Scientific name: Linepithema humile (Mayr) (Hymenoptera: Formicidae)
Common Name: Argentine Ant
Argentine ants are one of the world's worst ant pests. They were first reported in New South Wales at North Auburn, a suburb of Sydney, in April 1950 but there is no doubt that they had been present in several other suburbs for many years before that. They were the subject of an eradication campaign that ran from 1950 to 1983 which virtually eliminated the pest from the Sydney and South Coast districts in which it had been found. Unlike most species of ants, Argentine ants do not have a "nuptial" flight to establish new nests. New queens make new nests near the original nest and both nests remain connected, often sharing workers. Over time, this web of interconnecting nests grows into an infestation that may cover many hectares but which has identifiable limits. This characteristic allowed the eradication campaign to completely eliminate individual infestations by use of a long-lasting insecticide sprayed over the entire infested area. However, such insecticides sometimes have other effects in the environment and were eventually banned from use throughout New South Wales.

Well defined trails leading into 
a bag of garden clippings
 Identification:

Argentine ants are small (2.5-3mm) brown ants with a single node in the waist. They are typically found travelling in well defined trails between nests and food sources. Unlike other common small brown ant species, such as the coastal brown ant, Pheidole megacephala (Fabricius), Argentine ants do not have a soldier caste, so all the individuals out scavenging for food are the same size.

Biology:

The eggs of Argentine ants are elliptical in outline, pearly-white in colour, and take from 12 days to nearly 2 months to hatch. The larval stage may be completed in from 11 to 60 days. The pupal period may extend over 10 to 25 days. The minimum period from egg to adult is about 1 month but it may be 4 to 5 months and averages 2-3 months.
 
 



The population of a colony may vary from a dozen to many thousands and the number of queens from one to hundreds. During the warmer months, satellite nests are usually established close to food sources and these satellite nests are highly mobile, with the workers moving in with eggs and larvae over a short period and abandoning the nest if it is disturbed or the food used up. The adult winged males, which are larger than the workers but much smaller than the winged queens, usually fertilise the queens in the nest and the new queens establish nests nearby after losing their wings. In midsummer and early autumn there is a very large increase in numbers. It has been estimated that infestations grow at a rate of more than 200 metres per year.

The Argentine ant life cycle
illustrated by Emile Zeck
Argentine ants use all available food. 
Here they dismember a dead frog
 Damage:

Because of the nature of the infestation, Argentine ants need to utilise and monopolise every available food source in order to feed the vast numbers of ants that eventually build up in a single infestation. They also compete very effectively with all other ant species, both by fighting and by monopolising all available food sources. Virtually all other ants are replaced by Argentine ants, although study undertaken by NSW Agriculture in the early 1980s showed that some minute species of ants were able to persist with Argentine ants and some may even gain an advantage by the absence of other ants. These species were Solenopsis froggatti, Pheidole concentrica, Paratrechina sp. and Cardiocondyla nuda (Fletcher, Fitt and Holmes, unpublished data).
Argentine ants become a major domestic pest by invading houses and swarming over foodstuffs, including entering fridges, unopened packets and have even been known to follow the spiral down inside screw-top jars to get at the contents, particularly if the contents are sweet. They invade beds at night seeking moisture and can swarm over sleeping babies, although they are not known to do any damage.
Unlike many other domestic pest species, Argentine ants do not nest in houses but nest outside and enter houses in dense trails which can resemble miniature multi-lane highways.

Agricultural pest status:

From an agricultural viewpoint, Argentine ants have an impact in orchards and gardens by protecting honeydew producing insects such as aphids and scales. These sap-feeding insects can have very damaging effects on trees if allowed to increase in numbers and the protection they gain from predators and parasites by the presence of large numbers of Argentine ants allows them to increase to economically damaging levels. Integrated control programs, which rely heavily on the activities of natural enemies, are completely disrupted by the activities of Argentine ants. 
They can also become a serious problem with stored products, particularly those of a sweet nature and, in gardens and nurseries can damage flowers searching for nectar. They have also been reported affecting fruit set in orchards and collecting newly sown seed when the seed is small. There are overseas reports of the ants invading beehives and removing the honey. They are also known to swarm into birds' nests, including those of native birds, chickens and cage birds, to attack the helpless newborn chicks. In poultry runs, hens may be driven from their nests.

Argentine ants swarming over a citrus tree to collect honeydew from scale insects, which they protect from predators and parasites
Distribution in NSW: Scattered small infestations in the Sydney metropolitan area.
Control: Advice on control of insect pests is available from NSW Agriculture's Insect Enquiry Service by telephoning 1800 675821 during business hours.
Acknowledgement: Much of the information on this page was first published in New South Wales Department of Agriculture Entomology Branch Insect Pest Bulletin 111, 1977.