Members of the Solenopsis genus, fire ants are named after the fiery pain and burn-like effects of their stings. Fire ants are very aggressive when disturbed and rapidly run up any object placed into their nests. If this happens to be your leg then you are in trouble. Fire ants latch onto the skin of their victims with their barbed mandibles1 and then, pivoting around the bite point, each fire ant will rotate in circles, stinging repeatedly with the sting on the end of her gaster2 until the victim repels the ant or the ant dies. The venom the ant injects causes an intense burning sensation in the skin followed by blisters that persist for days.
To treat fire ant stings it is best to elevate the affected area if possible and then apply ice or a cold compress before cleaning the area with soap and water. Use topical ointments and oral histamines to control the itching.
Species of Fire Ants
Fire ants are tropical species of ant and are unable to survive if the temperature falls below freezing for any length of time. There are dozens of species of fire ant and they are all native to the Americas.
Red Imported Fire Ants
The species best known to the North American public is Solenopsis invicta, the Red Imported Fire Ant, often referred to by its acronym, RIFA. Of all the fire ants, this one causes the most damage to people, property and the economy and is therefore subject to the greatest scientific scrutiny. It has the most toxic venom of all US fire ants. Invicta means 'invincible.'
Red Imported Fire Ants are native to the Pantanal region of southern Brazil and the northern parts of Argentina but they were inadvertently imported into the port of Mobile, Alabama, USA in about 1930 hidden in wood used as ship ballast. From there, the RIFA spread rapidly throughout the Southern United States and now have a range extending throughout most of the South-eastern United States and west into Texas. Their adopted territory now extends over 275 million acres in 11 southern states and Puerto Rico. They have continued their exploration of the Earth by ship reaching Australia, where they were first identified in Brisbane, in February 2001.
Black Imported Fire Ants
The Red Fire Ant was not the first to invade the United States. As in the case of the RIFA the Black Imported Fire Ant, Solenopsis richteri, had been transported in ship ballast from Argentina to Alabama. This happened in 1918. Yet, despite this earlier start, the range of the Black Imported Fire Ant is less extensive than that of the red species and has been limited to the states of Alabama and Mississippi.
Native North American Species
Both imported species threaten the biodiversity of ants in the Southern States of the US and present a particular danger to the native fire ants of the region with which they compete for food and nest sites. The North American native fire ants are the Texas Fire Ant, S geminata, the Southern or California Fire Ant, S xyloni and the two species of fire ants still inhabiting the desert in the southwest, S aurea and S amblychila, that are both known by the common name of Desert Fire Ant. The Southern Fire Ant inhabits a wide arc of territory from North Carolina south to northern Florida, along the Gulf Coast and west to California. The Texas fire ant is sometimes called the Tropical Fire Ant. This ant ranges from South Carolina to Florida and Texas and is also to be found in many other parts of the tropics.
Other South American Species
Isolated populations of other fire ant species exists on the mainland and islands of South and Central America and there are concerns about the effects of accidental transport of these to the Southern United States where the red and black forms have already done so much damage. Some of these species are potentially a bigger threat to people and to the environment than those already present.
The workers of all fire ant species have some physical characteristics in common. They are between 2mm and 7mm long. Their bodies are a reddish brown and have two roughly spherical nodes between the thorax3 and the gaster whereas other species of ant usually only have the one node. These nodes are called the petiole and the post petiole4. The antennae of fire ants consist of ten segments with two sharply defined clubs5 and they share a particular facial feature - a depression running forwards across the vertex6.
The differences between the species are subtle and to distinguish between them it is best to use an identification key (488k PDF file).
Fire ants usually make underground nests in the soil but they sometimes nest in houses. In open ground, there is often a mound of loose soil on the surface of the grass marking the position of the nest. Each nest is a series of interlocking galleries, tunnels and chambers with openings in the surface of the mound from which the workers can emerge to search for food.
Ants are swarming insects in the same manner as bees and wasps. Some members of a nest will follow a fertile queen ant to a new nest site and start a new colony. Most species of fire ants swarm in late spring or summer.
A single fire ant colony may produce hundreds of queens per year but only a small percentage of new queens successfully start a colony. It is usually the case that only one queen survives. Within about a year, the colony expands into thousands of individuals.
The workers care for the virgin, winged reproductives until it is time for them to leave the colony. The reproductive queens and males are both winged and fly directly from the mound surface or climb on nearby plants before taking off. Nuptial flights7 generally occur in the afternoon following rain. The ants mate during flight and the females land to begin a new colony. The males die soon after mating.
Most females fly, or are blown, less than 1.5km from the nest but some may travel 20km or more. The newly mated queens select potential sites during flight and after landing. Selection is based on surface properties such as dampness, type of soil, topography and reflectance. This behaviour results in colonies being established along roadsides, in playgrounds and on golf courses. Large colonies are often found nesting next to fence posts, power poles, stumps and other objects that interrupt the flight of the queen.
The fertilized queens shed their wings and begin digging underground chambers in which to lay eggs and the newly established fire ant mound rapidly produces young workers. The queen lays 100 to 200 eggs during the first three to four weeks. Some of the eggs hatch in 8-12 days. The developing larvae eat the other eggs. The queen later lays eggs continuously and the workers produced earlier bring food to the larvae. The larvae pupate8 in 6-12 days and adults emerge in 9-16 days. The life cycle may be complete in three to six weeks depending on environmental conditions but the mounds built by newly established colonies may not be conspicuous until six months after the young queen starts laying eggs.
The ants forage at night. It is mainly the smaller workers that leave the nest through tunnels seeking out food. When a forager finds food, she returns to the nest and leaves a pheromone trail for others to follow to the food source. Fire ants produce 11 different known chemical signals and, in addition, communicate messages with their antennae.
The usual diet of fire ants is insects, spiders, myriapods9, earthworms, and other small invertebrates but they are also attracted to fruits, sugar and honeydew and feed on carrion. Fire ants also harvest seeds and obtain nectar from a variety of plants including many species of cactus.
Fire ants use their venom to kill prey for food. Fire ants are unusual among stinging insects in their venom contains alkaloids but relatively little protein making it possible to disperse the venom as an aerosol spray. The venom, which is more than 95 percent piperidine10 alkaloids, becomes both a weapon and an antibiotic.
Specialised worker ants, known as nurses, can spray a mist of venom from their gasters to protect the eggs and larvae from microorganisms. The nurse ants can also dispense the same venom in much larger quantities to repel other insect species from the nest. Fire ants make further use of the venom when a good food source is located in order to displace other ant species. They do this through a process known as 'gaster wagging' or 'flagging.' This is where a worker lifts her abdomen and sprays venom towards or even directly onto her enemy.
Problems Caused by Fire Ants
The Imported Fire Ant is undoubtedly the ant from Hell. It is a serious pest in both urban and rural areas causing billions of dollars of economic damage to the US annually. The Red Imported Fire Ant is a notifiable pest in Australia - landowners are legally required to tell the authorities of nests on their land. It is estimated that fire ants directly affect the lives of more than 40 million people. They often occupy the areas where we work, live and play where they create unsightly mounds, interfere with commerce, industry and farming and the imported species damage the ecosystem. It is important to be able to identify the native species and protect them while finding methods of controlling the Red Imported Fire Ant, the Black Imported Fire Ant and hybrid species.