Harvester Ant Mound (Nest)
Wild Burro Wash (11/26/23)
You have probably seen the perfectly shaped ant mounds that look like mini-volcanoes on your hikes in the Sonoran Desert. There are two common types of mounds (nests) constructed by either leafcutter ants or harvester ants. See Animals & Plants of the Tortolita Preserve-8/20/23-Desert Leafcutter Ant (Acromyrmex versicolor) [Insect #10] and Harvester Ant (Pogonomyrmex) [Insect #22].
I was not quite sure how to identify which mound belonged to which ant, so I contacted Shaku Nair, PhD-entomologist at the University of Arizona Extension Service. I sent Shaku some photos and she provided the following methods to identify ant mounds:
Leafcutter Ant Mound-crater-like with sharp edges at the top, steep slopes toward the hole, made from moist or firm soil and located near vegetation.
Harvester Ant Mound-smoother and rounded at the top, gentile slope toward the hole, made of loose soil and found in very dry bare areas. Another identifier is midden or grass husks around the outside perimeter of mound which is the waste after the ants strip the seeds from the grass.
This ant mound stuff piqued my curiosity so I did some more research on the Harvester Ant (HA) and Desert Leafcutter Ant (DLA). Here is what I gleaned.
References are denoted in parenthesis with links below.
Desert Leafcutter Ant
DLA (1)(2)(3)
DLA is one of 47 species of leafcutter ants that inhabit the Southern US, Mexico, Central and South America.
Color-dark reddish brown with spikes on the back.
Bite/Sting-typically do not sting but can bite if provoked.
Each DLA has a specific role in the colony:
Queen-only DLA that lays eggs and produces more ants including queens.
Forager-searches areas around the nest for plant material (leaves, grass, flowers, etc.) and alerts the other foragers to assist with cutting the plant material into smaller pieces and bring them back to the nest.
Guards-ride shotgun on the plant material being transported back to the nest and keeps watch for a parasitic fly that can kill a DLA.
Gardeners-DLA does not actually eat the plant material. They mix the plant material with their saliva and a fungus that converts the plant material cellulose into carbohydrates and the DLA then eats the fungus for nourishment.
Excavators-create the tunnels and chambers (fungal gardens, storage, waste dumps, etc.) in the nest by removing one soil granule at a time and depositing at the outside of the nest.
Soldiers-protect the nest and queen from threats.
DLA Foraging & The Nest (4) (5)
Nest colony can have upwards of 50,000 workers.
Nest has many chambers each serving a specific function:
Fungal Garden-this is where the gardeners do their work to make food for the colony.
Storage-excess plant material is stored for use in the fungal garden.
Waste-infectious fungus and pathogens waste is stored to insure colony does not get sick.
Foragers typically follow specific paths from the plant material source to the nest.
Foraging occurs nocturnally in summer due to high soil temperatures.
Foragers can make up to 300,000 trips in one 24-hour period.
DLA can sense changes in atmospheric pressure and cease foraging when it rains.
Harvester Ant
HA (6)(7)
There are 22 HA species in the US and many of them reside in Arizona.
Color-dark reddish brown or black depending on species.
Bite/Sting-very toxic venom and can produce intense pain in humans for 4 hours.
Each HA has a specific role in the colony:
Queen-only HA that lays eggs and produces more ants including alates (fertile males and females that leave nest and form new colonies).
Patroller-searches areas around the nest for grass to harvest seeds and alerts the Harvesters. Needle Gramma is the favored HA grass treat.
Harvester-collect and harvest seeds from the grass.
Midden Worker-deposits trash and grass husks around the outside of the mound.
Excavators-create the tunnels and chambers in the nest by removing one soil granule at a time and depositing at the outside of the nest.
Maintenance Workers-repair nest and take care of queen and young ants.
HA Foraging & The Nest (8) (9)
Nest colony can have upwards of 10,000 workers.
Nest has many chambers each serving a specific function:
Storage-seed is stored for food.
Brood-home for queen and young ants.
Foragers go to specific zones around the nest identified by the Patroller.
Foragers extend a maximum of 130 feet from the nest and stay out about 20-30 minutes per trip.
Foraging occurs during daylight hours.