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 man y intra- and interspecific differences in ecology. Recent taxonomic st udies question whether the Acromyrmex octospinosus populations on the Pacific and Atlantic slopes of Costa Rica are a single species. We the refore examined the foraging and nesting ecology of A. octospinosus in the tropical dry forest of Pale Verde National Park on the Pacific sl ope of Costa Rica and compared our findings with published data on the ecology of A. octospinosus in the tropical moist forest of La Selva B iological Station on the Atlantic slope. The Pacific A. octospinosus f oraged 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 sh allow and generally under a few centimeters of organic debris at the b ase 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 co llected considerable amounts of insect frass (11% of all loads), where as the Atlantic ants had no recorded loads of frass. This difference i n selectivity, however, may have been due simply to seasonal differenc es 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 de eply when dry, may not be suitable for major pest species of leaf-cutt ers in Costa Rica, Atta cephalotes and Atta colombica, which excavate nests deep underground.