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IN FOCUS: BIG-HEADED ANT

We take an in-depth look at big-headed ants, looking at their biology, behaviour and control options.

 

Big-headed ants are considered one of the 100 worst pests in the world. Not only do they create issues in agricultural areas and inside buildings, where they can cause faults with electrical services, as an invasive pest they readily out-compete and displace native wildlife, particularly invertebrates and especially ants. As a result they are seen as a major threat to ecosystems and biodiversity.

First recorded on the island of Mauritius in the Indian Ocean, the big-headed ant is thought to have originated in the tropical areas of Africa; indeed the ‘megacephala’ group seems quite diverse on the African island of Madagascar. Human-assisted transport has ensured the ant has spread to many sub-tropical and tropical regions throughout the world. Having arrived in Australia around 100 years ago, it is now found in many areas of the mainland.

 

Species snapshot

Latin name: Pheidole megacephala.

Common name(s): Big-headed ant, African big-headed ant, coastal brown ant.

Description: Brown ant, often with a darker gaster. They are readily identified by the dimorphic workers – the smaller workers are small (c. 2 mm long) and the larger workers (3-4 mm) have the oversized head, after which the species is named.

Native distribution: Tropical areas of Africa.Current distribution: Many tropical and sub-tropical areas globally.

Nest location: Typically a soil-nesting species, thriving in moist disturbed areas, particularly under logs, rocks and pavers. Will readily nest inside in ceilings, wall cavities and power outlets.

Nest structure: Multiple queens and multiple nests as part of the same colony. Budding can create ‘super colonies’.

Diet: Highly varied diet (protein, oils and sugar) including dead and live invertebrates, plant material, honeydew, food scraps.

Sting/bite: Big-headed ants do not sting and have a very mild bite (to humans).

 

Although big-headed ants are assumed to be Pheidole megacephala, it appears there are a number of species in the ‘megacephala’ group, including a number of unidentified species. They all have the unmistakable big-headed soldier. Their oversized heads contain the large muscles for dismembering prey, cracking seeds and colony defence.

Interestingly, it appears that the big- headed ants in Australia have the largest heads of any specimens collected globally.1 Researchers believe that the heads of soldiers in Australia have increased in size over a relatively short period of time since their arrival, in response to the presence of the more aggressive, competing ant species in Australia. However, this increased selection pressure does not appear to have influenced the number of soldiers produced – soldiers appear to comprise 1% of all nests analysed globally.

The colour of big-headed ants and size of the smaller worker sometimes cause them to be confused with the various Monomorium species, and sometimes even with the red important fire ant. But the presence of larger workers is generally a big giveaway and it can be readily differentiated from red imported fire ants, which have a continuous range of worker sizes, a more aggressive nature (and nasty sting!) and a unique nest mound of friable soil.

One of the main reasons why big-headed ants are so successful in invading new areas is because they have multiple queens per colony and produce brood year- round in their preferred tropical and sub-tropical areas. They expand their nest size through budding, which allows them to create ‘super colonies’ that can exclude other ant species from an area. It appears that in Australia, dispersal is exclusively through budding (no alates have been observed)2, although in Florida, nuptial flights are observed in winter and spring.

Their nests become obvious through the displacement of dirt, with obvious piles seen in lawns, around structural elements and particularly in and around pavers. Interestingly, big-headed ants can also building shelter tubes from dirt and other materials, which can look like termite mud tubes at first glance. They can also build partial shelter tubes over their foraging trails on the surface of the soil. Their foraging trails can cover significant distances to substantial food sources.

 

Big-headed ants can build shelter tubes that superficially look like termite activity (photo credit: Andrew Critchley, Instinct Pest Management)

 

Their omnivorous diet is another key reason for their success. Their food sources include: scavenging dead invertebrates; tackling live prey (mainly invertebrates but some small vertebrates); harvesting honeydew from sap-sucking pests; attacking plants, fruits and their roots directly; and feeding on food scraps left by humans. Big-headed ants do not sting, they overwhelm any live prey with sheer numbers. To humans their bite is virtually painless.

 

 

Complete elimination of big-headed ants is virtually impossible. When they become established in an area, even if they are removed from one property, it is only a matter of time before they will re-invade from the surrounding area. However, to temporarily eliminate from an area, or at least significantly suppress numbers, is certainly possible using a multi-product treatment program.

With their varied diet, big-headed ants will forage on a wide variety of baits. Granular corn grit-based baits are readily accepted by big-headed ants and as long as sufficient bait is applied, they can be very effective in eliminating nests in an area. Granular baits are certainly a good option for treating large external areas, but for internal areas sugar-based baits can also be considered as an option. Although they are accepting of a wide range of baits, as their preferences may vary, it may be necessary to trial several baits to determine acceptance.

Combining non-repellent sprays with a baiting program generally delivers the best results. They are certainly an option for both internal and external areas, applying to lawns and garden beds (if the label allows), as well as the perimeter of the building. Pyrethroid sprays and granular insecticides can also be included in a treatment program, particularly for exclusion of ants from between pavers and in garden beds.

It is very important to not over promise with big-headed ants – a 12-month warranty is almost certain to cause problems with callbacks. However, explaining the invasive nature of big-headed ants not only demonstrates your expertise but provides the opportunity to provide a regular service program (at least twice a year) in areas of heavy infestation.

 

1 Will, B.D et al (2014). Body size variation and caste ratios in geographically distinct populations of the invasive big-headed ant, Pheidole megacephala (Hymenoptera: Formicidae). Biological Journal of the Linnean Society 113: Issue 2, 423–438. https://doi.org/10.1111/bij.12386

2 Vanderwoude, C et al (2000). Response of an open-forest ant community to invasion by the introduced ant, Pheidole megacephala. Austral Ecology 25: 253-259.