The Western Harvester Ants: Their Density and Hill Size in Relation to Herbaceous Productivity and Big Sagebrush Cover

Abstract

Ant colony density decreased but the denuded disc area increased as big sagebrush crown cover increased and as herbaceous productivity decreased. The western harvester ants (Pogonomyrmex occident&s Cresson and P. owyheei Cole) have been of interest since the colonization of the Western range. McCook (1879, 1882 a&b, and 1883) made intensive and extensive studies of the harvester ant as far west as Reno, Nevada. Biological and ecological studies were sporadic through the close of that century and the beginning of this century. Agricultural interest relating to their impact on crop production resulted in studies by Johnson et al. (1938), Costello (1944), Hull and Killough (1951), Gilbert (1960), Sharp and Barr (1960), and Wight and Nichols (1966). Control of these ants was first reported in 1942 by Fritz and Vickers, but widespread control studies were not conducted until the 1950’s and those are documented by the works of Knowlton (1966) in Utah, Lavigne and Fisser (1966) in Wyoming, Race ( 1966) in New Mexico, and Crowell ( 1963) in Oregon. The association of ants with, and their impact on, grazed ranges has not been totally resolved by the studies reported. Rogers (1972) associated an increase in hill density with pastures heavily grazed but Kirkham and Fisser (1972) were unable to produce this result after 10 years of grazing with varrous levels ot grazing intensity. They did show, however, that soil texture was the most important factor influencing ant distribution. Similarly, while some workers have estimated productive losses due to the denuded disc, Rogers (1972) and Wight and Nichols ( 1966) reported increased growth on the disc perimeter which partially or wholly compensate for production loss on the disc area. This note presents information of effects of harvester ant activity that spans a 40-year period. It relates changes in ant hill density and disc area with other measured attributes of their environment.