Solenopsis invicta: Evidence for Recent Internal Immigration across Provinces in Argentina

Abstract

Ant tramp species refer to those with intercon tinental distributions that coincide with a likely predilection for hitchhiking with human move ments (Passera 1994). One such species, Solenop sis invicta Buren has been unintentionally trans ported across the globe from its native home in central South America over the last 70 years or more (Tschinkel 2006). Within South America it is found in a diverse set of biogeographic regions ex tending from its southern limit in the dry Chaco, across through the wet Mesopotamian region in Argentina and the Pampas of Uruguay and north wards through the Pantanal and other vegeta tional areas of Brazil (Pitts 2002). Its tramp sta tus as well as that of others in the genus renders 'original' or natural distributional limits difficult to interpret, with disjunct populations of this spe cies and others easily assigned to human trans port (Tr?ger 1991), rather than to relictual or other possible interpretations. Historical knowl edge of the biogeography of the genus in many of these areas is lacking and suggests that in many cases, only molecular analyses are likely to offer resolution to competing distributional interpreta tions. Here we note the recent, presumably intra continental immigration of Solenopsis invicta across provinces in Argentina. On 22 Aug 2006, while searching for Pseudacteon parasitoids ofSo lenopsis richteri in the Reserva Ecol?gica Costan era Sur (RECS) in Buenos Aires (S 34.6, W 58.3), we found 8 colonies of Solenopsis that we recog nized as S. invicta. Voucher specimens were sent to James Tr?ger who independently confirmed this diagnosis. Solenopsis invicta has never been recorded for the province (Pitts 2002) and the nearest source for the population is likely to be Rosario (ca. 250 km distant) in Santa Fe Province (Ross & Tr?ger 1990). The fauna for the area (in cluding that of the ants) is probably the best known for Argentina, as Buenos Aires is the de mographic and academic center of the country. If a pest species such as S. invicta had been found previously it is very likely to have been recorded. RECS is built on reclaimed land from the Rio de la Plata based on a polder-system. Initial rec lamation was begun in 1978 and the land was de clared a natural reserve in 1985 (Faggi & Czgononi 1987). The first date suggests the earli est possible time that S. invicta could have been present in the locality. Our group was visiting the Reserva on a weekly to monthly basis from 1996 through 2001, including very thorough searches of potential hosts of Pseudacteon (Bruzzone 2004). We had failed to find S. invicta, however, which suggests an even more recent invasion. We considered the hypothesis that this population had been moved in with the large volumes of soil used in current reclamation work in the park but have determined that this material was extracted