Genetic Structure Analyses of Red Imported Fire Ants in Zhejiang Reveal Multiple Introduction Sources in Eastern China

Red imported fire ants (Solenopsis invicta) have invaded Zhejiang Province in eastern China, but their introduction sources remain unclear. Using 42 microsatellite markers, we analyzed the genetic structure of 116 ant colonies from 35 locations across Zhejiang and seven other Chinese provinces. Our results reveal multiple, genetically distinct clusters within Zhejiang. The Ningbo population is highly differentiated from all other sampled Chinese populations, suggesting a possible overseas origin (e.g., via port entry), a hypothesis not previously considered. Despite geographical proximity, the Jinhua and Dongyang populations are genetically distinct, indicating separate sources: Jinhua from southern China via nursery stock transport, and Dongyang from imported logs. Populations in Wenzhou, Jiaxing and Hangzhou are genetically closer to samples from other Chinese provinces than to other Zhejiang populations. These findings demonstrate that S. invicta in Zhejiang originated from multiple independent introduction events, including both domestic and international sources. This complex genetic structure highlights the need for strengthened quarantine measures at ports (especially Ningbo) and reduced human-mediated dispersal within the province to prevent admixture that could enhance invasive potential. However, we acknowledge that the limited sample sizes (mostly three nests per population) may affect the precision of allele frequency-based estimates, and our findings should be interpreted with this caveat.