Aedes japonicus has established in many European countries. Our study had two objectives: (1) investigate the origin of new introductions in eastern Belgium; and (2) verify the origin of specimens that were recently recorded at Natoye, while the species was declared eliminated after control campaigns. To this end, genotypic variation at 7 microsatellite loci was surveyed in: (1) specimens collected along the border; (2) German reference samples; and (3) specimens collected before and after the eradication. The microsatellite data show a difference between specimens collected before and after the eradication, suggesting new introduction(s), but also indicate that some specimens of the Natoye population have survived. On the border, clustering results confirm a link between Belgian and German population specimens. Whether these introductions occur via passive human-mediated ground transport or by natural spread cannot be determined. Our results point to the complexity of controlling invasive species, with four years of intense control measures application found insufficient for eliminating Ae. japonicus