Fire ants use mounds to regulate temperature and moisture conditions in the colony. You may see mounds pop up in response to cooler weather or heavy rains. The mound is an above-ground nursery that provides optimal temperature and moisture conditions. The workers move the brood (larvae and pupae) and the queen(s) around the tunnels as needed to keep them in the best conditions.
On sunny winter days, the workers move the brood to the warmer, upper portions of the mound to take advantage of the extra heat. When it is too hot or too cold, fire ants move deep into the ground for protection from these weather extremes.
Mounds that appear to pop up are from colonies that have been living underground for some time. Mounds may also appear because the ants are cleaning out tunnels or preparing for a mating flight of reproductive ants.
Watch video of fire ants building a mound in an ant farm.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.