Fire Ants Are Disappearing From Florida — Scientists Found What's Already Stopping Them

[00:00:00]Sanford Porter had been running fire ant parasitoid research out of the USDA Agricultural Research Service Laboratory in Gainesville, Florida for more than two decades when his colony monitoring network in North Central Florida began producing survey results he had not seen before from release sites where his team had been working the longest.

[00:00:28]Fire ant foraging activity in three pasture zones outside Alachua County was running at suppressed levels that no pesticide application, no baiting program, and no mechanical disruption could account for.

[00:00:45]The data was not the result of a bad season.

[00:00:48]It was building year over year in a direction his models had said was possible, but had never before confirmed in the field at this scale.

[00:00:59]He ran the numbers against the fly establishment data his team had been accumulating across those same sites since the first successful release a decade and a half earlier.

[00:01:12]The correspondence was direct and it was consistent.

[00:01:16]He sat with both data sets for a long time before he filed the observation.

[00:01:23]"This is not what we released them to do this fast," he said, "but it is exactly what we hoped for."

[00:01:31]Stay with Terra Factor as we go deep into one of the most quietly significant ecological stories developing right now inside the grasslands, pastures, and pine flatwoods of Florida.

[00:01:46]Because something is happening to red imported fire ant populations across established monitoring sites in North and Central Florida that no bait program, no chemical treatment, no broadcast pesticide, and no federal eradication campaign has ever produced in the decades since fire ants first reached [music] this state.

[00:02:09]Colony densities in zones where a specific biological program has been operating for 15 years or more are declining at rates that the original release models described as optimistic projections.

[00:02:24]Foraging activity is suppressed at levels that are changing the behavior of every other species in the ant community around those colonies.

[00:02:35]And the explanation traces back to three native and naturalized biological forces that were working against fire ants in their home range in South America for millions of years before a single ant >> [music] >> ever arrived in Florida.

[00:02:53]Nobody designed this. Nobody managed them. Already working.

[00:02:59]To understand why the red imported fire ant was considered an essentially permanent feature of Florida's landscape for 70 years, you have to understand what made it so.

[00:03:13]Solenopsis invicta arrived in the United States through the port of Mobile, Alabama, sometime in the 1930s in soil ballast from ships traveling from the Rio de la Plata basin of South America.

[00:03:29]It entered a continent that had never encountered it and found a landscape with no predators, no parasites, and no competitors that had evolved to recognize [music] it as a target.

[00:03:44]It reached Florida by the 1950s.

[00:03:47]A single fire ant colony can contain [music] anywhere from 100,000 to 500,000 workers.

[00:03:55]In the multiple queen form of the colony, which dominates across much of Florida, there is no practical limit on colony expansion because workers freely move between queens.

[00:04:09]Mound densities in Florida pastureland have been documented at over 1,000 colonies per acre in heavy infestations.

[00:04:19]The sting of a single fire ant delivers a venom that causes immediate burning pain, a raised pustule, and in sensitive individuals, systemic reactions that send tens of thousands of Floridians to emergency rooms annually.

[00:04:38]University of Florida researchers estimated total fire ant damage in the state at over $1 billion annually in agricultural losses, medical costs, and control programs alone.

[00:04:54]Every bait program, every pesticide [music] rotation, every aerial treatment produced temporary reduction followed by full recovery within two breeding cycles.

[00:05:06]The ants' reproductive architecture was built by evolution to absorb exactly that kind of pressure.

[00:05:14]Sanford Porter had joined the USDA Agricultural Research Service Center for medical, agricultural, and veterinary entomology in Gainesville [music] in the early 1990s with a specific focus on one of the most unusual parasitoids ever documented in ant biology.

[00:05:35]In 1994, his team and collaborators at the University of Texas at Austin independently initiated the first scientific program to study Pseudacteon phorid as biological control agents of imported fire ants.

[00:05:54]The flies had been documented attacking Solenopsis fire ants across South America for decades. Nobody had attempted to establish them in North America.

[00:06:05]What his surveys in Argentina and Brazil were showing was a picture of fire ant ecology that looked nothing like what existed in Florida.

[00:06:16]In South America, fire ants are not dominant insects, and in many areas they are not common at all.

[00:06:25]The explanation for that discrepancy, Porter's research confirmed, sat inside a parasitoid complex that North America had never seen.

[00:06:36]Phorid fly, not a single species, a genus of at least 24 documented parasitoid species in the genus Pseudacteon.

[00:06:47]Each specialized to attack fire ant workers at different size classes, different times of day, different seasons, closing every behavioral window the ants had developed to avoid them.

[00:07:02]The phorid fly in the genus Pseudacteon is a small, humpbacked insect. Some species no larger than a fruit fly.

[00:07:12]The female locates active fire ant workers by detecting chemical signals produced during foraging.

[00:07:20]When she finds a target, she hovers briefly, then makes a rapid dive, and inserts an egg directly into the body of the ant worker.

[00:07:29]The larva that hatches migrates to the ant's head capsule.

[00:07:33]It feeds on the contents of the head while the ant remains alive and functional for days or weeks.

[00:07:40]The larva then severs the membrane between the head and thorax.

[00:07:45]The ant is decapitated.

[00:07:47]The larva uses the empty head capsule as a pupal case.

[00:07:52]The indirect behavioral effect turned out to be as significant as the direct kill rate.

[00:07:59]Fire ants are acutely sensitive to the presence of phorid flies.

[00:08:04]Within 60 seconds of fly activity above an active foraging trail, workers retreat into the nest. Foraging halts and the colony enters a suppressed activity state that can last for hours.

[00:08:19]The behavioral reflex that drives that response evolved across millions of years of coevolution between the flies and the ants in South America.

[00:08:32]The ants that arrived in Florida carried the same reflex.

[00:08:36]They simply had no flies to trigger it for 70 years.

[00:08:40]Research at sites with established phorid populations documented foraging reductions of 30 to 40% during fly activity periods.

[00:08:51]A colony that is not foraging is not gathering food, not recruiting workers at the rate its reproductive capacity would otherwise allow.

[00:09:03]The fly does not have to decapitate every ant in a colony to change the colony's competitive position in the landscape.

[00:09:12]It has to be present enough that the ant cannot afford to behave normally.

[00:09:17]In South America, where fire ants face as many as 12 different phorid species simultaneously in a single field, that pressure is constant across every time window the ant uses.

[00:09:34]In Florida before 1997, it was zero.

[00:09:38]The phorid fly had been separated from the fire ant's North American range, not through persecution or habitat destruction, but through simple biogeography.

[00:09:50]Pseudacteon flies evolved alongside fire ants in South America across millions of years of coevolution.

[00:09:59]When fire ants crossed the Atlantic in a cargo ship's ballast soil in the 1930s, they left every one of their natural enemies behind.

[00:10:09]The ants that arrived in Alabama had the same reproductive capacity, the same colony architecture, and the same aggressive foraging behavior as the ants in Brazil.

[00:10:21]What they did not have was the suite of parasites that kept them suppressed across their native range.

[00:10:29]North America offered them open habitat, no natural ceiling, and no biological memory >> [music] >> of what they were.

[00:10:39]The result was 70 years of unchecked expansion across 30 states, and the permanent ecological transformation of the American South.

[00:10:50]Porter's research in South America confirmed what the population dynamics had always suggested.

[00:10:58]The fire ant in its native range is not the fire ant Florida knows.

[00:11:04]It is a middle-level competitor >> [music] >> in a community that has spent millions of years developing the tools to manage it.

[00:11:12]The USDA Agricultural Research Service conducted the first successful establishment of Pseudacteon tricuspis in the United States in 1997 at release sites across the southeastern states.

[00:11:28]A second species, Pseudacteon curvatus, was established beginning in 2000. By 2010, four species had been established across the southeastern region.

[00:11:40]In Florida, Pseudacteon curvatus and Pseudacteon obtusus established and spread well beyond their initial release sites.

[00:11:50]Pseudacteon cultellatus was established at multiple Florida sites thereafter.

[00:11:56]Each species partitions the fire ant colony by worker size, time of day, and season.

[00:12:04]Together, they cover foraging windows that no single species could close alone.

[00:12:12]The program was designed for fire ant suppression through biological control.

[00:12:17]It was not designed in coordination with what two other biological forces were simultaneously applying to the same colonies.

[00:12:27]The nine-banded armadillo, Dasypus novemcinctus, has been expanding its Florida range steadily since the species naturally colonized the state in the mid-20th century.

[00:12:41]It is one of the most effective fire ant predators operating in the American South.

[00:12:48]An armadillo working an active fire ant mound does not forage selectively.

[00:12:54]It excavates the mound structure with powerful foreclaws, exposing chambers, brood, queens, and workers simultaneously, and consumes everything it uncovers.

[00:13:07]Documented consumption rates at active mounds run into the thousands of individual ants per hour.

[00:13:15]The armadillo targets fire ant mounds because they are consistently productive, reliably located food sources.

[00:13:24]In Florida, pasture habitat where phorid fly suppression has already reduced colony foraging activity, armadillo excavation pressure compounds the stress on colonies already operating below reproductive capacity.

[00:13:42]A colony under fly pressure can't simultaneously rebuild mound structure and brood output following excavation.

[00:13:51]The third layer accumulates slowest and holds longest. Across sites in North Central Florida where phorid fly populations have been established for 15 years or more, entomological surveys have documented something the original release models predicted, but that took years of monitoring to confirm in the field.

[00:14:15]Native ant diversity is returning.

[00:14:18]Dolichoderus mariae, a native species displaced by fire ant pressure, is reestablishing in patches around mature phorid release sites.

[00:14:30]Forelius pruinosus >> [music] >> and several other native species whose foraging is suppressed by active fire ant colonies are appearing in ant community surveys at frequencies not recorded in those areas since before the fire ant establishment.

[00:14:48]They do not defeat fire ants directly.

[00:14:51]What they do is occupy territory, food resources, >> [music] >> and nesting sites that fire ant colonies under phorid fly pressure can no longer hold with the same intensity as before.

[00:15:06]The competitive exclusion fire ants applied to native species for decades begins to reverse one foraging zone at a time when behavioral suppression from the flies reduces the ants' ability to patrol and defend its territory at full capacity.

[00:15:27]Three biological forces, three separate mechanisms, one ant colony unable to sustain its dominance against all of them simultaneously.

[00:15:40]Sanford Porter returned to his north central Florida monitoring sites four times across the most recent survey season, running colony density counts and foraging activity measurements across the same transects his team had worked since the first Pseudacteon release in the late 1990s.

[00:16:02]On his third visit, >> [music] >> he walked a pasture edge outside Alachua County where early release records showed fire ant colony densities that had been among the highest his team ever documented in the first years of monitoring.

[00:16:18]The mounds were still present, but their spacing had changed. The density had dropped in a pattern consistent across three consecutive survey cycles, moving in a direction that no pesticide rotation and no bait program running simultaneously in the surrounding areas could fully account for.

[00:16:40]He stood at the pasture edge before recording the transect data.

[00:16:45]Above the grass at the tree line, a phorid fly hovered over an active foraging trail.

[00:16:53]The ants below it stopped moving within 30 seconds.

[00:16:56]"Some data confirms itself while you stand in the field looking at it," he said. "This was that kind of data."

[00:17:05]Can the phorid fly complex continue expanding across Florida's fire ant range at the rate current establishment data suggests?

[00:17:14]And can the compound pressure from armadillo excavation and recovering native ant communities accumulate fast enough to push colony densities below the threshold where fire ant dominance over Florida's ground-level ecology begins to permanently reverse?

[00:17:35]The question Sanford Porter is still going back to his Florida monitoring sites to answer every season.

[00:17:43]Drop your answers in the comments below.

[00:17:46]And if this story changed the way you think about what is already happening to fire ants in Florida without a pesticide label attached to it, subscribe to Terra Factor.

[00:17:59]We will see you in the next one.