Yellow crazy ant

Anoplolepis gracilipes is a relatively large, yellow to orange ant with long legs, large eyes and extremely long antennal scapes.

Although A. gracilipes is the only invasive species in the genus Anoplolepis, there are several other genera that it can be mistaken for. Both Leptomyrmex and Oecophylla can be confused with Anoplolepis because of their similar sizes and very long limbs. Anoplolepis can be distinguished from Leptomyrmex by the presence of an acidopore. Anoplolepis can be distinguished from Oecophylla by the more compact petiole. Although both of these genera occur in the Pacific, neither contain any invasive species.

Several species of invasive ants belonging to the genera Camponotus and Paratrechina can appear similar to A. gracilipes. Although several invasive species of Pheidole can also be slender-bodied with long legs and long antennal scapes, they can be separated from A. gracilipes by their two-segmented waists.

A. gracilipes is widespread across the tropics, and populations are especially dense in the Pacific region. The species is most infamous for causing the ecological "meltdown" of Christmas Island.[6][5] Although widespread across the Pacific, A. gracilipes can cause significant damage to native biological diversity, and strong quarantine measures are encouraged to keep it from spreading to new localities.

A dead gecko being dragged by yellow crazy ants in India

The yellow crazy ant's natural habitat are the moist tropical lowlands of Southeast Asia, and surrounding areas and islands in the Indian and Pacific Oceans.[7] It has been introduced into a wide range of tropical and subtropical environments including northern Australia, some of the Caribbean islands, some Indian Ocean islands (Seychelles, Madagascar, Mauritius, Réunion, the Cocos Islands and the Christmas Islands)[5] and some Pacific islands (New Caledonia, Hawaii, French Polynesia, Okinawa, Vanuatu, Micronesia, Johnston Atoll, and the Galapagos archipelago).[8][9] The species has been known to occupy such agricultural systems as cinnamon, citrus, coffee and coconut plantations. Because the ant has generalized nesting habits, they are able to disperse via trucks, boats and other forms of human transport.[1]

Crazy ant colonies naturally disperse through “budding”, i.e. when mated queens and workers leave the nest to establish a new one, and only rarely through flight via female winged reproductive forms. Generally, colonies that disperse through “budding” have a lower rate of dispersal and need human intervention to reach distant areas. It has been recorded that A. gracilipes moves as much as 400 m (1,300 ft) a year in Seychelles.[8] A survey on Christmas Island, however, yielded an average spreading speed of 3 meters (9.8 ft) a day, the equivalent of one kilometer (0.6 mile) a year.[10]

A. gracilipes (yellow crazy ants) moving dead Blattidae sp. (cockroach) toward their nest in Pohnpei, Federated States of Micronesia

A. gracilipes has been described as a “scavenging predator” and has a broad diet, a characteristic of many invasive species. It consumes a wide variety of foods, including grains, seeds, arthropods, and decaying matter, including vertebrate corpses. They have been reported to attack and dismember invertebrates such as small isopods, myriapods, molluscs, arachnids, land crabs, earthworms and insects.[10]

Like all ants, A. gracilipes requires a protein-rich food source for the queen to lay eggs and carbohydrates as energy for the workers. They get their carbohydrates from plant nectar and honeydew producing insects, especially scale insects, aphids, and other Sternorrhyncha. Studies indicate that crazy ants rely so much on the scale insects that scarcity of them can actually limit ant population growth.[8]

Crazy ants get much of their food requirements from scale insects, serious plant pests that feed on sap of trees and release honeydew, a sugary liquid. Ants eat honeydew, and in return protect the scale from their enemies and spread them among trees, an example of mutualism. The honeydew not eaten by ants drips onto the trees and encourages the growth of sooty mold over the leaves and stems. This gives plants an ugly black appearance and reduces their health and vigour.

The ants protect the insects by "nannying" the mobile crawler stages and protecting them against their natural enemies.[11] Experiments have shown that this connection is so strong that, in environments where A. gracilipes was removed, the density of scale insects dropped by 67% within 11 weeks, and to zero after 12 months.[12]

In Australia, yellow crazy ants have been found at more than 30 sites in Queensland, and in Arnhem Land in the Northern Territory, where a large scattered population exists.[13] A single New South Wales infestation was detected and eradicated, and in Western Australia yellow crazy ants have been intercepted in shipping freight arriving at Fremantle.[14]

Queensland's main infestation is 830 hectares (2,100 acres) in and around Wet Tropics of Queensland rainforest, a World Heritage Site.[13] The Northern Territory infestation covers 2,500 square kilometres (970 sq mi),[14] an area larger than the Australian Capital Territory.[15]

Climate modelling indicates yellow crazy ants could spread across northern Australia from Queensland to Western Australia, across much of Queensland and into coastal and inland parts of New South Wales.[14] Areas with the most ideal habitat and climatic conditions, such as the Wet Tropics of Queensland rainforests, are likely to experience the highest impacts.[16]

A cost–benefit analysis by the Queensland government found that yellow crazy ants could cost Australia's economy over $3 billion (AUD) if the ants were not treated. This analysis did not take potential impacts on Australia's biodiversity into account.[16] The known impacts of crazy ants in tropical rainforests overseas may provide useful insights into these impacts, bearing in mind that the most significant impacts are associated with relatively small islands, such as Christmas Island.[14][5]

Crazy ants have had a profound impact on the biodiversity of Christmas Island.[14][5]

The crazy ant has a significant destructive impact on the island's ecosystem, killing and displacing crabs on the forest floor. The super-colonies also devastate crab numbers migrating to the coast. This has seen a rapid depletion in the number of land crabs [5] - killing up to 20 million of them[5] - which are vital to Christmas Island's biodiversity – land crabs are a keystone species in the forest ecology: they dig burrows, turn over the soil, and fertilize it with their droppings.[17]

Seedlings that were previously eaten by crabs started to grow and, as a result, changed the structure of the forest. Weeds also spread into the rainforest because there are no crabs to control them. One of the most noticeable changes in the forest is the increased numbers of the stinging tree Dendrocnide peltata, which now flourishes in many areas frequently visited by humans. The forest canopy also changed as the scale insects tended by yellow crazy ants multiplied and killed mature trees.[18][16]

Robber crabs, red crabs,[5] and blue crabs are completely wiped out in infested areas. Populations of other ground and canopy dwelling animals, such as reptiles and other leaf litter fauna, have also decreased. During crab migrations, many crabs move through areas infested with ants and are killed. Studies show that the ant has displaced an estimated 15–20 million[5] crabs by occupying their burrows, killing and eating resident crabs, and using their burrows as nest sites. This factor has greatly depleted red crabs, and made their annual land migrations far more perilous.[19][20]

Although crazy ants do not bite or sting, they spray formic acid as a defence mechanism and to subdue their prey. In areas of high ant density, the movement of a land crab disturbs the ants and, as a result, the ants instinctively spray formic acid as a form of defence. The high levels of formic acid at ground level eventually overwhelm the crabs, and they are usually blinded then eventually die from dehydration (while attempting to flush out the formic acid) and exhaustion. As the dead crabs decay, the protein becomes available to the ants.[16]

Crazy ants kill fauna, but encourage scale insects. Increased densities of scale insects cause forest die back, and even the death of large forest trees. These changes create a cascade of negative impacts, including weed invasion, significantly altering the forest landscape.