Foraging and nesting ecology of Acromyrmex octospinosus (Hymenoptera: Formicidae) in a costa Rican tropical dry forest

Leaf-cutting ants (Acromyrmex sp. and Atta sp.) in Costa Rica show many intra- and interspecific differences in ecology. Recent taxonomic studies question whether the Acromyrmex octospinosus populations on the Pacific and Atlantic slopes of Costa Rica are a single species. We therefore examined the foraging and nesting ecology of A. octospinosus in the tropical dry forest of Pale Verde National Park on the Pacific slope of Costa Rica and compared our findings with published data on the ecology of A. octospinosus in the tropical moist forest of La Selva Biological Station on the Atlantic slope. The Pacific A. octospinosus foraged primarily on the leaves of herbs and other small plants, fallen leaves, fruit, flowers, and insect frass, but does not cut the leaves of large trees. Worker size distribution within colonies was bimodal with only the larger workers leaving the nest to forage. Nests were shallow and generally under a few centimeters of organic debris at the base of trees and woody shrubs or in crevices. The foraging and nesting ecology of the Pacific A. octospinosus appeared to be very similar to that of the Atlantic A. octospinosus, except that the Pacific ants collected considerable amounts of insect frass (11% of all loads), whereas the Atlantic ants had no recorded loads of frass. This difference in selectivity, however, may have been due simply to seasonal differences in availability of frass at the sites. Acromyrmex octospinosus was the only species of leaf-cutting ant found at Pale Verde. The vertisol soil of the area, which has very poor drainage when wet and cracks deeply when dry, may not be suitable for major pest species of leaf-cutters in Costa Rica, Atta cephalotes and Atta colombica, which excavate nests deep underground.