Population genomics identifies Italian and North American source populations of Popillia japonica in Switzerland

The Japanese beetle (Popillia japonica), native to Japan, is a major invasive species in North America and Europe. Its colonization of Europe began on the Azores (1970s), followed by Italy (2014) and southern Switzerland (2017). In 2023, the presence of the pest was reported in three areas of Switzerland: Kloten (Canton of Z?rich, north-eastern Switzerland), Basel (Canton of Basel-Stadt, northern Switzerland), and in eastern Canton of Valais (South Switzerland) near the Italian border. In 2024, P. japonica individuals were discovered in several other Swiss Cantons. To trace P. japonica spread and origins in Switzerland, we investigated 42 individuals from infested areas, comprising scattered findings across Switzerland. Phylogenetic and population structure analyses using whole-genome resequencing, including data of previously sequenced samples from Japan, North America, the Azores, Italy, and Southern Switzerland, revealed distinct P. japonica migration patterns. Populations in Basel, Valais, and central Switzerland likely originated from Ticino/Northern Italy via road or rail transport. Conversely, the population near Z?rich Airport was identified as an independent introduction from North America, likely through unintentional air transport. These findings offer insights into P. japonica spread across Switzerland, highlighting the need for enhanced monitoring and identification of invasion pathways.

日本丽金龟（Popillia japonica）原产于日本，是北美与欧洲的主要入侵物种。该虫在欧洲的定殖始于亚速尔群岛（1970年代），随后蔓延至意大利（2014年）与瑞士南部（2017年）。2023年，瑞士境内三个区域报告发现该害虫：瑞士东北部苏黎世州的克洛滕、瑞士北部巴塞尔城市州的巴塞尔，以及瑞士南部与意大利边境邻近的瓦莱州东部区域。2024年，瑞士多个其他州又陆续发现了日本丽金龟个体。为追溯日本丽金龟在瑞士的扩散路径与起源，本研究对来自瑞士各受侵染区域的42个个体开展了调查，样本覆盖瑞士境内零散分布的各发现点位。通过全基因组重测序开展系统发育与种群结构分析，并纳入此前已测序的日本、北美、亚速尔群岛、意大利以及瑞士南部的样本数据，研究揭示了日本丽金龟独特的迁移模式。结果显示，巴塞尔、瓦莱及瑞士中部的种群大概率经由公路或铁路运输，起源于提契诺州/意大利北部；与之相反，苏黎世机场附近的种群被鉴定为独立于北美引入的种群，推测通过无意的航空运输传入。本研究结果为日本丽金龟在瑞士境内的扩散机制提供了新的认知，同时凸显了加强入侵路径监测与识别工作的必要性。