Much of the information for this page is from Walter R. Tschinkel’s The Fire Ants, 2006 Belknap Press. This 700 page tome covers 30 years of research on this species. Wikipedia is also an excellent source for general information and references.
Origins and Distribution
There are over 200 species of fireants around the world, but we’re concerned with one particular species, Solenopsis invicta Buren, or the red imported fire ant (RIFA).
They have a large range of 1900 by 220 miles in central South America, but the species we see in the US originated in Northeastern Argentina and arrived by boat in the seaport of Mobile, Alabama around 1940. By 1949 they had spread to Mississippi and Florida. The post World War II building boom involved nurseries shipping plants and soil throughout the Southeast, some of which included colonies of ants. This allowed them to spread more rapidly than they could on their own, reaching Texas in the early 50s, and the Texas Hill Country in the early 60s.
Fire ants prefer moist, disturbed soils and a low variety of plant life. Landscaping which replaces native trees, shrubs and grass with water loving ‘domestic’ species provides ideal habitat. Cleared ranch and farm land were early examples, while today golf courses and suburban lawns offer ideal nesting spots.
Pesticide based eradication efforts in the 50s and 60s temporarily reduced both RIFA and native ant populations. Freed form competition with other ants, these highly aggressive and prolific breeders soon recolonized the treated areas, becoming the dominant ant species in these areas. Today they have spread around the globe, and may be found in North and South America, the Bahamas, Asia and Australia.
Lifecycle
During the winter, a colony will produce alates, fertile winged male (“drone” or aner) and female (“queen” or gyne) ants. An average of 600 to 700 will emerge simultaneously from area nests in the spring (May through July) on a warm, humid day after a heavy rain for a single nuptial flight. Additional flights will emerge throughout the spring as groups of alates mature. The males will only live for a day or so after the mating flight, while the queens who survive to found a nest may live an average of around seven years. While they can fly for several miles, often the newly mated queen will select a site within around half a mile of her original nest.
The queen will initially dig a short burrow a few inches into the ground, then hollow out a claustral chamber which will grow into the ant nest. Once she has a suitable site, she seals the tunnel entrance and begins to lay eggs. In about a week, she will have produced a brood mass of between 20 and 100 eggs. These will hatch in about a week, and will become mature workers in about three weeks. Until this time, the queen must rely on fat reserves and ground moisture to sustain herself, and her body weight will drop by about half.
The initial group of workers, known as minim ants, are just large enough to begin the work of expanding the colony and foraging for food. At this point the queen is attempting to maximize the number of workers in order to establish her own territory. Fire ants accomplish this through a mix of egg laying by the queen and “brood raiding” by workers, which steal eggs (brood) from nearby nests in the area to be raised by their own queen. As workers are sterile, eggs from other nests do not compete with their abducting queen. Eventually the worker population reaches a point where raiding has little effect on population, and the raids cease. Population growth now depends on food, the number of eggs the queen can produce, and the temperature of the nest.
The queen can only produce eggs with the sperm stored from her single mating flight – around 7.3 million of them, or around 7.3 million workers and alates. The more quickly eggs are produced, the less time the queen and her colony will live. Once the queen runs out of sperm, she can no longer produce workers. As the population drops due to worker mortality, the nest becomes vulnerable, and may succumb to disease, predation, or invasion by neighboring colonies. On average, this happens around 7 years after the queen establishes the initial nest.
In ideal circumstances, the queen will lay one to two eggs a minute, 24 hours a day. At peak production, this would produce around 2,000 to 3,000 eggs in a single day. In practice, the frequency of egg laying varies by time of year, colony size, food availability, and mortality of worker ants. A 3 to 4 year old mature colony will have between 100,000 to 250,000 workers, with egg production regulated to maintain the size of the colony at a stable level. After the second year, the winter months will see the queen produce a mix of both workers and fertile alates, which will remain in the colony until the spring rains signal the start of nuptial flights, and the cycle begins again.
Nest Structure
Fire ants are noted for their large mounds that lack external openings. These hills are a special adaptation of the species, and serve as an incubator for their larvae. Each day the queen’s eggs are moved up to the hill, which is aboveground and exposed to the sun. As a result the mound temperature is several degrees higher than the rest of the nest, allowing the larvae to mature more quickly than they would at lower temperatures. At night the mound cools down, and the larvae are moved into the below-ground nest. This process is repeated each day, though the number of eggs and larvae varies with the time of year. Mounds are most active in late spring and early summer when the male and female children of the queen mature and leave the nest to establish their own colonies.
Nests may be extremely dense in areas that are well suited for fire ants, with densities of 50 nests per acre in some areas. These nests may be connected in a variety of fire ants which are more tolerant, and ‘share’ nests between queens. While each hill will have only one queen, the nests of individual queens are connected and may share workers, resources, and defense between them.
The nest itself may extend quite deep into the ground, with depths of over 15 feet found in sandy soils. More rocky soils will see nests that are less deep, and spread out horizontally. This is common in the Hill Country of Texas, where large flat sedimentary rock beds exist just a few feet beneath the surface, making deep nests difficult to build and maintain. In general nests will grow downwards as far as they can before spreading outwards.
The nest itself is usually a set of vertical columns with ‘rooms’ connected by tunnels. Outer columns will also have horizontal ‘foraging tunnels’, which run a few inches below the surface and radiate outwards from the nest to the surrounding area. These small tunnels will often have a very small (1/2″ to 1″) hill around it with an open center. The ants use these tunnels to allow their workers to get to food sources while lowering the chance of the workers being attacked or eaten. If you see a line of small open hills after a rain storm, following it will lead you to the central mound for that nest.
Diet
Fire ants are omnivorous scavengers, primarily consuming both plants (especially seeds) and animal remains. While not overtly predatory, they will attack and consume other organisms if encountered, especially other insects. Insects which are not immediately consumed may be stockpiled for later consumption. When entering a new territory the variety of other insects, especially other ant species, will dramatically decline or vanish altogether. While this is sometimes beneficial, as in the case of ticks, the overall impact on biodiversity is overwhelmingly negative.
Fire ants are also known to impact larger animals, especially the young, with rabbit, snake, turtle, alligators, and lizard hatchlings all dramatically impacted. Nesting birds are also victims, with ground nesting hatchlings being especially vulnerable. Of local note is their severe impact on the highly endangered Attwater’s Prairie Chicken.
Predators and Parasites
The primary impact of predators on fire ants occurs when the fertile winged male (“drone” or aner) and female (“queen” or gyne) ants emerge from the nest during mating season. Much larger than the worker caste, they are a tempting meal to dragonflies, chimney swifts, eastern kingbirds, and eastern bobwhites. On the ground, the wolf spider and southern black widow regularly consume fire ants which encounter their webs.
The dung beetle, has a unique ‘hunting’ technique. The thick armor of the beetle makes it highly resistant to fire ant attacks, especially when tucked into a defensive position. Dung beetles have been known to insert themselves into a nest and wait until they have absorbed the nest odor, which identifies ‘friendly’ ants of that nest to each other. Once disguised, the ants will ignore it, leaving it free to consume brood mass in the hill without consequence.
Armadillos, common throughout the Hill Country, are well adapted to consume fire ants. Their diet consists mainly of insects such as grubs, earthworms, beetles and ants. The last are notable, as armadillos are resistant to fire ant venom, with armadillos tearing apart the mound of the nest and consuming the brood mass, along with any worker, queen or fertile ants they come across. As fire ants reduce or eliminate other insect species, they have in turn become a reliable source of food for armadillos.
Phorid flies (genus Pseudacteon) are a group of small parasitic flies which target ants as part of their reproductive cycle. Some, such as Pseudacteon tricuspis and Pseudacteon obtusus are native to South America, and are adapted to target fire ants specifically. For example, to reproduce, the female obtusus will lay an egg on the back of the ant’s head, in a position where it cannot be removed. Upon hatching, the larvae bores into the head, consuming the fluids, nervous tissue and muscles for about two weeks. At that point they release an enzyme which breaks down the tissue connecting the head to the thorax, and the head falls off, offering a safe place for the larvae to develop. In another two weeks the mature fly emerges through the ant’s mouth. Because a single fly may lay hundreds of eggs, this can severely impact the size and health of a nest.