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"The Argentine Ants" (Iridomyrmex humilis)
The
Argentine ant probably made its entry into the United States at New
Orleans
via the coffee ships from Brazil. It was first noticed in 1891 by
Edward
Foster (1908), although it was undoubtely present some years before
Foster's
observation.
This insect is now established throughout the southern states and in California. Isolated infestations have been reported from Arizona, Missouri, Illinois, Maryland, Oregon and Washington. It is also known from Portugal, the Union of South Africa, France, Australia and from many islands, including Hawaii, where it first became established in 1940, according to Zimmerman (1941).
The eggs of the Argentine ant are white and 0.3 by 0.2 mm in size. Although the eggs are deposited throughout the year, the vast majority of them are laid during the summer. "In outdoor colonies, oviposition ceases when the daily mean temperature drops below 65 F, but is usually begun again when the mean temperature rises above this point, regardless of the time of the year." The shortest incubation period was 12 days, the longest was 55 days and the average incubation period was 28 days. The longer periods for the incubation of the eggs was in part due to cooler weather. The larval stage varies from 11 to 61 days, with an average of 31.4 days. The pupal stage ranges from 12 1/2 to 25 days with an average of 15 days.
Immediately after pupation, the ant is colorless and transparent and is commonly designated as a "callow". It is a clumsy creature and walks in a stumbling and uncertain manner. The integument of the callow darkens in color in 48 to 72 hours after emergence from the pupal stage. The time required for completion of the egg to adult stage varies from 33 to 141 days, with an average of 74 days.
In the spring, the nest will often be found in open ground with small piles of excavated earth a short distance from the nest holes. Form boards along walks and wooden objects of any kind are preferred as nesting sites and permanent runways, as are cracks and crevices in concrete walks. The area beneath a plant infested with "ant cows" often will be honey-combed with their tunnels. The ants may be encountered in enormous numbers in and under dead and decaying stumps. During warm weather they are partial to the under areas of houses and may use the mudsills as their runways. The nests even may be established in the house proper. The queen and her workers have been observed invading the second floor of a house and establishing themselves beneath a damp rag on a kitchen sink.
The nests during the summer are usually very shallow, only 1 to 2 inches beneath the surface. On one occasion, a nest was found that was more than 12 inches in depth about the roots of a tree, situated on a slope.
In the autumn, the insects aggregate into a virtual ant metropolis in which there are hundreds of queens. The huge nests may be found beneath boards, sheets of tin, buildings, etc., as well as in accumulations of dead plant material with its attendant heat of decay. The Argentine ant has been found overwintering in enormous numbers in tunnels containing hot conduit pipes. Smith (1936) noted in northern localities this ant may become established in buildings and spread from one structure to another, thereby avoiding the cold of winter. With the coming of spring, these colonies break up to smaller units consisting of one to several queens, with a large number of workers, all of whom migrate and establish themselves elsewhere. In fact, during the spring and summer months, an individual queen may take a stroll in the open and upon acquiring a retinue of workers, establish her own colony. This process is referred to as budding, and occurs in other species with multiple queens.
The Argentine ant has been distributed to all parts of the country, mainly by "riding the rails" in vehicles of all kinds. Its distribution by self movement is rather slow and Smith found this natural spread to be a few hundred feet per day. The Argentine ant often is introduced into new areas in balled nursery stock.
Nest Sites
Argentine ants usually nest outside near food sources, where moisture is present and light excluded. Nests are commonly located under mulch (usually at the base of trees), under stones or other protected sites, such as ground cover plantings at the edge of the sidewalks and streets. Argentine ants from neighboring nests can visit and intermingle without fighting. They only nest indoors occasionally.
Foraging Habits
Argentine ants explore as individuals, but once food and/or water are located, they quickly recruit large numbers of additional ants to the area. This "wolf-pack" strategy is one reason why the can outcompete other ants and become the dominant pest species. It appears that these ants are single-minded trail followers, and until they get their signal to stop and eat, they will often just move right past food placed in their path. They will move inside looking for both food and water, but will also move inside to escape hot, cold, dry or wet weather.
Reproductive Capacity
A single Argentine ant nest-site can contain one to hundreds of queens. Each can produce about 60 eggs a day. Egg-laying by the queens begins in the spring, and it takes about 50 days for the eggs to become mature ants. In Georgia, egg-laying begins in early May, and by early July, ant complaints start popping up everywhere. Egg-laying remains heavy until early fall.
A fully grown Argentine ant lives about two months. Therefore, high populations persist into the late fall, or until low winter temperatures help reduce their numbers. Whatever the reason, be it reduced egg population or winter kill, Argentine ant populations drop during the winter only to rebound each summer. Queens will over winter with a few workers in preparation for the springtime resurgence.
Feeding Preferences
Argentine ants are omnivorous, which means they eat just about anything. However, they do have a preference for sweets, especially liquid sweets. It is believed that the workers will eat sweet foods and carry back proteins to the queens and brood.
