Abstract: A supercolonial mound-building wood ant was intentionally introduced from the Italian Alps to Quebec, Canada, in 1971. This species was believed so far to represent Formica lugubris Zetterstedt, 1838. Yet, recent investigations on the distributions of F. lugubris and the closely related species F. paralugubris Seifert, 1996 in the Italian Alps showed presence of both species and also that the supercolonial social type is represented here mainly by the latter species. This raised doubts on the species identity of the Canadian ants and prompted a taxonomic re-investigation. Advanced exploratory and hypothesis-driven data analyses of worker phenotype of 152 nest samples of both species from the Alps and of two samples collected from the supercolony in Quebec convincingly confirmed the Canadian introduction to represent F. paralugubris. The Quebec samples were safely allocated to the F. paralugubris cluster in both Nest Centroid (NC)-Ward and NC-K-Means clustering, a nest-centroid based principal component analysis (PCA), and a linear discriminant analysis. The error of exploratory data analyses over all 154 samples varied between 0.6% (NC-KMeans) and 1.9% (NC-Ward, PCA). A new method calculating the size of nest populations of polygynous Formica rufa group ants is introduced, according to which the growth of the Valcartier introduction was estimated from about one million workers in 1971 to 19 million workers in 2005. Data on mating biology, strategy and speed of dispersal, colony structure, and ecological requirements indicate that active spreading of this ant to areas remote from the Valcartier beachhead is unlikely. There is also a low probability of passive dispersal by unintentional anthropogenic transfer of colony fragments. Although supercolonial, F. paralugubris lacks some of the essential properties of invasive tramp ants – its species-specific preadaptations are not comparable with the situation in the imported European Fire Ant Myrmica rubra (Linnaeus, 1758). A prediction of the role of F. paralugubris in the Nearctic forest ecosystems is presented. The concluded low risk of it becoming a dangerous invasive species does not refute the importance of keeping the situation in Quebec under careful observation.

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