Invasive Fire Ants Excavate Soil

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Solenopsis invicta, the red imported fire ant, is a native to South America, the United States, Australia and China. 80 years ago, the red fire ants dug their nests from Georgia to Los Angeles. According to Professor Goldman, these invasive fire ants can dig into anything, and are especially good at excavating soil.

Researchers used three-dimensional scanning and saw that these fire ants can build complicated nests, because they can move grains of any size. The fire ants also change their digging styles when excavating different types of soil.

Professor Dan Goldman from the Georgia Institute of Technology, was the lead researcher on this project. Findings from this project have been published in the Journal of Experimental Biology. Goldman says that the invasive red imported fire ants excavate the soil to create complicated underground structures using tiny extremities.

Humans have not designed anything that can manipulate soil or sand as refined as these fire ants can. The ants’ mining capabilities were tested, by allowing groups of 100 fire ants dig in cylinders that contained three different types of soil, small, medium or large-grained. The ants were also tested in soils with different moisture levels.

Making consistent imitation soil conditions for the ants to excavate was not simple. Different people would create different conditions. Then Daria Monaenkova, Nick Gravish and Sarah Sharpe, got the idea to strain the sand and water mixtures for uniform distribution of water in the soil. 14.5cm long tubes were filled with uniform soil samples, that ranged from tiny clay-like pieces to ant-head sized grains of sand. The moisture content ranged from completely saturated, to totally dry, and then researchers dropped mini-colonies of 100 fire ants on the top of the tubes, and left them to excavate for 20 hours. The challenge after that, was to observe the nests without destroying them. The tubes were x-rayed at 400 shots per nest. They were then reconstructed with three-dimensional structures with Greggory Rodriguez and Rachel Kutner, to be able to see what the diligent ants achieved.

An experiment was designed so that the researchers could x-ray the ants’ nests, as they have changed over the past 40 years. The red fire ants made tunnels faster in more coarse soil, but built complex structures with annexes in moist coarse soils. However, when the moisture level went above five percent, the fire ants dug tunnels.

A second experiment was put together to see individual fire ants at work. Monaenkova filmed the ants while they dug in clear cylinders that were full of different sized glass beads, and the fire ants treated them just like soil. This experiment showed that the ants had two different excavation methods. In the more coarse beads, they would grab onto a single particle, then shuffle backwards, up the tunnel, dragging the bead. When the ants were in smaller beads, they would grab and compress multiple grains, making them into a pellet, and then the ants would brace themselves against the side of the tunnel with their legs. Then they collected the pellet that they created with the beads, turned around, and then head upwards in the tunnel.

Goldman was mostly impressed with the ants’ creativity in making pellets out of the beads. The pellets were of the same size each time. The ants used their jaws, forelimbs and also their antennae. He said that it was a marvel to watch how well the red ants could dig.

Goldman said that studying these ants could help to create future designs of search and rescue robots, that would be capable of working in large groups or teams. The robots will be able to maneuver the environment in unstable conditions. The invasive red fire ants demonstrated how large groups can work together in tight spaces, such as in excavation of soil and created structures in loose materials.

By Jeanette Smith


Photo courtesy of Vishal R – Creativecommons Flickr License

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