P. salinus worker with large seed.

Known as 'harvester ants' [1], members of the genus Pogonomyrmex have long been known to collect a wide variety of seeds, grass spikelets, and other plant parts - but the list of potential food sources for these ants is much more extensive than this.

While seeds and other solid plant parts [2] seem to make up the bulk of most Pogos' diets, other plant-based resources like pollens, resins, and various plant exudates are also, at least for some species, occasionally collected. Many species also supplement this with living and dead insects/insect parts (including other ants), and sometimes other arthropods, like spiders. Pogos have also been observed to carry off small bits of dead mammals and reptiles, and to collect the feces of mammals and birds [3]. While not known to tend Hemipterans for their sugary excretions (like many other ants do) [4], there are at least two older records of Pogos obtaining honeydew from aphids [5], so it is possible that individual foragers may occasionally do this opportunistically. [also see [4] for a general note about trophallaxis in Pogos]

Many (but not all) Pogo species store a considerable amount of the gathered seeds in granaries, deep within the nest. These reserves probably serve the ants well during times of drought, and other periods when colony seed-intake is low. Evidence suggests, however, that at least for Pogonomyrmex salinus in central Oregon, the winter season (when the ants are dormant) is entered with very little, or no seed reserves. Apparently these ants do not build up a store of seeds in the fall, for winter consumption [6].

The exact manner in which Pogonomyrmex ants consume the seeds and other solid food items they harvest/scavenge is not a well-documented aspect of their biology. Since the actual consumption of this varied menu takes place within the subterranean confines of the nest, is not readily observable under natural conditions.

It is a well accepted fact that adult ants cannot consume/digest solid foods. The larvae of these and other ants, can, however, so much of this solid material is undoubtedly fed directly to these developing members of the colony. Exactly how the adults tap this nutritional resource is less clear.

McCook set up artificial nests and observed the eating habits of Pogonomyrmex barbatus. He determined that the adult ants manipulated the seeds, sometimes exerting considerable force with the mandibles, and then consumed the oils and 'juices' that oozed out (or were already present on the surface of the seed), with a licking action [7]. [IMAGE: a P. salinus worker dragging a relatively huge balsamroot seed (Balsamorhiza sp) back to her nest in central Washington state] [scroll down for additional notes/references]

ADDITIONAL NOTES/REFERENCES:

[1]
Many ants around the world share the life habit of gathering seeds (and related plant matter in many cases), and (along with Pogos) are referred to as harvester ants. These various species are often not closely related to one another, and the degree to which they rely on harvesting (and the particulars of their foraging/harvesting methods) vary greatly.

The bulk of these harvester ants are members of the Subfamily Myrmicinae, and include species in the following genera: Acanthomyrmex, Apheanogaster, Messor, Monomorium, Pheidole, and others. A few species in the Subfamilies Ponerinae and Formicinae are also considered to be harvesters. Like Pogos, most if not all of these other harvester ants also act as scavengers, and sometimes predators. NOTE: This is only a partial list of harvester ants - see reference works for more info.

·Hölldobler, B. & E.O. Wilson. 1990. The Ants. Cambridge, MA, Belknap/Harvard Press
·Johnson, R.A. 2000. Seed Harvesting Ants (Hymenoptera: Formicidae) of North America: An Overview of Ecology and Biogeography. Sociobiology Vol.36, No. 1, 2000
·Rico-Gray, V. & Oliveira, P.S. 2007. The Ecology and Evolution of Ant-Plant Interactions. University of Chicago Press, Interspecific Interactions Series, Chicago and London

[2]
Some species alternate in their reliance on seeds, as opposed to other plant parts, depending on the season, and availabilty of particular resources.

·MacKay, W.P. 1981. A Comparison of Nest Phenologies of Three Species of Pogonomyrmex Harvester Ants (Hymenoptera: Formicidae). Psyche, Vol. 88, No. 1-2, 1981

[3]
·Clark, W.H. & P.E. Blum. 1991. Observations of Ants (Hymenoptera: Formicidae: Myrmicinae, Formicinae, Dolichorderinae) Utilizing Carrion. The Southwestern Naturalist Vol.36, No.1, March 1991
·Cole, A.C. 1968. Pogonomyrmex Harvester Ants: A Study of the Genus in North America. Knoxville, University of Tennessee Press
·MacKay, W.P. 1981. A Comparison of Nest Phenologies of Three Species of Pogonomyrmex Harvester Ants (Hymenoptera: Formicidae). Psyche, Vol. 88, No. 1-2, 1981
·Taber, S.W. 1998. The World of the Harvester Ants. College Station, TX, Texas A&M University Press

[4]
In many cases, ants that have evolved the ability to exploit other major food sources, such as seed harvesters and those ants that cultivate fungus, seem to rely less on symbiotic relationships with sugary-secretion-providing insects (trophobionts) - even though they (the ants) may belong to sub-families in which these trophobiotic behaviors are highly developed.

·Hölldobler, B. & E.O. Wilson. 2009. The Superorganism. W.W. Norton & Company, New York and London

Many ants (including those of the sub-family Myrmicinae, to which Pogos belong) can store liquid or liquefied food in the crop where it can then be shared among colony members through regurgitation, in a process called trophallaxis (this process also involves the transfer of chemical information between members of the colony). It appears that trophallaxis does not play a major role in Pogonomyrmex colonies, and some species have apparently done away with it completely. We have not seen trophallaxis occurring in the Pogonomyrmex salinus workers we have observed.

·Hölldobler, B. & E.O. Wilson. 1990. The Ants. Cambridge, MA, Belknap/Harvard Press
·Taber, S.W. 1998. The World of the Harvester Ants. College Station, TX, Texas A&M University Press.
·Wagner, D., M.J.F. Brown, P. Broun, W. Cuevas, L.E. Moses, D.L. Chao & D.M. Gordon. 1998. Task related differences in the Cuticular Hydrocarbon Composition of Harvester Ants, Pogonomyrmex barbatus. Journal of Chemical Ecology, Vol. 24, No. 12
·Wilson, E.O.1975. Sociobiology: the new synthesis. Cambridge, MA, Harvard University Press

[5]
·Jones, C.R. 1929. Ants and their Relation to Aphids. Colorado Experiment Station, Colorado Agricultural College, Fort Collins Bulletin 341, February, 1929

[6]
·Willard, J.R. & H.H. Crowell. 1965. Biological Activities of the Harvester Ant, Pogonomyrmex owyheei, in Central Oregon. Journal of Economic Entomology Vol.58, No.3

[7]
·McCook, H.C. 1880. The Natural History of the Agricultural Ant of Texas. A Monograph of the Habits, Architecture, and Structure of Pogonomyrmex barbatus. Philadelphia: J.B. Lippincott & Co. 1880