Argentine ants in Auckland: how to tell, and what actually works

How do I know if I have Argentine ants?

Four field checks point to Argentine ants, although Landcare Research is clear that none of them is proof on its own. The workers are all the same size, 2 to 3 mm, and a uniform light brown rather than black; they move quickly in steady, continuous trails instead of wandering; they walk straight over anything placed on the trail; and crushed, they smell slightly greasy or of nothing at all.

The smell test matters because the common look-alike in Auckland gardens, Darwin's ant, is much the same size and colour but gives off a strong smell when it is squashed. Confirming the species properly takes a microscope, so treat a field identification as probable rather than certain, and if the ants you are seeing are black, orange or yellow, they are not Argentine ants.

In town, the trails are often found by pulling back the grass at the edge of the footpath, or along the smooth edge of a path or a driveway, and they run up flowering shrubs and citrus trees to the honeydew on them. Nests sit under timber, stones and piles of leaves and in pot plants, and thousands of ants pour out when one is disturbed. If you are still unsure, compare what you have against the other types of ants in New Zealand homes.

Are Argentine ants a problem in New Zealand?

Yes, as a nuisance and an environmental pest rather than a health or a structural one. They come indoors in very large numbers, get into cupboards and even sealed jars, swarm up arms and legs when a nest is disturbed, farm aphids and scale insects on garden plants, push other ant species out and rob beehives, and Landcare Research ranks them highly as a domestic nuisance species.

The Argentine ant, Linepithema humile, was first found in New Zealand at Mount Smart Stadium in Auckland, during opening-ceremony rehearsals for the 1990 Commonwealth Games, and genetic work suggests the population here came from a single source, most likely in Australia. It is now known from numerous places in the northern North Island and several in the South Island, and the warm, settled north is where it does best.

Auckland Council manages Argentine ants as a pest across the whole region under sustained control, which means you must not breed, distribute, release or sell them, and it is why the advice further down on moving plants and soil matters as much as anything you do in the kitchen.

What works against Argentine ants, and what makes them worse

Slow bait that the workers carry home works, placed beside the working trail and outside where the trail comes from, and then left alone. A repellent spray makes things worse, because disturbed Argentine ant nests bud and scatter, and Landcare Research reports nests fracturing after a dose of bait that was too weak to kill them, which is why the bait has to be the right kind, in the right place and in enough quantity.

The biology explains the choice of bait. Workers feed mostly on sugar while the larvae and queens are fed protein, so a sugar bait thins the workers and a protein bait reaches the brood. University of California guidance finds sugar baits attractive to Argentine ants all year and protein baits mainly in spring, when colonies are raising new offspring, and suggests offering a small amount of each first to see which they take. In New Zealand the approach Landcare Research describes as the commonly recommended one is protein bait laid at high density across the whole infestation, so that every part of the supercolony is within reach of it.

Scale is the catch. A few small bait stations on a windowsill against a supercolony is, in Landcare Research's words, like fighting a forest fire with "one bucket of water", which is why coordinated treatment beats one kitchen. Concrete paths and kerbs also act as ant highways, and their alkaline surface breaks down many insecticide sprays quickly, which is another reason a perimeter spray fades fast against this ant.

Where the bait goes matters as much as which bait. Put it mainly outdoors, near nests, along trails and along the foundations, because indoor baiting can draw more Argentine ants inside; keep it away from children and pets; and pair it with sealing the gaps they use, since bait on its own rarely controls them. We have set out which kind of ant bait suits Argentine ants separately.

Don't move them: pot plants, soil and bark

Within a region, Argentine ants spread mainly by hitching a ride with people, so check anything leaving a property that has them: pot plants, garden soil, bark, garden waste, camping gear and building materials. Auckland Council's simple test for a potted plant is to submerge the pot in water for two minutes and see whether ants come out, and it asks people to check vehicles and boats as well.

The Hauraki Gulf islands are treated more strictly, because Argentine ants are a pest the council works to keep off them, so take extra care before carrying plants, soil or gear out to an island, and check with Auckland Council first. If you are moving house, the same check applies to the pot plants that go on the truck.

When to stop doing it yourself

Stop and get help when trails turn up in several rooms or start coming out of the walls, when Argentine ants are on both sides of the fence so that you are treating one corner of a supercolony, when someone has sprayed and one trail has become several, or when two weeks of bait have made no difference to the numbers.

What we do differently is mostly a matter of scale and order. We confirm the species first, bait the trail outside as well as in, put out enough bait at enough points across the property to reach the nests, and deal with the aphids and scale insects whose honeydew is feeding the colony. We will also tell you plainly when a street-wide infestation means the best result anyone can sell you is a house kept clear. Our page on professional ant control in Auckland sets out how a treatment runs, or you can request a visit.