Biological activities of the harvester ant, Pogonomyrmex owyheei Cole, in central Oregon

Graduation date: 1964The mound-building harvester ant, Pogonmyrmex owyheei Cole,\ud (formerly included in the species occidentals) is widely distributed\ud on the semi-arid rangeland east of the Cascade mountains in Oregon.\ud Because of its seed foraging and vegetation clearing habits, this ant\ud has been considered as a possible competitor with livestock and\ud wildlife for the limited forage available.\ud The stages of development of P. owyheei include eggs, larvae,\ud pupae and adults. There are three castes: males, queens, and workers.\ud During the winter only workers and queens are found in the nest. In\ud 1963 egg laying began about May 15 and continued until mid-September.\ud The first larvae appeared in the mounds at Redmond, Oregon about\ud June 1. The first reproductive pupae appeared June 30, and numbers\ud increased until July 27. Swarming of the alates began about July 27\ud and continued until August 21. Worker pupae were found from July 6 to\ud October 5, but the largest number occurred between July 20 and\ud August 21. Callow workers were first observed July 11, and population\ud levels followed closely behind those of the worker pupae. The first\ud brood in the spring, requiring 38 to 55 days to develop, consisted almost entirely of reproductive forms. Later broods, consisting\ud entirely of workers, required 30 to 45 days to develop. The ants had\ud become dormant by November 2, 1963.\ud Ant colony population counts, made by excavation of nests in\ud late winter of 1963, showed numbers ranging from 173 to 3703 worker\ud ants per colony. During the summer of 1963 (July to October) numbers\ud of different immature forms present in a colony were determined. The\ud population of immatures ranged from 36 (October 5) to 1014 (July 27)\ud with an average of 376 per colony.\ud P. owyheei constructs a cone-shaped, pebble mound in the center\ud of a cleared area. Usually there is a single entrance on the east\ud or southeast side at the base of the cone. Mounds ranged from 8 to\ud 72 inches in diameter and from 2 to 14 inches in height. Excavations\ud of the nest showed that most of the galleries and chambers were\ud located on the entrance side of the mound and in a core directly\ud beneath the entrance, below ground level.\ud During the study 21 insect species, including both myrmecophiles\ud and predators, were found associated wlth p. owyheei. The insects\ud most commonly found in P. owyheei nests were larvae of Serica falli\ud Dawson, Phyllophaga sociata Horn, and Pelecyphorus densicollis Horn.\ud P. owyheei maintains an area cleared of all vegetation around\ud the mound, ranging in size from 3 to 30 feet in diameter. The ants\ud began clearing early in May and continued until late June, but did\ud not succeed in completely removing all plants present. Sisymbrium\ud altissimum, Descurainia pinnata, and Bromus tectorum were the most common plants, but a total of 25 species were observed being cleared\ud by the ants.\ud Foraging experiments, using colored grains, showed that workers\ud foraged heavily up to 50 feet from the mound, and the amount of grain\ud harvested increased substantially as the foraging distance decreased.\ud Results obtained on an isolated mound were similar to those from\ud mounds surrounded by other active colonies. The ants foraged the\ud seeds of Bromus tectorum, Phacelia linearis, Descurainia pinnata, Sisymbrium altissimum and Hordeum jubatum most heavily, but a total\ud of 29 different seed species were observed being carried into the\ud nests.\ud This study confirms the observed fact that the use of poisoned\ud baits for control is most effective when applied during the warmer\ud part of the summer when the ants are actively foraging. To reduce the\ud possibility of reinfestation by alate reproductives, application\ud should be made in June before pupation of the reproductive larvae