Controlling the Argentine Ant
The Argentine ant, Linepithema humile (formerly Iridomyrmex humilis), is a
dark ant native to parts of South America. It is an invasive species that has been established
in many Mediterranean climate areas, inadvertently introduced by humans to many other
places, including South Africa, New Zealand, Japan, Easter sland, Australia, Hawaii, Europe,
and the United States.
The Argentine Ant in Australia has been considered a pest species for over 40 years and is
well established in the suburbs of Melbourne – and elsewhere. Unlike many other indigenous
species that build their nests in the ground, the Argentine ant is migratory in that it will nest,
for example, under grass clippings, bark and wood piles and in suburban roofs and wall
space. When the environmental conditions become unfavourable, the ant colony simply
moves elsewhere. The ant population can increase rapidly due to the fact that there are
multiple breeding queens in the colony and populations from different colonies overlap
without the usual conflict we see in other species. They in fact can form what are called
One behavioural aspect of the Argentine ant that is detrimental to horticulturalists particularly
in a citrus growing region is that the ant protects sap sucking insects such as Aphids and
Lerps – like a scale insect, Mealy Bugs and White Fly. The Argentine ant worker uses its
antennae to stimulate the insect which excretes a drop of honeydew, a sugar-rich sticky liquid
and an ideal food source for the Argentine ant. Consequently, the populations of these sap
sucking insects increases and their sugary excreta on the stems and leaves of a plant become
infected with the spores of moulds (e.g. sooty mould) and mildews (e.g. powdery mildew).
Then of course, the need arises for the use of insecticidal and fungicidal sprays, all adding to
the cost of production. A modern pest management approach in a commercial situation may
include preventative measures such as releasing predatory mites, wasps and lady beetles to
help control Aphids and Scale insects or only spraying one side of a tree so as not to kill off
all of the beneficial insects.
The first sign of a home invasion by the Argentine ant is usually finding the ant on the
kitchen sink around moisture or more often clamouring over jam jars or any sugary substance
left on a kitchen bench. Sometimes, it is easy to see a trail of ants moving across a floor or
around the skirting board.
Outside the home, particularly around Melbourne where I did a few years researching this ant
species, investigating and testing various control measures, ant trails could be easily seen
along the edge of footpaths, kerbing, fences and external home walls. Extensive use of toxic
sprays such as the chlorinated hydrocarbon Heptachlor (now banned) in the early 1970’s and
later, were used to try and eradicate the Argentine ant from Melbourne suburbs, but to no
avail. Spray treatments using the newer biodegradable insecticides (e.g. synthetic
Pythrethroids) did not achieve eradication of the Argentine ant. But, what does seem to
achieve good control, at least in and around ones home is the use of ant bait that utilises the
Argentine ant’s weakness for sugary substances.
The bait that I have used very successfully for many years uses Boric acid, commonly called
Boracic acid, as the active ingredient. There are commercial ant baits that contain Borax ,
also known as sodium borate, sodium tetraborate, or disodium tetraborate, but in my
experience, they are not as attractive as bait containing the more acidic boric acid. As well,
boric acid is more soluble that borax in aqueous solution – water! The baiting system is easily
prepared and placed on active Argentine ant trails.
Firstly the ingredients:
1 cup of white sugar (250 mL)
1cup of hot water (250 mL)
1 level teaspoon of Boric Acid
1 Jar with a metal top (example: a pasta sauce jar approximately 6 cm x 16 cm or a large jam
1. Drill two holes approximately 2-3 cm apart need to be made close to the edge of the
jar lid. Alternatively, use a hammer and a large nail to make the holes.
2. Pour the hot water into a bowl followed by a level teaspoon of boric acid. Stir the
mixture until all of the boric acid is dissolved and the solution appears relatively clear.
3. Add the sugar and again stir until it is all dissolved. The bait solution is ready for use.
4. Pour about one third of the bait solution into the bait jar.
5. Add a piece of paper towelling or a small amount of absorbent cotton wool to the bait
jar to provide a feeding surface for the ants.
6. Replace the bait jar lid, tighten and then wrap adhesive tape around the edge of the lid
so that children or pets will not have easy access to the contents.
7. Place the jar on an ant trail with the lid holes at the top. Shake a few drops of the bait
solution onto the ant trail and the ants will begin feeding and enter the bait.
8. Cover the bait with a piece of wood or metal and secure it in position (e.g. a brick).
An alternative is to place the jar inside a small length of plastic water pipe and lay the
pipe along the ant trail.
9. If more baits are required then use more jars.
Note: Boric Acid is a poison and can be dangerous to animals and young children. If the bait
solution is not all used up, it should be correctly labelled, sealed with a tight waterproof lid
and stored in a high place away from pets and children.
Need more information...
1. The Argentine Ant - Linepithema humile (formerly Iridomyrmex humilis),
2. Garden for Wildlife Newsletter No. 15 Alice Springs NT – Lerp Facts sheet
3. Royal Botanic Gardens & Domain Trust - Mealy bugs Facts Sheet
4. SARDI Pests & Diseases - Aphids
5. Chemical Watch Fact Sheet – Synthetic Pythrethroids
6. Boric Acid -(General Fact Sheet)