Twenty-six species of insect pests belonging to thirteen families, eight orders were identified after the surveys in some gardens of the water apple (Syzygium samarangense) in Hoa An Commune of Caolanh city, Dongthap province, Southern Vietnam. Some species of the pests, e.g., Aleuroclava aucubae, Trialeurodes lauri, Trichopsocus clarus, are new recorded in Vietnam. The oriental fruit fly Bactrocera dorsalis is one of the major pest species devastating the water apple. Rainfall is a factor comprehensively influencing the population fluctuation of the oriental fruit fly. The pest population was exploded in the dry season, but reduced remarkably in the rainy season. The fruiting period from February to April was regarded as another major factor influencing the oriental fruit fly population in that area. The results provide scientific data for establishing practical management strategy for the fly under the particular geographical and climatic conditions, especially for the oriental fruit fly Bactrocera dorsalis.