In Biblical times, they might have called it a plague.
But the sudden eruption of new fire ant mounds after a day of soaking rain is normal behavior for one of nature’s most opportunistic pests.
“The typical mature imported fire ant colony has about 80,000 ants in it, but nests of up to 240,000 have been reported,” said University of Georgia professor and entomologist Nancy Hinkle.
Although the insects are on the move when rainfall softens hard, dry soil and accelerates mound building, they are active all the time.
“The majority of what goes on is underground,” Hinkle said. “While the above-ground mound may be up to 15 inches high, a mature colony can extend to a depth of five feet.”
Ants use mounds for thermoregulation and often move the larvae to the side of the mound that is warmest. When temperatures get too hot, the nursery ants carry the larvae deeper to keep them from getting overheated.
“You’ve no doubt observed these ants frantically carrying the larvae to safety after you’ve kicked open a fire ant mound,” she said.
Although new mounds sprout like mushrooms after a rain, it is the colonies – and not just the mounds – that should be targeted with control efforts.
“This is why we recommend using fire ant baits for control. No matter how carefully one searches an area, it is easy to miss these hidden (often called ‘incipient’) colonies. So individual mound treatment will miss a large portion of the fire ant population in a given area,” Hinkle said.
Spreading the commercially available bait throughout an area will encourage roaming worker fire ants to carry the material back to their deep nests, where it can poison their nestmates.
“Not only do the ants do the work for us, they also clean up all the toxic bait and carry it underground, removing it from the environment within 24 hours so that we don’t have to worry about it remaining on the ground and affecting other animals,” she said.
Fire ant baits, she added, contain a very low rate of insecticide because it is formulated for direct consumption by the ants.
“Each particle requires only a minuscule dose, meaning that we have to apply much less chemical than if we were using a dust or spray,” she said.
Imported fire ants, native to South America, arrived in the U.S. in the 1930s and began infesting Georgia in the 1950s.