Where to move when it gets cold: winter nesting sites attractive to the Argentine ant (Hymenoptera: Formicidae)

The Argentine ant, Linepithema humile (MAYR, 1868), shows a cycle of regrouping of its nests in winter. Nests are located in similar sites every year, with a high density of queens in winter. We measured the physical characteristics, temperature and water content of 90 winter nests and control points, from December 2008 to March 2010. Additionally, we carried out a bimonthly monitoring of nest site fidelity throughout 2009 and in winter of 2009 / 2010. All studies were conducted in two invaded areas of the north-eastern Iberian Peninsula. The objective was to determine the preferred places for this species to build its nests in winter. Our results showed that nest location was influenced primarily by soil moisture, temperature and the vegetation around. Soil moisture inside nests was regulated mainly by temperature which in turn was controlled by orientation, rock color and canopy cover above the nest site. Canopy cover was related to the distance at which the nests were from plant structures, providing them a close food resource. Additionally, canopy cover could also play a two-side role helping to avoid high levels of soil moisture and extreme temperatures inside nests. Nest orientation and rock color may also help to protect the colony from extreme temperatures as well as to capture sunlight to provide an additional heat source maintaining optimal environmental conditions inside the nest, but also for colony activity during winter. Knowledge of the Argentine ant's nesting behavior is essential to initiate control methods such as the extirpation of winter nests with the consequent elimination of high densities of queens and their brood in invaded natural areashis work was financed by the Spanish Ministry of Science and Innovation with the support of a predoctoral grant (BES-2008-005102) associated with research projects MEC / FEDER2007-64080-C02-02 / BOS and CGL2010-16451, and a FT grant from the European Social Fund and the DIUE of the Autonomous Government of Catalonia in support of ML