Initiation of swarming behavior and synchronization of mating flights in the leaf-cutting ant Atta vollenweideri FOREL, 1893 (Hymenoptera: Formicidae)

Abstract

Leaf-cutting ants of the genus Atta build giant nests, inhabited by millions of workers. During a few days in spring, thousands of alates leave their mature home colonies for their mating flights. These flights are synchronized on a large geographical scale, and weather conditions have been reported to play a crucial role in determining when mating flights occur. Nevertheless, many fundamental aspects of the fascinating swarming behavior in Atta are unknown. In this study we describe the three successive phases of the swarming behavior of A. vollenweideri Forel, 1893, the initiation phase, the aggregation phase, and the mating flight. Prior to take-off, alates of both sexes exhibit distinct pre-flight behaviors. Atta vollenweideri is a day-flying species, with mating flights occurring in the late afternoon before dusk, and it is the southernmost species of the genus, experiencing strong seasonal climate. In order to identify climatic parameters that induce swarming behavior and elicit a synchronized mating flight, we analyzed 23 swarm events and the corresponding climate data from 2004 - 2010 recorded in northern Argentina. Colonies prepare for mating flights in the spring after a cumulative precipitation of at least 64 mm in the last month before the first mating flight. Only if temperatures rise above 26°C on the days following a major rainfall, alates may leave the nest mound, although they prefer temperatures of about 32°C. When accounted for together, rainfall and a subsequent temperature increase are highly predictive and thus prerequisites for swarming behavior in this species. We propose that Atta species have based on the preferred depth of the founding chamber and local soil conditions speciesspecific thresholds for cumulative precipitation. In A. vollenweideri, the heavy clay soils that are desiccated after the austral winter select for mating flights and subsequent colony founding only after very high precipitation.