[2] Like other harvester ants in the genus Pogonomyrmex, it is so called because of its habit of collecting edible seeds and other food items.

The specific epithet "occidentalis", meaning "of the west", refers to the fact that it is characteristic of the interior of the Western United States; its mounds of gravel, surrounded by areas denuded of plant life, are a conspicuous feature of rangeland.

When numerous, they may cause such loss of grazing plants and seeds, as to constitute both a severe ecological and economic burden.

Survivorship of colonies in the first year is negatively correlated with increasing density of foundresses.

[11] Workers' bodies are usually dark red and those in a mature colony are on average 6 mm long.

[15] Workers vary in size, but are not subdivided into groups of different-sized individuals with special roles.

A worker lives for an average of six months, and as it ages, it usually progresses through different roles within the colony.

[18] Workers usually forage one kind of item each day, but change their specialty daily.

Foragers produce more period mRNA during darkness, the timing of which varies seasonally.

[20] Cole et al. surveyed the distribution of Myrmecocystus mexicanus nests, (a species of North American honey ant) relative to the distribution of P. occidentalis nests at a site in western Colorado and found that there was a definite spatial association between the two species.

Thus, the M. mexicanus situates nests nearby P. occidentalis colonies to collect this source of nutrition.

There was no substantial evidence that M. mexicanus actively predates healthy P. occidentalis workers.

[25][26] Major mating swarms are about 1.4 kilometres (0.87 mi) apart,[27] and queens can fly no more than 800 metres (2,600 ft).

[15] Mating swarms that are consistently present and large determine much of the spatial variability in colony density and emerge over the long term.

[11][13] The population may self-thin through direct interference competition[11] resulting in a uniformly overdispersed distribution pattern.