Aedes japonicus has expanded beyond its native range and has established populations in several European countries, including Belgium where the species was first reported in 2002 at Natoye. During a recent nationwide monitoring programme (MEMO, Monitoring of Exotic Mosquitoes, 2017-2020), Natoye was monitored to check for the efficiency of the extensive elimination campaign undertaken between 2012-2015. Also the species was surveyed along the Belgian border with Germany where Ae. japonicus has established and has been expanding. The objectives were: (1) to verify the origin of specimens that were collected each year at Natoye during MEMO and (2) to investigate the origin of new introductions into eastern Belgium, assuming these derive from established populations in western Germany. To this end, genotypic variation at seven microsatellite loci was determined in specimens collected before and after the eradication at Natoye and from specimens collected along the shared Belgo-German border. German reference samples were included. The microsatellite data shows a difference between specimens collected before and after the eradication campaign, suggesting new introduction(s). The genotype results also indicate that some specimens survived the eradication. At the border, clustering results confirm a link between the specimens collected in Belgium and the western German populations. Whether these introductions from Germany occur via passive human-mediated ground transport or by natural spread cannot be determined at this moment. Our results indicate the complexity of controlling invasive species, since four years of intense control measures were insufficient to eradicate Ae. japonicus which are entering Belgium via Germany.