Since 2019, entomologist Sara Colby has conducted fire ant density surveys across agricultural corridors in southwestern Georgia, working from the University of Georgia campus in Tifton, within the state’s peanut and cotton belt. Her monitoring protocol was straightforward: fixed survey lines along the same field margins, shrub lines, and pasture edges each growing season, using standard mound counts per hectare to track population pressure on collaborating farms.
In the fall of 2022, three farms in Colquitt County began showing mound density figures she had not recorded at any of those sites in four years of surveys. The numbers were not a dramatic collapse to zero, which would have suggested chemical treatment, but rather a targeted, measurable, and consistent decline. All three farms showed the same trend in the same season. She spent two months reviewing treatment records, rainfall data, and tillage history before ruling out every management variable she could measure.
She then returned to the insect trap archives from those sites and began counting what was present in the samples alongside the fire ants. What she found in the trap records from those three farms was not what anyone in Georgia’s agricultural entomology field would have expected for those numbers in those locations in 2022.
What is happening to fire ant populations in certain agricultural areas of southwestern Georgia is not the result of any current fire ant management program. The explanation points to three separate biological mechanisms operating simultaneously, without coordination, funding, or approved permits.
To understand what Colby’s data showed, it helps to understand what the red imported fire ant has meant for Georgia agriculture for nearly a century. Fire ants arrived in the United States through the port of Mobile, Alabama, in the 1930s, likely in soil ballast discharged from cargo ships coming from South America. In their native range in the Pantanal plains and the Paraná River system of Argentina and Brazil, fire ants remain in ecological balance, checked by a group of predators, parasites, and native competitors that evolved alongside them over millions of years.
In the southeastern United States, none of those constraints existed. Fire ants encountered a landscape of agricultural fields, pasture margins, and roadsides that provided unrestricted foraging areas. Winters were mild compared to their South American home, and no native predator, parasite, or competitor could overcome the colony’s defensive system. They spread north and west from Mobile at a pace documented in extension service records.
By the 1990s, fire ants had established themselves in all 159 Georgia counties. By 2000, estimated economic losses to Georgia agriculture, from fire ant mounds interfering with equipment, injuring livestock, and directly damaging crops, were in the hundreds of millions of dollars annually.
What made fire ant control so difficult was not the venom or aggression, though both mattered. It was colony structure. A mature fire ant mound can contain 200,000 to 500,000 workers, multiple queens in polygyne colonies, and a chemical communication system sophisticated enough to mount coordinated defense within seconds. When farmers used broadcast baits, the colony queen survived underground in protected chambers and rebuilt the surface mound within one foraging season. When tillage destroyed surface mounds, the colony relocated and rebuilt. When chemical treatments killed ants, surviving queens established satellite colonies in untreated adjacent land.
Fire ants’ reproduction rate, colony resilience, and ability to survive underground made them functionally resistant to any single management intervention at the farm level. Georgia farmers who had struggled with fire ants for 30 years described them as a tax on doing business in the Southeast, the kind of annual problem budgeted for each year like fuel or seed. A permanent cost that was reduced but never eliminated.
The first biological mechanism in the trap records involves phorid flies, specifically species in the genus Pseudacteon. These parasitic flies are native to South America, having evolved in the same Pantanal river system where fire ants originated. In their native environment, phorid flies are one of the main biological pressures keeping fire ant colony densities in check, causing ecological disturbance without ecological dominance.
The flies are small, with adult females measuring only a few millimeters long. What they do with those few millimeters is why fire ant colonies in South America never reach the mound densities that became normal in Georgia. The fly hovers above a disturbed fire ant mound, targets a worker ant, and in a motion taking less than a tenth of a second, inserts its ovipositor through the outer layer of the ant’s thorax to deposit a single egg. The worker ant walks away with no immediate behavioral change. Over the following weeks, the egg hatches into a larva that migrates to the ant’s head capsule and begins consuming the neural and muscle tissue inside. When the larva is fully developed, the ant’s head separates from its body and falls to the ground. The phorid larva pupates inside the detached head capsule and emerges as an adult three to six weeks later, immediately beginning to search for another fire ant colony to parasitize.
The USDA released multiple Pseudacteon species into fire ant populations across the Southeast starting in 1997, targeting sites in Florida, Texas, and Georgia as initial release points. That part of the story was planned and funded. But Colby’s trap data from the Colquitt County farms showed something different: densities and geographic locations of Pseudacteon flies that do not appear in any USDA release documentation for those sites. The flies had spread well beyond their original release areas into agricultural habitats they reached on their own, and were appearing in field-edge traps in numbers that matched the decline in mound density in her survey data.
Direct parasitism rates alone, however, did not explain the scale of population suppression she was measuring. The mechanism operated on two levels. In areas where phorid flies were present, fire ant workers significantly reduced foraging activity during the hours when fly numbers peaked, typically from mid-morning to early afternoon when temperature and light conditions favored fly activity. A fire ant colony that retreated underground for four to six hours per day lost 30 to 50 percent of its effective foraging time. Reduced foraging meant fewer calories for the colony. Fewer calories meant slower queen reproduction and reduced mound expansion. The phorid flies were not just killing fire ants; they were suppressing the colony’s ability to operate at full capacity during the entire foraging period.
The armadillo data in the same trap records required a different explanation and a phone call to the Georgia Department of Natural Resources. The nine-banded armadillo, Dasypus novemcinctus, is not native to Georgia. It is a South American species that reached Florida in the early twentieth century through a combination of zoo escapes and deliberate releases from the exotic pet trade. Its northward expansion across the Southeast has been one of the most consistently documented wildlife range shifts in North American conservation biology over the past 30 years.
Georgia DNR records showed that armadillo detections across southwestern Georgia counties had increased significantly in the years before her mound density declines, and that the confirmed armadillo presence front tracked closely with the geographic distribution of the state’s high-density fire ant zones. The connection was not incidental. Fire ant mounds in Georgia’s clay-rich agricultural soil create underground infrastructure of excavated chambers where protein is concentrated at densities a highly specialized armadillo olfactory system can detect through several inches of soil. A single armadillo working field edges at night can excavate 15 to 20 fire ant mounds in one foraging session. The armadillo does not need to consume every ant in the mound. The excavation itself destroys queen chambers, exposes brood to thermal stress, and forces the colony to direct resources toward rebuilding rather than expansion. In areas with intensive armadillo activity documented in DNR records, Colby’s mound density surveys showed suppression patterns not fully explained by phorid fly data alone.
The third signal in the trap records was the most significant in terms of what it suggests for how agricultural ecologists in Georgia view fire ant management. In the same field-edge transects where fire ant mound density was declining, trap records showed increasing numbers of harvester ants, Pogonomyrmex badius, and four species of native ants in the genus Messor, native to Georgia’s coastal plain. These were ant species that fire ant expansion had effectively eliminated from most of southwestern Georgia’s agricultural landscape over the previous 40 years.
Fire ants do not simply outcompete native ant species for food resources. They overwhelm them chemically. The chemical signals of fire ant colonies trigger alarm and evacuation responses in native ant species that have no evolutionary countermeasures to fire ant defensive chemistry. In areas with high fire ant mound densities, native ants abandon the surface entirely. Colby’s data from the Colquitt County farms showed that in areas where behavioral suppression by phorid flies and mound destruction by armadillos had reduced fire ant surface dominance below a certain density threshold, native harvester ants were returning to territory they had not occupied in decades.
The native ants were not just moving into vacant space. They were competing for it, establishing foraging trails, and creating competitive pressure on fire ant colony stability at the edges of suppressed zones. Any ground fire ants could not defend consistently was ground they could not reliably hold.
Raymond Tanner has farmed peanuts and cotton on the same 700 acres in Colquitt County since 1989. He described fire ant mounds as a permanent feature of every field edge he had ever managed, the kind of problem budgeted for each year like fuel or seed. In the fall of 2022, he noticed his tractor was no longer hitting mounds in parts of his field edges that had consistently contained them for as long as he could remember working that land. He did not call anyone about it immediately or file any report. He assumed the previous spring’s bait application had worked better than usual. When the next season produced the same result in the same areas with no bait application at all, he contacted the University of Georgia extension office.
The extension agent who came to inspect the fields that evening pulled Colby’s contact information from a recent publication and made the call. Colby was already standing at a field edge three counties north, counting the same signal in her traps. Two people who had never communicated, in adjacent counties, were observing the same change in the same direction at the same time, for reasons neither farm management records nor extension treatment logs could explain.
The phorid flies did not wait for a permit reauthorization for their release. The armadillos did not apply for a pest management exemption before pushing their range north into Colquitt County. The native harvester ants did not consult a restoration plan before reestablishing foraging trails in margins the fire ants could no longer continuously defend. Three species, three mechanisms, three separate arrival timelines. None of them aware that the others were operating in the same field margins.
Fire ants reached Georgia without their South American biological constraints and spent 80 years operating without them. What is happening now in southwestern Georgia’s peanut and cotton belt is what happens when those constraints eventually follow.
