Effects of colony-level variation on competitive ability in the invasive Argentine ant

Although colony size and nest number are believed to influence competitive ability in social insects, experimental studies testing this idea are rare. Here, we experimentally manipulated worker number and nest number in laboratory colonies of Argentine ants, Linepithema humile, to test how these attributes, working alone or in combination, affected different components of exploitative and interference ability. As expected, every measure of competitive performance tested increased with worker number. In contrast, the influence of nest number was more complex, with colony-level performance increasing, decreasing, or remaining constant depending on the type of competition-related test or colony-size category being considered. In the exploitation of randomly distributed food items, retrieval rates decreased with nest number for 50-worker colonies yet increased with nest number for 2500-worker colonies. In contrast, retrieval rates decreased with nest number across all colony sizes when resources were clumped. In two experiments on interference competition, only colonies with more than 1000 workers and that occupied single nests were able to maintain more than 10 workers, on average, at baits in the presence of a competitor, Forelius mccooki. Argentine ant workers initiated a majority of pairwise fights against F. mccooki workers but often lost (i.e. either retreated or were injured), in part because their pre-emptive attacks left them vulnerable to chemical defensive compounds used against them by Forelius. These findings promise to promote a better understanding of the causes of invasion success not only for the Argentine ant but for other invasive ants as well, given that most have colony structures resembling that of L. humile. (C) 2001 The Association for the Study of Animal Behaviour.