The Argentine ant (Linepithema humile, formerly Iridomyrmex humilis) is a tiny dark ant native to northern Argentina, Uruguay, Paraguay, and southern Brazil. It is an invasive species that has been established in many mediterranean climate areas, inadvertently introduced by humans to many places, including South Africa, New Zealand, Japan, Easter Island, Australia, Hawaii, Europe, and the United States. They have many locally used common names such as grease ants.
The worker ants are only about 3 mm (1/8th inch) long and can easily squeeze through cracks and holes no more than 1 mm (0.040 inch) in size. Queens are two to four times the length of workers. These tiny ants will set up quarters in the ground, in cracks in concrete walls, in spaces between boards and timbers, even among belongings in human dwellings.
German entomologist Dr.Gustav L. Mayr identified the first specimens of Hypoclinea humilis in the vicinity of Buenos Aires, Argentina in 1866. This species was shortly transferred to the genus Iridomyrmex, and finally to Linepithema in the early 1990s.
The native range of Argentine ants is limited to around major waterways in the lowland areas of the Paraná River drainage; They have recently spread into parts of Argentina, Brazil, Chile, Colombia, Ecuador, and Peru. The species has become established in at least 15 countries throughout the world, on six continents as well as many oceanic islands.
They have been extraordinarily successful, in part, because different nests of the introduced Argentine ants seldom attack or compete with each other, unlike most other species of ant. In their introduced range, their genetic makeup is so uniform that individuals from one nest can mingle in a neighboring nest without being attacked. Thus, in most of their introduced range they form "supercolonies". "Some ants have an extraordinary social organization, called unicoloniality, whereby individuals mix freely among physically separated nests. This type of social organization is not only a key attribute responsible for the ecological domination of these ants, but also an evolutionary paradox and a potential problem for kin selection theory because relatedness between nest mates is effectively zero. In contrast, native populations are more genetically diverse, genetically differentiated (among colonies and across space), and form colonies that are much smaller than the supercolonies that dominate the introduced range. Argentine ants in their native South America also co-exist with many other species of ants, and do not attain the high population densities that characterize introduced populations.
Argentine ant workers are unable to lay reproductive eggs, but can direct the development of eggs into reproductive females; the production of males appears to be controlled by the amount of food available to the larvae. The queens seldom or never disperse in winged form. Instead, colonies spread by budding off into new units, with Argentine ants routinely moving their nests. As few as ten workers and a single queen can establish a new colony.
The ants are ranked among the world's 100 worst animal invaders. In its introduced range, the Argentine ant often displaces most or all native ants. This can, in turn, imperil other species in the ecosystem, such as native plants that depend on native ants for seed dispersal, or lizards that depend on native ants for food. For example, the recent severe decline in coastal horned lizards in southern California is closely tied to Argentine ants displacing native ant species on which the lizards feed.
Argentine ants also cause problems in agricultural areas by protecting plant pests, such as aphids and scale insects, from predators and parasitoids. In return for this protection, the ants receive a sweet excretion, known as "honeydew". Thus, when Argentine ants invade an agricultural area, the population densities of these plant parasites increase, and so too does the damage they cause to crops.
Due to their nesting behavior and presence of numerous queens in each colony, it is generally impractical to spray Argentine ants with pesticides or to use boiling water as with mound building ants. Indeed, spraying with pesticides will stimulate increased egg-laying by the queens, compounding the problem. Pest control usually requires exploiting their omnivorous dietary habits. They prefer sweet foods such as the honeydew produced by aphids and mealybugs (see also) The most effective control is through use of slow-acting poison bait, which will be carried back to the nest by the workers, eventually killing all the individuals, including the queens. It may take four to five days to eradicate a colony in this manner. An effective homemade recipe consists of a solution of granulated white table sugar and boric acid, placed in a shallow dish in the area being invaded:
The boric acid will dissolve only if the water is hot, or one can mix the ingredients cold, then place the container in a microwave oven to bring the water to boiling temperature. When mixed in small quantities, the solution can be stored in a dropper bottle and dispensed as needed to replenish the bait dish. Although the solution isn't particularly hazardous when used in small quantities as described here, the bait dish should be placed out of reach of pets and children.
This formula works by desiccation and laceration. The solution begins to draw water from the ant's body, causing slow dehydration. Also, as the ants or their larvae transpire water, the solution becomes more concentrated, causing the boric acid to crystallize and lacerate the digestive tract.
In areas where the use of liquids or poisons are not desirable, Argentine ants may be repelled by ordinary talcum powder (baby powder) which contains talc. The powder does not appear to kill the ants, but they will try to avoid it after contact.
Researchers from the University of California, Irvine, have developed a way to use the scent of Argentine ants against them. The exoskeletons of the ants are covered with a hydrocarbon-laced secretion. They made a compound that is different, but similar, to the one that coats the ants. If the chemical is applied to an ant, the other members of the colony will kill it. The success of the chemical for controlling Argentine ants will depend on the cost, the ease of application, toxicity to non-target organisms and the frequency of reapplication. The chemical may work best in combination with other methods.