Global invasion history of the Tropical Fire Ant: a stowaway on the first global trade routes

Biological invasions are largely thought to be contemporary, having recently increased sharply in the wake of globalization. However, human commerce had already become global by the mid-16^th century when the Spanish connected the New World with Europe and Asia via their Manila galleon and West Indies trade routes. We use genetic data to trace the global invasion of one of the world's most widespread and invasive pest ants, the Tropical Fire Ant, Solenopsis geminata. Our results reveal a pattern of introduction of Old World populations that is highly consistent with historic trading routes suggesting that Spanish trade introduced the Tropical Fire Ant to Asia in the 16^th century. We identify southwestern Mexico as the most likely source for the invasive populations, which is consistent with the use of Acapulco as the major Spanish port on the Pacific Ocean. From there, the Spanish galleons brought silver to Manila, which served as a hub for trade with China. The genetic data document a corresponding spread of S. geminata from Mexico via Manila to Taiwan and from there, throughout the Old World. Our descriptions of the worldwide spread of S. geminata represent a rare documented case of a biological invasion of a highly invasive and globally distributed pest species due to the earliest stages of global commerce.