New Zealand has about 28 foreign species of ant, which have become established mostly through accidental human transport. The majority have come from Australia. Others are invasive species that have colonised many countries. Several are bothersome pests around the house, and some have the potential to invade native habitats, which would have a severe impact on native species and ecosystems. Most are found in the warmer parts of the North Island.
Because ants can be easily carried around the world with cargo on planes and ships, the Ministry of Agriculture and Forestry established a national invasive ant surveillance programme in 2001. This monitors the points of entry to New Zealand, particularly ports.
White-footed house ant
The most common introduced ant is the white-footed house ant (Technomyrmex jocsus). It often nests in walls and forages in houses for water and food, particularly sweet items. This ant also protects plant pests such as scale insects and mealybugs, in order to feed on the honeydew they produce.
The colonies have several thousand workers and many functional queens, so their numbers can build rapidly.
The Argentine ant (Linepithema humile) is one of the world’s 100 most invasive species. This light brown ant (2.2–2.6 millimetres long) is well established in much of the North Island and parts of the South Island. It thrives in open areas and bush with open canopy, but does not penetrate far into native forest.
An invasive ant known as the red imported fire ant (Solenopsis invicta) is spreading around the world. It has been found and eradicated on three occasions in New Zealand. In some parts of the USA and Queensland it’s impossible to walk or sit on the grass because the ants swarm from their underground nests when disturbed, and attack and sting anything nearby. The sting is as painful as a bee sting, and produces a pustule on the skin.
Argentine ants form super-colonies with many queens, and displace other ant species. Competitive, and eating a broad range of food, they could have far-reaching effects on natural ecosystems, as they have in other countries. The Department of Conservation established an eradication programme on the wildlife reserve island of Tiritiri Matangi in the Hauraki Gulf, but colonies on the mainland present a greater challenge.
In urban areas the Argentine ant is a pest because it forages indoors, swarms over pot plants and feeds on honeydew produced by mealybugs and scale insects – plant pests that they protect from predation. They swarm up your arms and legs if you disturb their nest. They bite, but they cannot sting.