One of the world's 100 worst animal invaders has arrived in Whanganui, pest control technician Gary Clark says.

Argentine ants have been as close as Patea for years. He's had a microscope and identification book ready, and thought surely there must be some in the city.

He took home a sample and identified some for the first time two weeks ago. They have "really gone mad" in Whanganui's old town area, in Bell St, Church Place and Market Place. The biggest nest he knows is behind Whanganui Police Station.

"I haven't struck much in domestic properties. I think they must have been introduced downtown and that's where I'm striking them."


The ants are smaller than usual species, only 2-3mm long and a honey brown colour rather than black. They eat honeydew, honey, meat, eggs, oils and fats.

They climb trees and have been known to kill baby birds, infest beehives and kill bees. They can crawl through 1mm holes. They also bite or sting, probably as a defence. Some people react to it, but Mr Clark said it wasn't too severe.

"It's nothing like a bee sting or wasp sting."

The serious thing about the ants is their numbers. They can join together in supercolonies with many queens.

Foraging as far as 60m from their nests, they walk four and five abreast, rather than single file or "darting around like a 6-year-old girl on Christmas morning" like common pavement ants.

From South America, the ants have spread to many countries. They arrived in Auckland in 1990 and are now in most coastal North Island centres - especially in the north. In some countries they have prevented children playing outdoors, killed pet birds and lizards and caused people to sell up and move elsewhere.

They generally kill off other ant species and some other insects in their areas. They also protect insects like aphids that produce honeydew, and may cause them to proliferate.

Mr Clark said they liked a moist environment, which humans were good at providing. They nest in waterpipes, pot plants, walls and under leaf litter in gardens.

"Dry conditions will drive them into your shower base."

They are often moved to new places in people's vehicles, in pot plants and in soil and bark.

They are best handled by a registered pest control technician, because their large numbers need 70-100 bait stations on an average property.

There are effective contact sprays and granules that technicians can access. More than half Mr Clark's work in summer is controlling ants, and he's getting a lot of calls about them this year.

He said white-tailed spiders, which have a bite that can cause a painful swelling for some people, were also especially common in Whanganui.