Floating balls of fire ants!!!

We have had quite a bit of flooding here in South Carolina this week and many people have reported seeing floating masses of fire ants. Such masses were also reported during the floods in Texas this summer and most people in Louisiana are familiar enough with floods to be aware of this phenomenon.

Fire ants and their brood floating in a man-made flood.

Fire ants coalescing into a raft.

Close up of fire ant raft. You can see the white grubs and pupae scattered throughout the raft.

 

You may have noticed that fire ant mounds are much more common in open areas with plenty of sunlight. Fire ant mounds are rare in wooded environments. This is because the red imported fire ant, Solenopsis invicta, is native to the flood plains and river banks of Brazil and Argentina. Disturbances, especially floods, are frequent enough on flood plains and river banks that many trees never get very big before their roots are waterlogged and they die or they are physically knocked down by fast flowing water.

Nesting in open environments may be important in regulating colony temperature. Direct insolation of the mound surface can increase the temperature inside the upper parts of the nest. When it is cold, ants can move brood to warmer parts of the nest on sunny days. Extreme winter cold is a key cause of colony mortality in the red imported fire ant’s northernmost range. They cannot survive conditions that freeze the soil to the depth they are nesting. Although fire ant nests have been measured at 10 feet deep this is probably very rare. Based on fire ant mounds cast on the Ant Hill Art website (anthillart.com; If you have not seen them, they have some great footage of casting fire ant nests with molten aluminum), 21 nests ranged in depth from 17.8 cm to 54.61 cm, with a mean of 34 cm deep. At these depths the soil temperature can still drop dramatically, though more slowly than air temperature.

Cold is not something Solenopsis invicta has had to deal with in its native range, but flooding is a regular occurrence. Fire ants, like almost all insects, have a cuticle that strongly repels water. When it floods, the ants gather their eggs and larvae and hold on to each other, forming a hydrophobic raft that floats. The ants can survive in these rafts for weeks. When they arrive at dry land they can begin to construct their nest again.

This behavior is often exploited by scientists studying fire ants and the organisms that live with them in their nests. A large cooler or wash tub is filled with water near the mound (you don’t want to have to carry the tub of water). Shovelfuls of ant mound, soil and all, are dropped into the tub. The soil sinks, but the ants and their guests float.

Once I was contacted by a documentary film crew that wanted to film fire ant rafting behavior. I told them about the tub trick and they weren’t impressed. They wanted to get a fire truck to pump water over fire ant mounds and film that!