Gall-inducing insects develop within the galls they induce, and generally remain undetected. In recent years, many instances of gall-inducing insects turning into invasive organisms internationally have come to light. Anselmella miltoni induces seed-like galls within the fruits of Syzygium samarangense was found in Taiwan recently, thus impacting severely on the country’s earnings made out of the export of fresh fruits of S. samarangense. Biological details of A. miltoni are sparsely known, thus limiting an effective management. We studied A. miltoni populations from an orchard in Jhongpu, Chiayi and report in this paper its biology, including notes on field populations, synchronization between fruit development in S. samarangense and gall development, emergence, and adult longevity. Anselmella miltoni lays eggs during the anthesis stage of fruit phenology of S. samarangense. The egg stage of A. miltoni overlaps with the petal fall stage in S. samarangense. Immature stage of A. miltoni develope through the rapid stage to middle fruit stage. Anselmella miltoni enters into pupal stage and adults emerge, when the fruits of S. samarangense in mature stage. A gall inside the fruit includes many compartments, each compartment with one individual of A. miltoni inside. Average number of gall compartments within a fruit is 20.08 ± 11.5. The emergent adults leave its fruit unsynchronized and the period of adult emergence from one gall last for 3~7 days. Adult longevity is 2.36 ± 1.16 days. Based on biological details, we offer suggestions for the management of A. miltoni.