Supporters of Tiritiri Matangi Archives, Dawn Chorus, Autumn 2000

Ten years ago a new immigrant species slipped quietly into New Zealand, arriving in Auckland and establishing itself in Onehunga, just prior to the Commonwealth Games in 1990. That immigrant was the Argentine ant (linepithema humile) and initial surveys quickly revealed it to be quiet widespread. It was decided that there would be no welcoming committee dishing out pesticide sprays, or free food laced with insecticide, as has been the scenario when insect pests such as fruit fly or tussock moth arrived. Those entomologists “in the know” however, knew that New Zealand had come of age with this arrival of one of the world’s most invasive pest ant species.

The Argentine ant is a native of South America and has been invading overseas countries, including North America, Hawaii, South Africa, and Australia, for more than 50 years. Since its arrival in New Zealand, Argentine ant has spread to many areas in the Auckland Region, established itself in Tauranga and Morrinsville, and has recently been reported from Christchurch, Gisbourne and west of Dargaville. Now it has been found on Tiritiri.

Argentine ants are small-two to three millimetres long-are a pale red-brown and cannot spread by flying, only by walking or being carried. The ant is particularly successful because it develops large multi-nest colonies with huge numbers of workers that swamp food sources. Like most other ants they feed on sweet, surgery solutions such as nectar, as well as protein-based foods such as insects. Overseas research has shown that after these ants invade a site most other ant species disappear, any many other insect groups suffer a significant decline.

Argentine ant has several attributes that make it much more successful than other ants. A key feature that sets the species apart from most other ant species is that workers from neighbouring Argentine ant colonies cooperate with each other. Thus when a new food source is located, such as a tree coming into flower, all surrounding nests will be able to partake. Because it is very active, fast-moving ant, the species often locate new food sources ahead of other species and can thus more efficiently dominate all available sources in the area occupied. The species also features a highly developed chemical defence secretion, which will force the retreat of most other ants and invertebrates, even when these other species are much larger than Argentine ants. Thus despite their small size, Argentine ants often frequently win one-on-one contests.

Unlike virtually any other ants in New Zealand, Argentine ant trails feature huge number of ants, moving in a stream of up to five to six ants wide, like a busy six-lane motorway. These huge trails can be seen frequently moving up trunks of flowering trees, where the ants feed on nectar from flowers. They are also well known to exploit or “farm” honeydrew from other insects such as the mealybugs, which are common on flax Tiritiri. The sheet numerical superiority of the species tends to lock up these food resources and prevent other fauna feeding on them. This has implications for many species of invertebrates, lizards and birds that would normally feed on nectar and honeydew. As well as being extremely successful competitors the ants are predators of many invertebrate groups and there are even reports of them killing recently hatched chickens and invading broken eggs.

With an international reputation like this, it is a species that the Department of Conservation would prefer not to see in our nature reserves, especially our offshore islands used as safe havens for endangered species. Therefore, several years ago I put up a bid to have research undertaken in Auckland to determine if Argentine ant is likely to have such a severe impact on our native ecosystem as has been reported overseas. As a result two studies were instigated in Auckland last year, one by Landcare Research Ltd, and another by a PhD student. Results of this research will be some years away, but in the interim the decision has been made by the Auckland Conservator that the any should be eradicated from Tiritiri.

The Landcare contact includes research on a new insecticide ant bait, and early trials elsewhere in Auckland indicate it is very effective against the ant. Landcare has agreed to take part in an eradication campaign using the bait.

Surveys carried our over April indicate that the ant is present over about 5% of the island, centred on the wharf area. It is fortuitous that it has not reached the nursery or buildings at the top of the island, and special precautions are being taken to reduce the risk of the ant being carried up there. Other areas of the island are still being checked in detail.

It is extremely important that the ants are left undisturbed so they do not spread out even further before poisoning over the winter period. If visitors to the island see any ants please do not touch them. When disturbed, the multi-queened nests are likely to fragment, and potentially each queen can set up a new separate nest, thus spreading the problem over a greater area.

It appears as though Argentine ant has been on Tiritiri for some time, possibly several years. The best guess is that is that is probably came in on heavy machinery as a complete nest. The Department if moving quickly to eradicate the species as soon as possible.

Supporters of Tiritiri Matangi Archives, Dawn Chorus, February 2005

Argentine ants were first discovered on Tiritiri in March 2000. The species is recognised as one of the 100 most important pests of the world due to the impact it has on other wildlife, particularly invertebrates, but also lizards, frogs and birds. The ant has many attributes which make it such a successful pest (see Bulletin f or details) but three key features stand out. Each nest can have huge numbers, up to hundreds of thousands, foragers are active 24 hours a day, unlike most ants which are either diurnal or nocturnal (not both), and all Argentine ant nests co-operate with each other. These, and other features, mean the species can dominate whole ecosystems where conditions are suitable.

During the summer of 2001 a group of 14 ant specialists and volunteers f rom around New Zealand assembled on Tiritiri to administer the first treatment of poison baits over the whole 11 hectare area mostly centred around the wharf but also including a small population at Northeast Bay. Insecticide paste baits were placed every 2 – 3 metres in a grid fashion, as described in Bulletin 45. Intensive post-bait monitoring revealed that a massive 99% of the ants were killed. No longer were swarms of ants seen on trees and concrete edges by visitors waiting f or the afternoon ferry.

Following on from that first treatment a second treatment was applied in December 2001, again covering the entire previously infested area. As in the first season, there was a good kill but, frustratingly, once again a small number survived. However, there was no sign of any at Northeast Bay. In fact there hasn’t been an Argentine ants seen there since autumn 2001. Yeah!!!

During the 2002-03 season, with the assistance of Landcare Research, there was a lot of research into monitoring methods to detect small nests of surviving ants. Sticky traps, pitfall traps and various non-toxic baits were trialled. In the end we chose non-toxic baits placed in tubes with netting covers to prevent interference from other wildlife. Monitoring from December 2002 through to February 2003 showed surviving colonies of Argentine ants at just eight sites (see red areas on photo). All of these were initially small colonies but two, around the wharf shelter and above the wharf pond, grew rapidly in size during summer with the ants covering about 300 square metres at each site by early March. All eight sites, plus buffer areas (blue areas on photo), were treated with insecticide baits during March – April. Compared to the large scale treatment of the whole 11 hectare area (inside the black line on photo) the spot treatment of these sites was straightforward. However, particular attention was paid to getting optimum weather conditions and ensuring there were no gaps in bait coverage. Each site was baited twice, a month apart. Intensive monitoring during January – March 2004 showed no surviving Argentine ants at any of the sites spot treated during 2003 so the attention to detail really paid off. Other sites were monitored and only two small nests were found. Both were spot treated twice, as per the previous season. Therefore, after four years of the eradication programme I feel we are very close to eradication but now the real grind starts. In any eradication programme the last individual is always the hardest and most expensive to kill. Making sure there are no survivors is the tricky part – that is the current challenge. Two years Argentine ant-free status is required before the programme can be declared successful. Argentine ant was first found in New Zealand in 1990 and is now widespread in many parts of Auckland, as well as some towns to the north and south. Thus we all need to be forever vigilant to ensure no new nests of Argentine ants are transported to Tiritiri. As with rodents and other pests, prevention is better than cure.