Formica obscuripes worker.

Formica obscuripes, also known as the 'western thatching ant' is found throughout the central and western US, and western Canada. F. obscuripes inhabits semi-arid sagebrush scrub lands, prairies, and various forest ecosystems.

These ants are obvious because of their large size, sometimes very large, organic nest-mounds, and colony populations in the tens, or hundreds of thousands. The worker caste is polymorphic, so large and small individuals (majors and minors) can be observed foraging, and performing other tasks.

In some western US states, mounds can reach over one meter in height, with tunnels and galleries extending several feet into the soil below the mound. The large nest-mounds collect solar radiation, warming the ants during cooler periods. The large mass of organic material may also help to generally moderate temperatures and humidity levels within the nest's interior. Workers are constantly adding bits of twigs, cut grass stems, conifer needles, and other material to the mound. Damage caused by high winds, rain, or even predators, is repaired by hordes of workers. Nests of Formica rufa, a closely-related European species, have been observed to be active for as long as forty years.

In some cases, groupings of related nests form enormous supercolonies. One of these assemblages in Oregon, USA, included 201 active nests, and an estimated population of over 56 million ants.

Food is obtained primarily by scavenging or preying upon insects and other arthropods, and by harvesting honeydew from aphids. In addition to food-gathering activities on the ground (and in shrubs), F. obscuripes workers forage high in the foliage of trees, and are important predators of western spruce budworm, and other forest 'pests'.

The founding of new colonies is usually carried out by the process of 'temporary social parasitism'. An inseminated F. obscuripes queen will enter the nest of another Formica species, and gain the acceptance of the 'host' workers. At some point the host queen is killed or driven off, and the host workers raise the brood of the invading queen. Eventually, only the invading species remains, as the original host workers die off. Many, if not most F.obscuripes nests eventually house more than one queen.

[IMAGE: Formica obscuripes worker - western Washington state, USA] [scroll down for additional notes/references]


·Cole, A.C. 1932. The Thatching Ant, Formica obscuripes Forel. Psyche 39:30-33, 1932.
·Conway, J.R. 1996. Nuptial, Pre-, and Postnuptial Activity of the Thatching Ant, Formica obscuripes Forel, in Colorado. Great Basin Naturalist, 56(1), 1996, pp. 54-58
·Jurgensen, M.F., Storer, A.J. & Risch, A.C. 2005. Red Wood Ants in North America. Ann. Zool. Fennici 42:235-242, Helsinki, 28 June, 2005
·McIver, J.D., Torgersen, T.R. & Cimon, N.J. 1997. A Supercolony of the Thatch Ant Formica obscuripes Forel (Hymenoptera: Formicidae) from the Blue Mountains of Oregon. Northwest Science, Vol. 71.No.1, 1997
·Weber, N.A. 1935. The Biology of the Thatching Ant, Formica obscuripes Forel, in North Dakota. Ecological Monographs, Vol. 5, No. 2, pp. 165-206