A supercolony of the thatch ant Formica obscuripes forel (Hymenoptera: Formicidae) from the Blue Mountains of Oregon
McIver, James D.
Torgersen, Torolf R.
Cimon, Norman J.
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Foliage-foraging ants have often been demonstrated to be important predators of tree defoliating insects such as western spruce budworm. Because of the importance of ants as pest suppression agents, we were interested in describing a rare supercolony of western thatching ants (Formica obscuripes Forel) from the Blue Mountains of northeastern Oregon. The supercolony occupies a site within a second-growth mixed-conifer stand near Lehman Hot Springs, approximately 20 km east of Ukiah, Umatilla County, Oregon. In October 1993, the 4-hectare colony had 210 active nests, with the largest nest having an aboveground volume of 6 m-3. Location of primary nests was not related to canopy cover, although nests tended to be associated with stumps and down woody debris. Nest excavations in May and June 1995 allowed estimation of the total nestbound population for the supercolony at 56 million individuals; addition of the foraging population would increase this estimate substantially. We estimate that to maintain the current worker population the supercolony would require at least 470 kg dry weight of food from its foraging territory annually, or about 11 times the total dry weight of western spruce budworm that might typically occupy the site at any given time during an outbreak. Thatching ants may have had a significant effect on populations of defoliating insects at Lehman Hot Springs during the last western spruce budworm outbreak (1980-1992), since the forest stand occupied by the supercolony was visibly less defoliated than the surrounding mixed-conifer stands