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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
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