Mango blossom gall midge, Procontarinia mangiferae (=Erosomyia mangiferae Felt), is an invasive pest that causes economic damage worldwide. The objectives of our study were to highlight the genetic and ecological abilities of this monophagous gall midge to invade new habitats and to evaluate its genetic structure on an isolated island. This study, carried out in subtropical Reunion Island, is based on data from population dynamics surveys and from molecular analyses (mitochondrial DNA and microsatellites). Using 11 microsatellite loci and an extensive sampling of 27 populations at 17 sites, we tested the genetic differentiation between populations sampled on different mango organs and cultivars at different seasons and under different climatic and cultural environments. We checked for the existence of a seasonal bottleneck. Our results showed that a single species, P. mangiferae, was present all year round with no genetic bottleneck at any of the sites sampled, regardless of the climatic and cultural conditions, and that it fed on inflorescences and young leaves. These characteristics showed the ecological plasticity of P. mangiferae, despite its low genetic diversity and, consequently, the invasive potential of this species. Populations in Reunion Island are structured into two clusters in sympatry and present in different proportions at each site. One cluster was more frequently found in the cultivated mango area. This work provides insights into the relationships between gall midges and tree host plants in a subtropical agro-ecosystem, as well as into the role of the population genetic structure in the establishment process of a monophagous invasive cecid fly.