The coastal brown or big-headed ant is a pest of houses, gardens and natural environments in the Northern Territory. It is thought to be a native of southern Africa but has spread over much of the tropical and sub-tropical world. The ant is spread mainly by human activity, such as the transfer of infested pot plants and landscaping materials to un-infested houses and gardens. The coastal brown ant also poses an ecological threat to Top End rainforests by destroying native fauna and flora.
The coastal brown or big-headed ant is a small ant 1.5 to 2.5 mm long. The colour of the ant varies from light yellowish brown to dark brown. Coastal brown ants travel in trails from the nest to food. The ant has four castes or body forms. The largest form is the queen, which lays eggs. Unfertilised eggs produce males and fertilised eggs produce females. There are many queens in coastal brown ant nests. Males are the next largest form and are elongated in shape. By far the most numerous forms are the workers. The large headed form defends the colony and is sometimes known as the soldier caste. One of the ant’s common names is taken from this form. Queens and males are rarely seen outside the nest.
The ant is thought to have originated in southern Africa and has now spread all over the Old-World tropics and into many temperate areas. In Australia, the ant has long been established along the east coast and the south-western corner of WA, around Perth. It is commonly found in the urban areas of Alice Springs, Katherine and Darwin, where it is a domestic nuisance feeding on food scraps and invading kitchens and bathrooms in search of food.
In the Top End, the coastal brown ant avoids bright sunlight and prefers lower temperatures and high humidity. As a result, the ant is more numerous in moist shaded areas. When temperatures are high workers forage only at night. Covered galleries or runways made of soil and organic matter are constructed on the trunks of shrubs and trees. This allows workers to travel up and down the trunk without being exposed to the high temperatures and lower humidity levels outside the runways. Heavy rainfall and waterlogged soils are unfavourable to the coastal brown ant. Ant activity and populations are lowest during the wet season. To avoid waterlogged soil the ant will invade houses and make subsidiary nests in trees and shrubs. Since the coastal brown ant does not have a mating flight, it is spread mainly by human activity. In suburban areas, the transfer of infested pot plants and landscaping materials to un-infested houses and gardens appears to be the main method by which the ant is spread over longer distances.
Nests are constructed in gardens, (particularly when landscaping materials and mulch are placed over garden soil), in pot plants and in crevices between brickwork, in cavity walls, under pavers, under concrete paths and under poorly constructed concrete slabs.
The ant prefers food of animal origin such as meat particles, urine, fat and grease, and dead insects. A fine layer of grease over kitchen walls, stoves, dish cloths and kitchen utensils is attractive to the workers. Soiled clothing may also attract the ant.
The coastal brown ant enters houses in search of food. Kitchens and bathrooms are the most attractive to the ant, although other parts of houses are also invaded. Ant trails inside the house usually lead to cracks and crevices between brickwork and tiles, around skirting boards and gaps in wooden floors. Once inside houses the ant can nest in light switches, computers and household appliances. In the garden, the ant will excavate nests in soil at the base of trees and shrubs. The excavation disturbs the root system of the plant, sometimes resulting in the death of the tree or shrub.
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