Coastal Brown Ant Colony Structure and its Impact on Control Programs

Notoriously difficult to control, understanding the biology and nest-building habits of coastal brown ants is the key to successful ant management. 

The coastal brown ant or big-headed ant (Pheidole megacephala) is one of the world’s worst invasive ants. Like many invasive ants it has high reproductive rates and a multi-queen, multi-nest colony structure. Understanding its reproductive behaviour and colony structure is important in order to achieve optimal control from treatment programs.

Coastal brown nests can develop into ‘super colonies’, spreading over many kilometres, sometimes taking over whole islands. It appears that this phenomenon is more apparent in countries where they have invaded, such as Australia, rather than within its native range (Africa). With little genetic variability in an invading population, there is none of the intra-specific competition between neighbouring colonies that normally limits colony size in its native region. As such, nests continue to grow year on year.

Although big-headed ants in other geographies are observed to have defined alate flights, in Australia, founding by newly mated queens is rare. Instead, coastal brown ant nests in Australia primarily expand through budding, whereby a mated queen or queens leave an existing nest with some workers or brood to set up a new nest in a nearby location. These nests are often interconnected. In northern Australia, the rate of nest expansion has been estimated at 15 metres per year (source: Pacific Invasive Ant Toolkit).

Coastal brown ants have a prolific reproductive rate. Queens mate once, within the colony, laying up to 300 eggs a month, although up to 40% of the eggs may be eaten by workers and developing larvae. However, 180 surviving eggs per month, multiplied by the numerous queens in the colony, means the reproductive capacity of the colony is significant. New queens are recruited from daughters, and mating takes place between related individuals, thus keeping genetic variation low – a key feature of super colonies.

Although coastal brown ants can be a problem year-round, they are certainly more of a problem in the warmer months, when foraging activity and reproduction rates are high. In Australia, the peak period for coastal brown populations is in the late summer through to early winter. However, although this is likely to be the period when pest managers receive an increasing number of calls about coastal brown ants, in areas of known coastal brown infestations, it’s best to have a year-round management strategy.

If customers aren’t sure whether they should invest in a treatment for coastal brown ants, as often they are an outdoor ant problem, it’s worth pointing out that they can actually dig to a depth of up to four metres and their extensive digging activity can undermine paths, driveways and even foundations, as well as kill plants and damage lawns.

Clearly the main issue in dealing with coastal brown ants is the sheer size of the population and the fact that infestations will cover the whole neighbourhood. As a result, even if you clear the infestation from one property, there will be ants in the neighbouring property ready to move in. The good news is that baiting can be an effective strategy to keep populations in check, providing you use the correct type of bait.

Coastal brown ants are oil and protein feeders, so a dual food matrix such as Synergy Pro, which has both oil and protein granules, is ideal. Coupled with the fact Synergy Pro contains two actives, both hydramethylnon and pyriproxyfen, it provides optimal control on invasive ants such as coastal browns. The hydramethylnon is an effective killing agent and the pyriproxyfen acts to sterilise queens at very low doses.

In infested areas, it may be necessary to apply more than one application per year to keep activity at bay. A spring application, when ant activity starts to build, is ideal to nip any population growth in the bud. This setback to colony growth means they are unlikely to become a problem again until autumn, when a second application will be required.

It is always important to ensure sufficient bait is applied, especially with high ant numbers. Scattering bait along trails and across the area of activity will maximise uptake. Also, applying additional bait along the property fence line will help eliminate nests bordering the customer’s property, thus increasing the time before the ants re-invade.

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