Putative native source of the invasive fire ant Solenopsis invicta in the USA

Identifying the source population(s) of invasive species is crucial for understanding the ecological and evolutionary dynamics that occur when introduced species become invasive. This is especially important, as many species vary behaviorally, morphologically, and genetically among populations. In this study, we attempt to identify the source population(s) of the Red Imported Fire Ant (Solenopsis invicta) in the southern U.S.A. We utilize three classes of molecular genetic markers (Allozymes, microsatellites, and mitochondrial DNA sequences) and employ Bayesian clustering simulations, assignment and exclusion tests, and phylogenetic analyses to locate the source population(s) of S. invicta throughout the native rage in South America. We putatively assign the Mesopotamia flood planes around Formosa, Argentina, as the source region of introduced S. invicta. This result confirms previous suspicion that the source population(s) resides in northern Argentina, while discounting earlier claims that the Pantanal region of Brazil is the source area. Several lines of evidence suggest that S. invicta in the southern U.S.A. have colonized from a single region of Argentina (i.e. there have not been multiple invasions from different localities). Additional genetic analyses and finer scale sampling (particularly around the regions of northern Argentina and Paraguay) are ongoing and necessary to provide a more precise source population assignment. The results presented here have major implications for biological control efforts.