Coastal brown ants, big-headed ants

Page last updated: Tuesday, 21 July 2020 - 3:00pm

Please note: This content may be out of date and is currently under review.

Coastal brown ants (Pheidole megacephala), also called big-headed ants, are a major nuisance ant species in Western Australia. They are an urban pest and are often seen in lawns and in brick paving which they tend to undermine.

This page describes ways to identify this ant species, discusses their biology and advises on effective control methods. Control procedures outlined here are specific to this ant and may not be effective on other ant species.


The coastal brown is an introduced ant species of African origin. Look for:

  • Small, light ginger-brown coloured ants, with shiny dark brown abdomens.
  • The presence of two very different-sized ‘castes’ (types determined by their function) of worker ants, that is:
    • Smaller ‘minors’ — 2 to 3mm long.
    • A larger ‘major’ caste — 3.5 to 4.5mm long, which has a very obvious, much larger head and which makes up about 1% of the population. The ‘major’ caste of worker ants are not primarily soldiers for defence. Instead, their powerful jaws are used for cutting up large pieces of food into small pieces which can more easily be transported back to the nest by the more numerous minors.
  • Ants with no obvious odour when crushed.


There are multiple queens in the nests which are interconnected. New colonies are formed by budding whereby one or more queens with attendant workers leave an existing nest and walk to a nearby location. Rarely are new nests established by flying, mated queen ants.

While these ants can sting, the sting does not cause discomfort to people. Big-headed ants are particularly active in late summer, autumn and early winter. They nest outside in the ground and only occasionally invade buildings when populations outside are very high. However, invasions of buildings can be severe. These ants prefer meat or fat/oil-based foods.

Big-headed ants can form ‘super-colonies’ when their interconnected nests act as a single colony. The worker ants of such an infestation can occupy many hectares and they cooperate in ways similar to a single colony. In these situations they can displace native ant species and be the only ant species present in a heavily infested area. Big-headed ants are specialists in invading disturbed areas and are therefore ideally suited to urban developments. There are native species belonging to the genus Pheidole but these are rarely ever pests, and tend to be found in undisturbed habitats where they are predominantly seed-harvesters.


Infestations of big-headed ants are characterized by lines of inter-connected holes and small mounds of excavated soil. Excavations can be so extensive that brick paving is destabilised and the roots of plants and the lawn can become so aerated that the plants subsequently die by drying out. Often the small worker ants are hard to see, but food put out for pets can become covered in ants.

Look for two distinctly different-sized ants on a food source, with the larger worker ants having a disproportionately larger head.


Correct identification of the ants is crucial before commencing any control procedures. The Department of Primary Industries and Regional Development provides a free identification and advisory service.

Spray some ants with fly spray and stick about a dozen specimens, with clear sticky-tape, to a piece of paper containing your contact details. Ensure the surface the ants are collected from is clean to prevent picking up sand and other debris.

Send specimens to the address listed at the end of this page.

You can also report or send an image of a suspect pest or disease using the MyPestGuide suite of apps.

Control treatments

The most effective treatment is to use oil-based baits containing the active ingredient hydramethylnon. They are available under various trade names from a chemical retailer or hardware store. The ants collect the small particles and take them back to their numerous nests where complete control of the colony can be achieved.

These baits must be broadcast over the infested areas as per instructions on the product label. The baits should never be heaped around ant mounds as this not only increases the risk to non-target organisms but is far less effective in the control of big-headed ants. While expensive, a small 170g container is sufficient to treat a normal-sized urban block. The entire area should be treated as infestations exist in lawns and garden beds and can quickly re-invade treated areas if only a small area of obvious infestation has been treated. If the entire property is treated it may take two to three months or longer before it is re-colonised from untreated neighbouring properties.

Mechanical hand-held rotary or spinning-disc spreaders, designed to distribute small granules like fertiliser, are ideal for evenly applying the baits. It may be necessary to dilute the bait evenly with clean, dry sand to ensure that the application rate is sufficiently low to allow coverage of the entire area. If using a fertiliser spreader, ensure it is clean so that the bait is not tainted with fertiliser residue which might stop the ants collecting the bait and taking it back to the nest.

Keep the bait in a cool, dry location when not in use. Since the bait attractant is oil, the bait can become rancid and unattractive to the ants if it is not stored correctly.

Bait application

Bait should be broadcast when the ants are most active – this varies with the season:

  • Hot weather in summer — apply the baits before 9am or after 4pm.
  • Cold weather in winter— apply baits between 10am and 3pm.

Baits should remain dry for 12 hours after being applied. Therefore, do not apply if rain is expected and ensure irrigation does not come on within 12 hours of application.

Correct application of bait outside the building will also cure indoor ant infestations but do not apply the bait inside the home.

Protection of pets and wildlife

When applied correctly (broadcast) the baits should not present a risk to pets and birds. The bait should not be applied directly inside bird cages, and they can be toxic to fish and aquatic invertebrates, so take care not to contaminate ponds and waterways.

Don't spread ants

Apart from the natural radial expansion of existing colonies, big-headed ants are spread unintentionally through transport by people via a wide range of commodities, including soil, pot plants, foodstuff and rubbish. Several plant nurseries in metropolitan and country areas are infested with big-headed ants and present a risk of spread. Be sure to check that pot plants or any other plant material you may receive, even from friends, are free of ants.

If you are moving house and take pot plants with you, or are giving pot plants to friends, check that the pots are free of ants or treat the pots by immersing the pot for 30 seconds in a solution of an insecticide registered for controling ants.

Specimen identification

When sending or delivering samples for identification, the following information is required:

  • collectors name
  • location (where the specimen was found)
  • full address, telphone number and email address
  • description of the damage
  • date collected.

Please send specimens to:

Department of Primary Industries and Regional Development
Pest and Disease Infomation Service
3 Baron-Hay Court, South Perth WA 6151

The department needs your help to detect animal and plant pests, diseases and weeds that could pose a threat to WA's agricultural industries.

If you discover something unfamiliar please contact the Pest and Disease Information Service (PaDIS) on 1800 084 881.

You can also report or send an image of a suspect pest or disease using the MyPestGuide suite of apps.

Contact information

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080