Classification and distribution
Ants are insects, belonging to the order Hymenoptera and the family Formicidae. Worldwide, there are 16 subfamilies, about 300 genera and 15,000 described species and subspecies of ants. Australia is currently known to have 10 subfamilies, 101 genera and 1275 described species and subspecies.
Fire ants (Solenopsis invicta) are native to the South American countries of Brazil, Paraguay, Uruguay and Argentina. They also occur in the United States, where they are believed to have arrived in the ballast of ships carrying agricultural goods from an infested country. First noticed in 1929 at Mobile, Alabama, fire ants have spread as far as California and infest about 110 million hectares of the southern United States.
Fire ants were first detected in Brisbane, Queensland on 22 February 2001, but it is thought that they may have arrived up to 20 years earlier. How they entered Australia is not known.
Workers, alates and queens
A fire ant colony consists of three types of ants - workers, alates and queens.
A distinctive feature of fire ant workers is their range of sizes. Within one nest, adult workers can vary in size from 2-6 mm. All worker ants are sterile females and live for two to three months, major workers may live for up to six months.
Workers' functions are determined by their age, size and the colony's needs. Newly emerged workers act as nurses and feed the brood and queen. As worker ants age, their role changes to that of colony maintenance, sanitation and defence. This group is generally referred to as reserves. The oldest workers function as foragers, leaving the colony to search for food.
Fire ant alate and workers.
Alates and reproduction
Mature colonies produce a number of winged, fertile males and females called alates. These are cared for by workers until they leave the colony to begin their mating flights. These flights usually occur during the warmer summer months, when environmental conditions such as temperature and humidity are right, but they can occur throughout the year.
Females mate with a male in flight. After mating the male dies and the newly mated female sheds her wings and begins a new colony if an appropriate nest site is found. New queens are particularly vulnerable at this stage and only a very small percentage (maybe less than 1%) survive to start a new colony.
Fire ant lifecycle. Images Courtesy of SARE and Texas A & M University
New fire ant colonies
The new queen digs an underground chamber in which to lay her first eggs. Within the first 24 hours she lays 10-20 eggs, which hatch after 6-10 days. The newly hatched larvae develop through four stages, or instars, over the next 12-14 days, before becoming pupae. After 9-16 days the newly developed ants emerge. Development from egg to ant takes approximately one month.
During this time the queen continues to lay eggs, which are tended by the newly emerged workers. The new workers also expand the nest and gather food. With the queen capable of laying up to 800 eggs a day, numbers build up quickly, and a year-old colony can often have over 100,000 workers. Queens can live for over seven years and are capable of producing over two milllion eggs in their lifetime. Queens also produce chemicals called pheromones, which affect the behaviour of the workers in the colony.
The lifecycle of a fire ant can be broken down as follows:
- alate mating flights
- males die
- females go on to start new colonies
- lays 10-20 eggs within first 24 hours
- eggs hatch in 6-10 days and more are laid
- within 1 month first workers open colony and tend queen (who lays more eggs)
- mature colony producing alates within 6-12 months.
Fire ant nests are mounded with no obvious entry/exit hole.
Fire ant nests
Fire ants build a dirt nest or mound, which can be up to 40 cm high. The shape and size of the nest varies depending on soil type and colony size. An unusual feature of the mound is that it has no obvious entry or exit hole. The ants enter and leave the mound via underground tunnels which radiate outwards from the nest. These tunnels can be up to 30 m long.
Nests can also develop under logs, rocks or other materials lying on the ground. These materials absorb heat from the sun in the same way that the fire ant mounds absorb heat. Fire ants appear to have an attraction to electricity, and nests have been found in buildings and equipment around electrical systems.
Internally, the nest consists of many interconnecting galleries, which give it a honeycomb appearance.
If a nest is disturbed, the workers may very quickly move the queen and the brood (eggs, larvae and pupae) to a new location.
Monogyne and polygyne colonies
Fire ant colonies are of two basic types: monogyne colonies with only one queen and polygyne colonies with more than one queen.
Workers from monogyne colonies are territorial and defend the area around their mound from fire ants from other colonies, new queens that land and attempt to start new colonies nearby, or from other types of ant.
Ants from polygyne fire ant colonies are able to move from one mound to another without hostile reaction from other fire ant workers. This type of colony is more stable and has a much greater mound density per unit area.
In Texas, the average number of monogyne and polygyne mounds per hectare has been recorded at 300 and 680, respectively. However, polygyne mound densities up to 2600/ha have been found.
Mature fire ant colonies can contain 200,000 to 400,000 workers.
Fire ants are omnivorous, eating plant material, insects and small animals. They will also scavenge dead animals. Workers forage during the warmer months, typically when temperatures are between 22°C and 36°C. Workers can consume only liquids. Nearly half of the resources that are returned to the nest are liquids, which are stored in the ant's crop (stomach) or a gland in the head. Ants pass regurgitated food to each other through a process known as trophallaxis.
Foragers collect both liquid and solid foods. Liquids, usually oils or carbohydrates, are transported in the crop and fed to reserves. The reserves feed the nurses, and the nurses feed the brood and queen.
Fire ants have four larval stages, or instars. Only the first three larval instars are fed liquids. Solid food, such as parts of insects, are given to fourth instar larvae. The fourth instar larvae break down the solid food into proteins, and the nurses then distribute the digested protein, through trophallaxis, to the younger larvae (instars I - III) and to the queen.