The Coastal Brown Ant (Pheidole megacephala), commonly known as the Big-headed Ant, is one of the most invasive ant species found in Australia. Originating from Africa, it has successfully spread across tropical and subtropical regions worldwide, including much of coastal Australia.

This species is recognised internationally as one of the “World’s 100 Worst Invasive Species”, mainly because of its ability to displace native ants and alter local ecosystems. In Melbourne and other Australian cities, coastal brown ants are common household and garden pests, often seen nesting in lawns, paving, and soil around buildings.

Identification

Coastal brown ants are easy to recognise by their two worker types — larger soldiers with strong jaws for crushing seeds and insects, and smaller workers responsible for foraging. They range in colour from yellowish-brown to dark reddish-brown, with twelve-segmented antennae and sparse long hairs on their bodies. Major workers are about 4 mm long, while minor workers are around 2 mm long. These ants usually nest in sandy soil, under pavers, or along garden edges, often leaving small soil mounds. Highly adaptable, they form multi-queen colonies that spread quickly and remain active year-round, frequently foraging in kitchens, bathrooms, and outdoor areas.

Threats

Coastal brown ants don’t sting or damage structures, but cause major environmental and domestic issues, making them a common urban pest in Australia. They outcompete native ants, affecting birds, lizards, and insects that depend on them for food. Their spread alters soil ecology and seed movement, disrupting plant growth.

In cities, they invade kitchens, bathrooms, and food areas, attracted to sugary and oily substances. Outdoors, they build small soil mounds along driveways, paving, and lawns — harmless but unsightly and signs of underground colonies. These ants are sometimes mistaken for termites because of their fragile soil tubes, causing unnecessary concern, though they don’t damage wood.

Preventions

Effective control requires area-wide management, as colonies connect and reinvade from untreated zones. Regularly check gardens and paved areas for trails or mounds, remove food waste, seal entry points, trim plants touching walls, and fix leaks around foundations.

Use oil-based baits with hydramethylnon, which ants share through the colony for lasting control. Spread bait evenly — not directly on mounds — and treat the whole property. Reapply if ants return and avoid using sprays beforehand, as they repel ants from the bait.

For large infestations, seek professional pest technicians for correct baiting, full coverage, and follow-up inspections to prevent re-colonisation.

Summary

The Coastal Brown Ant is a persistent invasive pest that quickly spreads through gardens and homes. Though not dangerous, it disrupts ecosystems and causes household nuisance. Combining good sanitation, proper baiting, and monitoring ensures effective, long-term control.