A closer look at yellow crazy ant, an invasive species that is spreading across tropical regions of Australia.
The yellow crazy ant is the largest of the global invasive ant species. It is a serious invasive pest and is named on the top 100 invasive species list compiled from the Global Invasive Species Database. When they become established in an area, they decimate local wildlife (invertebrates, reptiles and birds) through their sheer numbers and their ability to spray formic acid.
In addition to their environmental impact, they can damage crops, destroy electrical wiring in buildings and generally impact outdoor activities. They are a challenging species to control, not only because they persist as ‘super colonies’, but because they have a very mobile nesting habit.
The yellow crazy ant is thought to have originated in the tropical lowland forests of Southeast Asia, but through human-mediated transport it is found in an increasing number of tropical and sub-tropical regions. In Australia it is found on Christmas Island and Queensland coastal regions (especially the tropical north). An incursion in Lismore (NSW) is claimed to have been eradicated.
It is a relatively large ant (5 mm body length), but notable for its very long antennae and legs, which are also about 5 mm long. It exhibits an erratic movement when disturbed, from which it gets its name. There are two other ‘crazy’ ants which show similar movement – Nylanderia fulva (tawny crazy ant, raspberry crazy ant) and Paratrechina longicornis (longhorn crazy ant, black crazy ant). The yellow crazy ant is light brown/yellow, with a darker gaster. Although it does not bite or sting, it sprays harmful formic acid. It is not considered a health risk to humans unless there is a major infestation, which poses an increasing risk of burns to the skin and eyes from formic acid spray, especially to children and the elderly when sleeping.
One of the reasons for the successful establishment of yellow crazy ants when invading new areas is its colony structure and fluid nesting habits. Yellow crazy ants develop polygyne colonies and expand by budding – when a mated queen and a group of workers move away to start a new nest (as part of the same colony). Nuptial flights in yellow crazy ants are rare. They are able to nest in a wide variety of locations and they develop very loose nests, often under leave litter, which allows them to move as required. They are capable of nesting in cracks and crevices around structures.
Like many invasive ants, yellow crazy ants have a very broad diet – they forage on plant material, animals (both alive and dead) and in particular, honeydew. There is some evidence to suggest that the presence of honeydew-producing scale insects can accelerate the domination of yellow crazy ants in a given area. Similarly, the nature of this mutualistic relationship means the presence of yellow crazy ants can allow scale insects to proliferate, which can become a significant issue in agricultural areas.
Control of yellow crazy ants can be achieved through a combination of direct nest treatments and baiting. As the nests tend to be at the ground surface (under leaf litter), if the next is located and exposed, spraying the nest directly as well as the surrounding area can be very effective. However, if the nest cannot be located, or in cases of larger infestations, baiting should be considered. The challenge with baiting yellow crazy ants is that they have a very broad and changing diet. It is therefore often necessary to try more than one bait to determine which is likely to deliver the best results. Granular baits are generally recommended (to treat large areas) with a protein/fish meal bait matrix suggested as the preferred option. Actions to eliminate scale insects should also be considered if detected in the area.
In Australia, the Yellow Crazy Ant Eradication Program (YCAEP) is charged with eradicating yellow crazy ants from within and adjacent to the Wet Tropics World Heritage Area (between Cooktown and Townsville in QLD). Yellow crazy ant infestations outside of tropical north Queensland may fall under state responsibilities (e.g. Northern Territory and NSW), although some incursions will be left to local council programs. However, with limited funding at the lower levels of government, many of these infestations go unchecked or need to be dealt with by pest managers when called in by homeowners or businesses.
Species snapshot
Latin name: Anoplolepis gracilipes
Common name(s): Yellow crazy ant, long-legged ant, Maldive ant
Description: A light brown/yellow ant, with darker gaster. 5 mm long. Notable features include its long legs and antennae that are as long as the body. Tends to make fast, erratic movements when disturbed i.e. “crazy”.
Native distribution: Tropical lowland areas of Southeast Asia, both mainland and surrounding islands.
Current distribution: Many tropical and sub-tropical areas globally.
Nest location: Very adaptable nesting habits, from urban areas, to disturbed environments, agricultural areas and undisturbed forests. They typically create shallow nests under leaf litter and in cracks and crevices around buildings.
Nest structure: Multiple queens and multiple nests as part of the same colony, with the potential to create ‘super colonies’. Colonies expand by budding; nuptial flights are rare.
Diet: Highly varied diet (protein, oils and sugar) including dead and live invertebrates, plant material, honeydew, food scraps. The presence of honeydew-producing scale insects in the environment seems to be a key element for colony success.
Sting/bite: Yellow crazy ants do not bite or sting but can spray formic acid.
