P. salinus ants cutting down a desurainia plant

Pogonomyrmex salinus is just one of the Pogo species that will often actively remove plants (including grasses) that grow on, or adjacent to the nest mound.

Removal of vegetation can deprive predators (e.g. spiders, horned lizards) of a convenient hiding place from which to prey on the ants. Also, plants close to, and on the mound itself, could block sunlight from reaching the surface of the nest mound in the morning. This can result in cooler temperatures in the upper nest (cone or mound) interior, thus reducing activity/foraging time for the workers - or possibly having an effect on brood development (and placement, or movement of brood within the nest interior).

Other theories as to the function of these plant-removing activities, include the idea that plant roots could intrude upon and damage the tunnels and galleries below. There is also the possibility that large peripheral clearings could serve as firebreaks - protecting the colony from the ravages of wildfires.

It also likely that these cleared areas (in conjunction with chemical markers deposited by the ants) might aid in the delineation of a particular colony's territory. There are undoubtedly a number of factors that compel these ants to make clearings around their nests, or to simply remove plants growing directly on the nest mound itself.

We, and others, have observed P. salinus workers (in OR and WA) engaged in this plant-removal behavior. Most of this activity takes place in early spring. It seems that in many (or most) cases, a number of workers will climb a plant, and cut the leaves off with their mandibles - starting from the top (we most commonly observe the ants accomplishing this in a 'head down' orientation). In some instances, a denuded stalk is left standing - in others, the entire plant is systematically dismembered until only a tiny stump remains (in April 2010, we observed P. salinus workers digging down a few millimeters into the soil, in order to cut the stumps of grass stems off below ground level). Sometimes it is obvious to us that a certain plant is too large, or robust for the ants to successfully remove - so it remains standing.

It is not uncommon for us to observe a P. salinus nest surrounded by a large defoliated area, while just a dozen meters away, another active nest of the same species has no discernable clearing at all, and may in fact have some plants growing on the nest mound itself.

In some species, when large clearings are present, they can occasionally reach diameters of up to 10m/33ft or more, though diameters of 3m/10 ft are more typical. The seed harvesting, and nest clearing habits of Pogos (such as P. occidentalis, and P. salinus), have been cited as a cause of significant damage to western rangelands, though some believe that areas of high nest density (and consequently high incidence of plant removal) are a symptom of damaged (overgrazed) lands. Also, these activities, while reducing the cover of some plant species, often indirectly increase the cover of others.

[IMAGE: Pogonomyrmex salinus workers systematically cutting down a Descurainia plant growing too close to their nest mound, in central Washington state] [scroll down for additional notes/references]


·Anderson, K. E. & J. C. Munger. 2003. Effect of Temperature on Brood Relocation in Pogonomyrmex salinus (Hymenoptera: Formicidae) Western North American Naturalist 63(1), 2003, pp. 122-128
·Cole, B. J. 1994. Nest Architecture in the Western Harvester Ant, Pogonomyrmex occidentalis (Cresson) Ins.Soc. 41:401-410 (1994)
·Soulé, P. T. & Knapp, P. A. 1996. The Influence of Vegetation Removal by Western Harvester Ants (Pogonomyrmex owyheei) in a Relict Area of Sagebrush-Steppe in Central Oregon. American Midland Naturalist 136(2): 336-345
·Soulé, P.T. & P.A. Knapp. 1996. Pogonomyrmex owyheei Nest Site Density and Size on a Minimally Impacted Site in Central Oregon. Great Basin Naturalist 56(2) pp, 162-166
·Taber, S.W. 1998. The World of the Harvester Ants. College Station, TX, Texas A&M University Press.
·Wheeler, W.M. 1910. Ants: Their Structure, Development and Behavior. New York, Columbia University Press
·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