A SUPERCOLONY OF THE THATCH-ANT FORMICA-OBSCURIPES FOREL (HYMENOPTERA, FORMICIDAE) FROM THE BLUE-MOUNTAINS OF OREGON

Foliage-foraging ants have often been demonstrated to be important pre dators of tree defoliating insects such as western spruce budworm. Bec ause of the importance of ants as pest suppression agents, we were int erested in describing a rare supercolony of western thatching ants (Fo rmica 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, Uma tilla County, Oregon. In October 1993, the 4-hectare colony had 210 ac tive nests, with the largest nest having an aboveground volume of 6 m( 3). Location of primary nests was not related to canopy cover, althoug h nests tended to be associated with stumps and down woody debris. Nes t excavations in May and June 1995 allowed estimation of the total nes tbound population for the supercolony at 56 million individuals; addit ion of the foraging population would increase this estimate substantia lly. We estimate that to maintain the current worker population the su percolony would require at least 470 kg dry weight of food from its fo raging territory annually, or about 11 times the total dry weight of w estern spruce budworm that might typically occupy the site at any give n time during an outbreak. Thatching ants may have had a significant e ffect on populations of defoliating insects at Lehman Hot Springs duri ng the last western spruce budworm outbreak (1980-1992), since the for est stand occupied by the supercolony was visibly less defoliated than the surrounding mixed-conifer stands.