It is early afternoon on February 19th, and after weeks of cold, harsh winter, the sun is bright and the temperature is 72 F. Upland Chorus Frogs are calling from the wetlands, gnats swarm in circles in the meadows, and…large masses of fire ants bask in the sun. Displaying an unusual and little studied behavior, entire colonies of the Red Imported Fire Ant (Solenopsis invicta) leave their underground chambers to lie in tangled masses in the warm sun.
The following photographs were taken during an identical occurrence in January 2010.
Fire ants are highly temperature sensitive. Because they are incapable of surviving long, cold winters, their range is limited. In North Carolina, they have not spread into the northern border counties or most of the mountains. In Virginia, they inhabit only a few southern coastal counties.
Thermoregulation is an important biological imperative for fire ants, as demonstrated by their willingness to completely expose most of the colony during sudden, short warm spells in the wintertime. One or two days of warm weather in mid winter is not enough to warm the soil at the surface of the mound, so the only way for the ants to warm themselves is to come outside the mound into the sun. Even queen ants have been reported on the outside surface during these basking episodes. This would appear to be a risk to the colony, but apparently the benefit of warming outweighs the risks.