Assessment of the Argentine ant invasion management by means of manual removal of winter nests in mixed cork oak and pine forests

The dynamic in Argentine ant colonies varies seasonally, influenced by biotic and abiotic factors. In winter the spatial range of the colony is contracted in large formations (winter nests) containing a large number of queens and workers. Winter nests are the clue to the species' dispersion power and the invasion of new habitats. For this reason a yearly elimination of queens and workers in winter at the edge (front) of the invasion could be a useful tool for weakening the species' dispersion and therefore limiting its establishment in new areas. Here, we determined the spatial dynamics of the Argentine ant nests during 1 year, and we assessed the invasion management by means of manual removal of winter nests for two consecutive winters, determining its effects during the following 3 years. We mapped nests found in 18 plots divided into two groups: extirpated (with removal of nests) and non-extirpated (control), along the fronts of three locations. Seasonal variation in the abundance of nests and workers, together with the two-year extirpation effects were evaluated. We found that colonies tended to follow an annual cycle of contraction and dispersion, with a decrease in the number of nests as we approached the invasion front. The extirpation was effective only in the front area, where it promoted smaller, less lasting and aggregated nests, as well as a decrease in the abundance of queens and workers. Nests also experienced a decrease during the two first winters but a recovery in the third, when no extirpation was done. Thus, a yearly perturbation should be performed to keep the expansion of the Argentine ant at a low rate, and to limit its establishment in new areas.