Large and dense feeding aggregations of O. ferruginea directly over large trays of black pepper berries drying in the sun were studied near La Virgen. Heredia Province, Costa Rica. Several years of general observation on this species in northeastern Costa Rica revealed adult densities of less than one dragonfly/m2 at roadside ditches and clearings in secondary growth forest. Yet the densities observed over the black pepper ranged from 1.4 to 1.8 dragonflies/m2, a large increase over nearby habitats. Daily abundance of O. ferruginea over the black pepper was 30-50 individuals, predominantly males. Adults arrived each morning by 07.00 hours reaching peak densities by 07.30 hours, and flying over the fruit and capturing small flies as prey. The deliberate removal of the trays of fruit results in a sudden exodus of the dragonflies from the area. Undisturbed departures are usually by 08.00 hours. O. ferruginea opportunistically aggregates in areas of habitat where large densities of prey occur, although the mechanism underlying such behavior was not determined.