Determing Behavior in Harvester Ant Colonies

Here at the Social Insect Behavior lab one of our projects is understanding how the native Pogonomyrmex californicus (California harvester ants) are still surviving while the invasive Linepithima humile (Argentine ants) wreak ecological havoc in the Bernard Field Station. For some background information, harvester ants are about 8mm of an inch or almost a centimeter long, live in colonies of up to 20,000 ants, and have some intraspecies aggression; meaning they may attack members of other harvester ant colonies. Argentine ants, on the other hand, come in at about 2mm long and create large supercolonies with up to a million ants because of their extremely low intraspecies aggression. The research project I’m working on aims to understand how Argentine ant presence near a harvester ant colony impacts that colony's behavior, and to do that we need to be able to identify categories of ant behavior to see changes in colonies with or without a strong Argentine ant presence. We decided on using four different types of behavior categories for the harvester ants: standing, wandering, excavating, and foraging. I’ll tell you what those behaviors do for the ant colony and how we tell them apart.

A relevant part of understanding how we categorize behavior is understanding our filming setup. We set up GoPros centered above the nest entrance 9 inches above the ground and filmed in intervals throughout the day. A filming height of 9 inches created enough space to identify if ants were going far away from the entrance or just exiting and reentering. For the first and last hour of the day we would film for 30 seconds every 5 minutes to try to capture the nest entrance opening and closing. Then for the rest of the day we filmed for 5 minutes every hour. The camera setup is pictured below.

A photo of our filming setup above a harvester colony [1]

    

The first behavior I’ll talk about is standing. We define standing as ants going right outside the entrance and literally standing around; they don’t move very far or fast and they don’t bring anything in or out of the nest entrance. My guess for why harvester ants do the standing behavior is to guard their nest entrance and to warm up in the mornings (ants are ectotherms, meaning they get heat from their environment like crocodiles). Standing is shown in the video below by the ants who are standing near the nest entrance, there are also excavators and foragers which you'll see more later in this article.

                                    Colony 17 on 10/29/25: look for ants not moving much around nest [2]

Next I’ll talk about excavating; this is when the harvester ants open their nest or change the size of the entrance. At night they close their entrance entirely and re-open it in the morning. I’d often see them open their entrance in the morning when I’d set up the cameras. When ants are excavating they move back and forth in and out of the nest and can be seen carrying debris. This is a typical behavior at the beginning or end of the day, although I have also seen it at midday. It will often be accompanied by a change in size of the entrance. A video is below of the nest entrance being opened in the morning.

             Video from colony 17 taken 10/1/25: watch out for ants carrying material from entrance [3]

Foraging and wandering are the toughest to sort out because they both involve ants walking far from the nest entrance, usually out of view of the GoPros we filmed the colonies with. According to Deborah Gordon, a biology professor at Stanford, wandering (which she calls midden work) is how harvester ants sort the midden pile. The midden pile is a refuse pile for the colony that contains seed husks, parts of plants, and other waste that doesn’t have nutritional or structural value to the colony. Foragers on the other hand will come back to nest with seeds, which are harvester ants’ primary food source. Sometimes foragers will bring back a seed that's too big to fit into the nest. A video of foraging is below; I am confident it's foraging because of the fast and somewhat coordinated movements of the ants. 

        Colony 17 on 9/30/25: look for ants rapidly carrying material into the entrance (especially seeds) [4]

In conclusion, when we can categorize ant behavior it means we can understand and compare these behaviors across colonies and times. For the project I’m working on we are using these categories to understand how harvester ant behaviors change when exposed to invasive Argentine ants. However, categorizing behavior can also be used for understanding how other external factors like climate change are affecting harvester ants.

Further Reading 

1. Daugherty, Matt. “Argentine Ant.” Center for Invasive Species Research, cisr.ucr.edu/invasive-species/argentine-ant.
2. Gordon, Deborah M., and Natasha J. Mehdiabadi. “Encounter Rate and Task Allocation in Harvester Ants.” Behavioral Ecology and Sociobiology, vol. 45, no. 5, 15 Apr. 1999, pp. 370–377, https://doi.org/10.1007/s002650050573.
3. from Ironwood | Magazine of Santa Barbara Botanic Garden | Issue 36by Santa Barbara Botanic Garden. (2024, December 7). Ants! at the Disc Golf Course. Issuu. https://issuu.com/sbgarden/docs/sbb_8157_ironwood_fall_winter_2024_final/s/62456223
4. Writer, M. P. C. (2025, March 5). Harvester Ants. Jackson County Herald Tribune. https://www.jacksonconews.com/article/599,harvester-ants

Media Credits

1. Natalie Minor, HMC Bee Lab
2. HMC Bee Lab video GX010409
3. HMC Bee Lab video GX010127
4. HMC Bee Lab video GX010100