Florida’s Invaders: Fire Ant

If you’ve ever spent some time outside in Florida, then you’ve probably met the Fire Ant. The numerous sandy mounds, often several to a colony, are a familiar site in rural fields and urban lawns, and encounters often end with a painful raised welt that lasts for several days. The Fire Ant may be our most unwelcome invader of all.  🙂

Fire Ant mound

There are about 280 species of fire ant, all in the genus Solenopsis. They can be found in virtually every tropical and subtropical area on earth, and get their name from their ability to deliver painful stings when disturbed. Biologically, “ants” are wasps that have shed their wings and taken to living underground. Many ant species, including theSolenopsis, have retained the stinging apparatus of their wasp ancestors. The stings can deliver a venom, made mostly of alkaloids, that varies in potency from species to species.

In Florida, our native species of fire ant is Solenopsis geminata. These live in underground colonies excavated in the sandy soil, marked on the surface by a mound of dirt. Inside this mound are chambers where, on warm days, the larvae and pupae are brought to warm themselves in the sun-heated sand. If you disturb the mound, you will see the worker ants frantically carrying the young back into the subterranean nest.S geminata is capable of stinging, but their venom is not all that potent, and they prefer to defend the nest with a special caste of soldiers–individual ants with disproportionately large heads and strong jaws, who attack intruders by biting. Native Florida fire ants are not much bother to people.

Sometime in the 1930’s, however, a new species arrived in Florida, probably by stowing away in a cargo shipment. The Red Fire Ant, Solenopsis invicta, is native to the Amazon forests of Brazil. (The Latin name “invicta” means “unbeatable”, which may turn out to be sadly prophetic.) It was originally believed to be just a color morph of the Black Fire Ant (S. saevissima richteri), but subsequent study has shown it to be a separate species. (The Black Fire Ant is also an invader in the US, but so far its range is limited to parts of Alabama and Mississippi.)

Like our native fire ants, the Red Fire Ant lives in a large underground colony with tens of thousands of individuals. After a new queen has mated, she will land in a suitable spot, tear off her wings, and dig out a small nesting chamber. The first batch of workers are very small (called “minims”)–they live just long enough to help raise a couple generations of new workers. The vast majority of nests have only one reproductive queen, and all the worker ants are her daughters. For reasons that are not well-understood, however, a certain small percentage of Fire Ant colonies will have more than one queen, sometimes several dozen, sharing a single colony. These queens can be entirely unrelated–apparently, some solitary queens are able to move into an existing colony and set up housekeeping, with the existing ants accepting the new queen and her new daughters as members of the group.

The workers come in three different sizes. The “minors” are the ones who tend the young and maintain the nest. The “media” are larger in size, and do most of the foraging for food. The “majors” are the biggest, and they play the role of “soldiers”, defending the nest from intruders. When the mound is disturbed (such as when a human inadvertently stands or sits on one) the soldiers will emerge from their underground chambers, stealthily climb onto the intruder, then, at a chemical signal, they will all sting at once. Latching onto the intruder’s skin with their jaws, they will plunge the stinger at the end of their abdomen into the enemy and release their venom. Since the invader gets no warning, it may be suddenly stung by dozens or even hundreds of ants. In small animals like mice or lizards, this often brings death. In humans, each sting will produce a swelling, as the necrotoxic venom eats away a small bit of flesh, usually resulting in a distinctive white pustule the next day, which will then burn and itch painfully for several days afterwards. In some cases, such as incapacitated patients in nursing homes or small toddlers, such attacks have been fatal. A number of people are also allergic to the venom, like bee stings, and even a single sting can cause a lethal case of anaphylactic shock.

As a “superorganism” containing thousands or tens of thousands of individuals, Fire Ant colonies are also efficient hunters, scouring the areas around the nest for anything edible. They will harvest seeds and young plant shoots (which can cause significant damage in agricultural areas) and can also attack and eat animal prey such as ground-dwelling birds, lizards, toads and rodents. This has made them a conservation threat to a number of Florida species, who have no previous evolutionary experience with the ants and therefore have no effective defense against them.

It is not known exactly when or where Solenopsis invicta first entered the US. It is known that Black Fire Ants, S. saevissima richteri, became established around Mobile AL in 1918, likely from a shipment originating from Brazil. Sometime in the late 1920’s or early 1930’s, the closely-related Red Fire Ant followed, being first detected in the Florida Panhandle. It may have entered either in Pensacola FL or Mobile AL. By 1945 the species was firmly established, and began a slow but steady expansion. Today, the Red Fire Ant is found throughout the entire Southeast and into Texas, with colonies found locally all the way to California and up into Virginia and Maryland. The tropical ants cannot tolerate freezing conditions, so it is likely that climate conditions will prevent them from reaching much further. They have also become established in the Caribbean, Australia, and Southeast Asia. In Florida, they can be found in every county.

The Florida folk remedy to dispose of Fire Ants is to pour boiling water down the mound to kill them. Since the underground colonies are quite large, this can take as much as three or four gallons of water. Commercial fire ant treatments consist of insecticides contained inside granules of corn powder. These are scattered on the mounds, where the workers carry them into the nest and feed them to the others. Both of these methods will fail, however, unless the queen is killed, and this can be a particular challenge in multi-queen colonies. Some states, including Florida, have also been attempting biological warfare against the ants, by releasing a species ofPseudacteon fly known as the “brain-eating fly”, which parasityzes the ants and kills the colony. A type of protozoan parasite and a fungus which attacks the ants is also under investigation as a possible biological control.

There are some indications that the Florida population of colonies has been decreasing in recent years, though this may be just a temporary response to weather conditions. So far, it seems that S invicta is indeed “unbeatable”.


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