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How to identify it
- Workers are moderately large ants, about 5 to 7 millimeters (roughly 1/5 to 1/4 inch) long, and reddish brown overall.[1]
- Their relatively large heads hold powerful jaws for cracking seeds, and brushy hairs on the front of the head help them carry food and other materials.[1]
- Look for the mound: broad and flat, built over a network of underground tunnels, often topped with small excavated pebbles and surrounded by a wide area cleared of plants. The main entrance most often faces southwest.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Rough harvester ant | In Colorado, the rough harvester ant, found mainly in the southeast of the state, has a black head and thorax; the western harvester ant is reddish brown all over.[1] |
Where it lives
It is a grassland ant of open country. In Colorado, harvester ants are especially common on the eastern Plains and also live in the San Luis Valley and the grasslands of western Colorado, and the western harvester ant's swarming has been studied in Utah.[1][2]
Biology and life cycle
Harvester ants feed mainly on seeds, though they sometimes collect dead insects.[1]
The mound, its pebble covering and the cleared ring that keeps it from being shaded all help warm the nest.[1]
Winged males and would-be queens swarm to mate in summer, usually a few days after a heavy rain, and they gather at high points: swarms have been seen on hilltops, tall buildings, farmhouses and farm equipment, and mating balls have fallen down chimneys.[1]
Before a swarm, workers groom the winged ants outside the mound and guard them aggressively; a Utah study watched swarming and mating in July.[2]
Males die within 24 hours of leaving the colony. A newly mated queen breaks off her wings and lives on her flight muscles while she tries to found a colony, and if she succeeds she may live a decade or more as it grows.[1]
Newly mated queens probably ride on moving vehicles, which extends how far they disperse.[2]
Workers are long-lived and easy to keep in captivity, which is why harvester ants have long been sold in novelty ant farms for children.[1]
Damage and signs
- Each colony keeps an extensive area around its mound cleared of vegetation.[1]
Health risk
When the nest is directly disturbed, workers defend it vigorously and give very painful stings, though the stinger is relatively short and can't get through thicker skin. Workers also guard the winged ants aggressively before summer swarms.[1][2]
Control
Prevention
Give mounds a wide berth, especially in summer when workers are guarding winged ants before a swarm; the ants sting when their nest is directly disturbed.[1][2]
Common questions
- Why is there a bare circle around the ant mound?
- Western harvester ants clear the plants around their mounds. The cleared ring, the low mound and its pebble covering keep the nest from being shaded and help warm it.[1]
- Do harvester ants sting?
- Yes, and painfully, mainly when their nest is directly disturbed. The stinger is short, so it can't get through thicker skin.[1]
- Why are swarms of winged ants on my roof after a summer rain?
- Harvester ants hold their mating flights in summer, usually a few days after a heavy rain, and the swarms head for high points such as hilltops, tall buildings and farmhouses.[1]
Sources
- Colorado State University Extension (Western Colorado Insects), “Harvester Ants (Colorado Insects of Interest).” wci.extension.colostate.edu/wp-content/uploads/sites/14/2017/04/HarvesterAnts.pdf (accessed 2026-11-08)
- Great Basin Naturalist (Brigham Young University ScholarsArchive), “Swarming of the western harvester ant, Pogonomyrmex occidentalis (D. M. Allred, Great Basin Naturalist 40(2), Article 13, 1980; abstract).” scholarsarchive.byu.edu/gbn/vol40/iss2/13 (accessed 2026-11-08)
- iNaturalist, “Pogonomyrmex occidentalis taxon page (order and family).” www.inaturalist.org/taxa/69514 (accessed 2026-11-08)