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Oregon State University (1964)

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

Willard, John Royal Alexander

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

Auteur : Willard, John Royal Alexander

Université de soutenance : Oregon State University

Grade : Master of Arts (M.A.) 1964

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


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