ANNUAL FORAGING OF THE LEAF-CUTTING ANT ATTA-COLOMBICA IN A SEMIDECIDUOUS RAIN-FOREST IN PANAMA

In a 1-y study of vegetation harvested by the leaf-cutting ants, Atta Colombica Guerin, daily harvesting activity of two nests was observed for 24 h at c. I-wk intervals (colony I: June 1993-June 1994; colony I I: February-June 1994) on Barro Colorado Island. The average daily qua ntity of green leaves harvested by colony I was higher during the wet season (11.4 m(2) d(-1)) than during the dry season (9.0 m(2) d(-1)), but was highly variable between survey days. Total annual herbivory of green leaves was estimated to be 3,855 m(2) foliage area for colony I and 1,707 m(2) for colony II. Total dry weight of biomass harvested w as higher in the dry season because most material collected during the wet season consisted of green leaves, while during the dry season, mo re than 50% of the total collected biomass was non-green plant materia l (stipules of Ficus sp., fruits, seeds, and flower parts of a variety of other species) which represented c. one third (ill kg y(-1)) of th e total annual intake (370 kg y(-1)) of plant material. Total daily bi omass intake was negatively correlated with daytime rainfall. The peak of daily foraging was affected by timing and duration of rainfall eve nts. Highest input rates normally occurred between 15:00 and 16:00 h ( colony I). Dry weight and surface area of harvested leaf fragments dif fered between plant species, with thicker leaves generally being cut i nto smaller pieces. Significant linear correlations were found between total daily harvest of fragments and the respective harvesting rate a t the maximum of daily activity. High correspondence was found between estimates using this relationship and the measured daily leaf harvest of four other Atta colonies and of two colonies reported in published literature. The use of this relationship as a research tool is discus sed.