Worldwide spread of the Argentine ant, Linepithema humile (Hymenoptera: Formicidae)

The Argentine ant, Linepithema humile (Mayr, 1868), originally from subtropical South America, is an important pest in many parts of the world. To evaluate its worldwide distribution and potential for further spread, we mapped records of L. humile from > 2100 sites. Because several South and Central American Linepithema species have been often misidentified as L. humile, we excluded all unconfirmed South and Central American records. We documented the earliest known L. humile records for 95 geographic areas (countries, island groups, major islands, and US states), including several for which we found no previously published records. We could not confirm any L. humile records from several South and Central American countries with published reports. Most records of L. humile come from the subtropics, particularly from regions with Mediterranean-like climates (i.e., warm dry summers and cool moist winters), including its native range in South America and exotic populations in California, the Mediterranean, southern Africa, Australia, New Zealand, and Japan. In more humid subtropical areas, such as the southeast US, L. humile rarely dominates outside urban areas. In tropical latitudes, L. humile dominates only at higher elevations, most notably in Hawaii. In temperate areas, L. humile is almost exclusively an indoor pest. Linepithema humile has already spread to most subtropical lowland regions with Mediterranean-like climates, but is not known yet from most tropical highland areas with suitable climates. In the past, L. humile probably arrived in tropical regions by sea accompanying human commerce and had to survive coastal lowland conditions before spreading to higher, cooler elevations. Nowadays air travel allows L. humile to stowaway in cargo delivered almost anywhere in the world. Therefore, a wider spread of this pest is expected in the future.