (Illiciaceae) was studied at the population level from 1977 to 1995 in Kagoshima, Japan. Most males emerged between 0:00 and 08:00 h and females between 05:00 and 11:00 h. Males swarmed around the host trees between 05:00 and 11:00 h. Mating occurred on the host leaves mainly between 06:00 and 08:00 h. Females then left the host trees for somewhere else, possibly to collect symbiont fungal conidia. From 08:00 to 16:00 h, females were observed ovipositing into the host shoots. The low development threshold temperature for overwintered larvae was 14°C, while the thermal constant for emergence differed with individuals. Thermal totals above 14°C up to the 50% emergence date varied yearly from 33.1 to 68.7 degree-days. The 50% emergence date varied from 9 to 18 May. The thermal totals significantly correlated with the 50% emergence date but did not correlate with the date when 50% of shoots became suitable for oviposition. Thus, the host-plant responded to thermal effects differently from the gall midge. Illiciomyia yukawai has been synchronizing well with the host-plant phenology but will suffer from asynchrony when global warming becomes more conspicuous.