Observations on a little-known leaf-cutting ant, Acromyrmex volcanus (Hymenoptera: Formicidae)

Most well-studied leaf-cutting ant species (e.g., Atta cephalotes, Atta colombica, and Atta sexdens) have mature colonies with more than one million workers, nest on the ground, and primarily attack the leaves of trees. We examined the nesting and foraging ecology of an inconspicuous and little-studied leaf-cutting ant species, Acromyrmex volcanus, at La Selva Biological Station in Costa Rica. AcromyrTnex volcanus is known from the Atlantic side of Costa Rica and Nicaragua up to an elevation of 1000 m. Previous studies found that mature A. volcanus colonies have fewer than 20,000 workers, commonly nest in trees, and primarily harvest the leaves of herbaceous plants and fallen leaves, fruit, and flowers. By following laden foragers, we found the nest sites for nine A. volcanus colonies: seven in trees and two on the ground. In addition, we observed one founding A. volcanus queen starting a fungus garden on the ground, under a coconut husk. We examined foraging at seven A. volcanus colonies and discerned no consistent differences in foraging selectivity between day and night, nor between wet and dry season. Our observations generally confirm and augment earlier results, though we made a number of novel findings, including the first record of A. volcanus cutting the fresh leaves of a large tree. One A. volcanus colony, whose ground nest we disrupted, abandoned the nest site and moved their entire colony and fungus garden elsewhere less than one day later. Acromyrmex volcanus may generally start colonies on the ground, but move if disturbed. Disruption of ground nests may be a driving force behind the primarily arboreal nesting habits of A. volcanus.