Argentine Ants on Great Barrier Island

They may be tiny but these ants are a serious potential threat and they are already here.

by Joanne O'Reilly
 

Background

Argentine ants originated in South America. They are listed by the World Conservation Union as one of the world’s 100 worst invasive species. They were first discovered in NZ, in Auckland in 1990. Since then they have spread to a number of places around New Zealand, from Canterbury to Northland. In areas where the area of infestation is considered small enough, a coordinated approach to their control is undertaken as is the case on Great Barrier Island.

An argentine worker ant
(Linepithema humile).

Argentine ants were identified in Tryphena 5 years ago after a call to DOC from a resident concerned about the number of ants in her garden and their aggressive behavior. After surveillance Auckland Council (then ARC) and DOC decided to undertake a coordinated control plan with an objective to eradicate Argentine ants from the Barrier. This program has been ongoing. Auckland Council has taken the lead role in this project because the majority of the infested area is private land. DOC has taken a strong support role because of the risk Argentine ants present to biodiversity values and because if left untreated they could inhabit most of the island.
From the survey work undertaken in the first year ants were found, they were identified, and assumed to be confined to, the Mulberry Grove area of Tryphena, Ocean View Road in Claris, and the southern end of Sandhills Road in Medlands. Additionally, several smaller sites were also found including at Puriri Bay transfer station, Barrier Building ITM, a garden rubbish dumping site on the Sugar Loaf hill, and a property on Mohunga Peninsula.

What is the problem?

So what is all the fuss about these little brown manuka honey colored ants? First of all, unlike other ant species these guys co-operate between nests creating super colonies of millions of ants. While one ant is relatively harmless, other than being able to give a little bite, in numbers they have a huge combined appetite and nuisance factor.

Argentine ants are most often associated with human habitation. Untreated, they may spread to all parts of the island where people live. Residents from infected areas have told horror stories about when ant numbers were at their highest - towels hanging in bathrooms being covered with ants, of ants swarming over them in their beds at night. Evidently in Auckland, people will place their BBQ legs in buckets of water to prevent the ants swarming up their BBQ and onto the food. They have been known to make their way into refrigerators, microwaves and screw top jars.

In the garden, Argentine ants interfere with pollination, feed on crops and flowers and will transfer aphids from one plant to another to farm them for their honeydew. In terms of biodiversity these critters will out compete other ants and other invertebrates – good and bad. They are a nuisance to nesting birds and have been known to kill chicks as they emerge from their eggs as well as annoying them so much that they leave their nests. Argentine ants are most aggressive in warm sunny climates and over the summer months. They tend to go to ground over winter and are not so obvious.

What has been happening?

Early in 2006 after survey work had been undertaken, treatment of the infestation sites was undertaken by volunteers led by an ARC-DOC contractor – Des Casey.

At the end of 2006, i.e. the beginning of the next ‘summer treatment season’, monitoring of the infestation sites was undertaken to determine the effectiveness of the operation. Argentine ant numbers were found to be markedly reduced in the infested areas but still scattered throughout them.

Since 2006, monitoring has been undertaken each year by local company ‘Envirokiwi’, under contract to ARC (AC). This monitoring has been followed by treatment of the areas where Argentine ants are found. Additionally, areas of likely invasion i.e. wharves, refuse transfer stations, rubbish tips, nurseries and timber yards, have been surveyed each year.

Envirokiwi has employed teams of up to 12 people each summer to undertake the work. Most of the staff is local youth either returning to the island for holidays or residing on the island. The crew is out on the hottest days when the ants are most active. There is a need for accurate recording of results and attention to detail to retrieve pottles placed out the previous day. This may sound like unpleasant, hard work but there is great team spirit.

The Argentine ant’s colouration can extend from light to dark honey brown.  This highly successful “tramp ant” invader has a varied diet of nectar, insects, carrion, seeds and honeydew.  The ant is small but what they lack in size they make up in numbers.


So what happens in the field?

Approximately 67ha was monitored this year, and a considerably smaller area was treated.
Pottles are placed out in the field in a 3m x 3m grid pattern over the whole of the monitoring area excluding buildings. Wherever possible, pottles are placed out of direct sunlight and close to the ground. Pottles (marked with flagging tape) are usually left overnight and collected the next day.

The location of ants is mapped to allow treatment to occur at the site and within a 20m radius of the site. Bait is placed on the ground in a grid pattern and four to six weeks later this is repeated to kill any ants that were in the pupae stage at the first treatment.

How are we doing?

The boundary of the areas where Argentine ants are found has expanded in the three main infestation areas but the density of ants within those boundaries has been significantly reduced. Several sites including Barrier Building supplies, Sugar Loaf, Puriri Bay recycling bins and Mohunga have been eradicated. One new site was found this year.

What can we all do to help this project to succeed?

To prevent Argentine ants being re-introduced to Great Barrier or moved around Great Barrier, it is important to check all pot plants, soil media and building materials for Argentine ants. This can be done by dunking any pot plants in a container of water until the soil is fully saturated. If ants are in the soil, they should come out of the soil and fly spray can then be used to kill them. Be aware that there may be some pupae surviving in the soil, so where ants are seen, this process should be repeated in a week’s time. Any vehicles that have been standing for a period of time should be shaken around a bit to disturb any ant nests that may be present. The ants are likely to desert a moving object but any ants seen should be sprayed again with fly spray. This process should be done whenever moving materials from a known infestation area, on or off the island. Additionally, if purchasing materials from a nursery, check the supplier’s hygiene standards – is their soil sterilized etc?

Argentine ants are sophisticated little critters that have ingenious methods of ensuring not just survival but dominance in many situations. Eradication of Argentine ants is still in the experimental stages and a review of the Great Barrier Island project will be undertaken this year.
A lot of information about Argentine ants is available from the web, otherwise, if you would like more information or are concerned that you may have them on your property (or elsewhere) please call the DOC field centre 09 4290 044 or Jeff Cook at the Auckland Council, 027 5553451 jeff.cook@aucklandcouncil.govt.nz