Yellow crazy ant

Yellow crazy ant
There is also a different genus of ant called "crazy ants", Paratrechina.
Yellow crazy ant
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Genus: Anoplolepis
Species: A. gracilipes
Binomial name
Anoplolepis gracilipes
F. Smith, 1857

The yellow crazy ant (Anoplolepis gracilipes) is a species of ant, introduced accidentally to northern Australia and Christmas Island in the Indian Ocean, that has wreaked ecological damage in both locations. It is colloquially called "crazy" because of its erratic movements when disturbed, with its long legs and antennae making it one of the largest invasive ant species in the world.[1]

Along with the red imported fire ant (Solenopsis invicta), the big-headed ant (Pheidole megacephala), the little fire ant (Wasmannia auropunctata), and the Argentine ant (Linepithema humile), the yellow crazy ant is one of the five species of tramp ants, known for invasive behavior and devastating ecological effects.[2] Also known as the long-legged or Maldive ant, it is listed among the 100 most devastating invaders of the world. It has invaded ecosystems from Hawaii to Seychelles, and formed supercolonies on Christmas Island in the Indian Ocean.



Worker ants are monomorphic and have a life cycle of 76–84 days. They have a body length of approximately 4mm, a yellow brownish colour with a darker brown gaster and remarkably long legs and antennae. They are weakly sclerotized, have an oval-shaped head, mandibles with 8 teeth and 11-segmented antennae. The mesosoma is slender, pronotum narrow, with an almost-straight dorsum in profile. The anterior portion of the mesonotal dorsum, back to the propodeum, is gently concave in profile. The propodeal dorsum is convex in profile. The petiole is thick, with an inverted-U-shaped crest.[1] Although they cannot sting, the ants spray formic acid in defence and, while this in itself does not kill the target, it can cause blindness and lead to death by starvation in many vertebrate victims.


Yellow crazy ants exhibit unicolonial behaviour and have formed multiple large high-density supercolonies on Christmas Island. Although many ant species exhibit unicoloniality, the supercolonies reach new heights in the case of invasive species in their introduced environment. Literally meaning "one colony", unicolonial behaviour is characterized by the cooperation and lack of aggressive behaviour between the foraging ants of multiple colonies, each with their own queen. The lack of competition between colonies gives the ants a competitive edge, and enables them to reach unusually high densities. Densities of up to 2,254 foraging ants per m2 with a biomass of 1.85 g per m2, and densities reached 10.5 nests entrances per m2. This represents the highest density of foraging ants ever recorded.[3]

Furthermore, this multi-nest (polydomous) and multi-queen (polygynous) structure increases the probability of colony survival. Nests can contain up to 300 queens and 2,500-3,600 workers over areas as large as 150 hectares.[4]

Although introduced to Christmas Island between 1915 and 1934, the first supercolony was discovered in 1989, and after 1996 supercolonies have been forming rapidly. By September 2002, 28% of the 10,000 hectares of rainforest on the island were infested. ( [5] in [3])

Three different mechanisms contribute to maintaining the unusually high numbers of yellow crazy ants on Christmas Island. The enemy release hypothesis, suggests that lack of pressure from co-evolved natural enemies may allow organisms to achieve a large colonial size.[6] Another hypothesis relies on the fact that reduced intracolony aggression enables foraging ants to use the energy allocated to colony defense on expanding their colony.[7]

Finally, the superior efficiency compared to native species at exploiting a diverse array of resources enables the ants to maintain a high foraging rate and thus increase in numbers.[8]

Geographical range and dispersal

A dead lizard being dragged away by Yellow Crazy Ants in Kerala, India

The yellow crazy ant’s natural habitat is not known, but it has been speculated that the species originated in West Africa. It has been introduced into a wide range of tropical and subtropical environments including Caribbean islands, some Indian Ocean islands (Seychelles, Madagascar, Mauritius, Reunion, the Cocos Islands and the Christmas Islands) and some Pacific islands (New Caledonia, Hawaii, French Polynesia, Okinawa, Vanuatu, Micronesia and the Galapagos archipelago)[8][9] The species has been known to occupy agricultural systems such 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.[2]

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 at about 37–400 meters a year in Seychelles.[8] A survey on Christmas Island, however, yielded an average spreading speed of three meters a day, the equivalent of one kilometre a year.[10]


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]


Mutualistic associations with several species of honeydew-producing scale insects allows the ants to obtain energy-rich carbohydrates. The ants protect the insects by ‘nannying’ the mobile crawler stages and protecting them against their natural enemies.[4] Recent 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.[11]

Impact on Christmas Island

Christmas Island, located in the Indian Ocean, has a unique ecosystem and until the invasion of the crazy ants it was almost untouched. The island has a tropical climate and most of its surface is covered by rainforest. It hosts a unique assemblage of endemic animals and many species of land crabs and seabirds. It is a key breeding point for seabirds in the area and the diversity and abundance of crab species is not matched by any other island in the world.[12]

A. gracilipes was accidentally introduced to Christmas Island between 1915 and 1934.[10] Since the early 1990s it has formed supercolonies and occupied more than 30% of the rainforest on the island.[13] There are three main ways in which the yellow crazy ant has affected the Christmas Island ecosystem. The first is direct predation by the ants, especially of the red ground crab. The great numbers of red ground crabs that cross ant territories during migration disturb A. gracilipes and the ants react by spraying formic acid on the crabs. Due to their high densities, ants are capable of killing red ground crabs within 24 hours and crab population was almost zero in areas occupied by supercolonies.[14] The ants have reportedly killed up to 20 million crabs (approximately 30% of the initial crab population) providing extra protein for the ants, and helping them expand their colonies by establishing nests in the crab burrows. Secondly, by eliminating the red ground crab, a keystone species in the Christmas Island ecosystem, A. gracilipes indirectly alters the rain forest structure and affects the habitat of other organisms. The endemic red ground crab (Gecarcoidea natalis) is the major seed, seedling, and litter consumer in the island’s ecosystem.[15] In areas where the red ground crab was eliminated litter cover was doubled, seedling density 30 times higher, and seedling species richness 3.5 times higher.[14] Thirdly, A.gracilipes favours scale insect populations which reach unusually high densities in areas occupied by supercolonies. While in their naturalized state scale insects live in the canopy and cause little, if any, damage. In large numbers, however,they promote the growth of sooty moulds, and cause canopy dieback and tree death.[14]

In conjunction with the increase in seedling species, due to the disappearance of the red ground crab, this results in holes in the canopy and a change in rain forest structure.

Recent studies indicate a correlation between areas infested by ant supercolonies and reduced population numbers for some endemic species on Christmas Island such as Abbott's Booby, Christmas Island Frigatebird, Christmas Island Gecko, Christmas Island Hawk-Owl, Christmas Island Thrush, and Christmas Island Shrew.[16] There have been reports of ants swarming over grounded birds and animals, but it is not known whether the attack was on an already injured animal. Reports also suggest that A. gracilipes harms many bird species by attacking hatchlings and harassing the adults in their nests.[12] The red ground crab provides “biotic resistance” to invasive species, such as the introduced Giant African Snail (Achatina fulica) and some weed plants. By removing it from the ecosystem, A. gracilipes facilitates secondary invasions.[14] There is also evidence suggesting that the absence of the red ground crab favours other introduced species such as rats and cats.[12] Scale insects also have a dramatic effect on Inocarpus fagifer, which have lower rates of seed production, slower rates of growth, and higher rates of mortality in infested areas.[14]

Other threats

Yellow crazy ants have also been recorded in human communities, where they are seen as agricultural pests, causing outbreaks of sap-sucking insects. They may also cause blindness in humans, especially infants, as people can get formic acid on their hands and then accidentally touch their eyes.[17]

Ants also have a detrimental effect on tourism by threatening endemic species and altering the habitat. This was the case on Bird Island after the ants eliminated the island’s main attraction, the sooty terns (Sterna fuscata).[18] More worrying is the fact that a recent study indicates that A. gracilipes has the potential to inhabit vast areas of continental Australia. By using potential distribution and climate matching researchers concluded that the ant is capable of occupying most of northern and north-eastern Australia.[19]


The main method of dealing with ant invasion is baiting. This requires special considerations. Finding a suitable bait that does not affect other species, is contagious, and lets the worker ants live long enough to carry the poison back to the colony is difficult. Different approaches include the so called “stomach” poisons and the use of pheromones.

Fipronil in a fish-protein base proved to be such a substance for the Christmas Island ecosystem. Ground baiting in 2000 followed by an aerial baiting program in 2002 proved efficient in reducing ant population. While yellow crazy ants are still present in low numbers, and require surveillance, it is hoped that ground baiting will be sufficient.[2] The crazy ant infestation in Cairns and Townsville has been eliminated through a government programme. In New South Wales the yellow crazy ant invasion is listed as a key threat according to the Environment Protection and Biodiversity Conservation Act 1999. In Northeast Arnhem Land, a project is being conducted in collaboration by the Dhimurru Land Management Aboriginal Corporation, the Department of Environment and Heritage, and others. The project began in 2004 and is scheduled to last 4 years. It is the biggest ant eradication project in mainland Australia.[2]


  1. ^ a b "Pests and Diseases Image Library: Anoplolepis gracilipes". 
  2. ^ a b c d "Global Invasive Species Database: Anoplolepis gracilipes". 
  3. ^ a b Abbott, K.L. (2005). "Supercolonies of the invasive yellow crazy ant, Anteplolepis gracilipes, on a oceanic island: Forager patterns, density and biomass". Insectes Sociaux 52 (3): 266–273. doi:10.1007/s00040-005-0800-6. 
  4. ^ a b Ness et al., J.H.; Bronstein, J.L. (2004). "The Effects of Invasive Ants on Prospective ant Mutualists". Biological Invasions 6 (4): 445–461. doi:10.1023/B:BINV.0000041556.88920.dd. 
  5. ^ Green et al. (2004). "The Management and Control of the Invasive Alien Crazy Ant on Christmas Island, Indian Ocean: The Aerial Baiting Campaign September 2002". Unpublished final report to Environment Australia and the Crazy Ant Steering Committee, Monash University: 79. 
  6. ^ Crawley M.J (1997). Plant Ecology. Blackwell Scientific. pp. 736. 
  7. ^ Holoway D.A., David A. (1998). "Effects of Argentine Ant invasions on ground-dwelling arthropods in Northern Californian riparian woodlands". Oecologia 116: 252–258. doi:10.1007/s004420050586. 
  8. ^ a b c d Holoway D.A., David A.; Lach, Lori; Suarez, Andrew V.; Tsutsui, Neil D.; Case, Ted J. (2002). "The causes and consequences of ant invasions". Annual Review of Ecology and Systematics 33: 181–233. doi:10.1146/annurev.ecolsys.33.010802.150444. 
  9. ^ McGlynn T.P., Terrence P. (1999). "The Worldwide Transfer of Ants: Geographical Distribution and Ecological Invasions". Journal of Biogeography 26 (3): 535–548. doi:10.1046/j.1365-2699.1999.00310.x. 
  10. ^ a b c O’Dowd D.J. (1999). "Crazy Ant Attack". Wingspan 9 (2): 7. 
  11. ^ Abbott et al., Kirsti L.; Green, Peter T. (2007). "Collapse of an ant-scale mutualism in a rainforest on Christmas Island". Oikos 116 (7): 1238–1246. doi:10.1111/j.0030-1299.2007.15629.x. 
  12. ^ a b c "Loss of biodiversity and ecosystem integrity following invasion by the Yellow Crazy Ant (Anoplolepis gracilipes) on Christmas Island, Indian Ocean". 
  13. ^ Abbott et al., K. L. (2006). "Spatial dynamics of supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on Christmas Island, Indian Ocean". Diversity and Distributions 12: 101–110. doi:10.1111/j.1366-9516.2006.00193.x. 
  14. ^ a b c d e O’Dowd D.J., Dennis J.; Green, Peter T.; Lake, P. S. (2003). "Invasional 'meltdown' on an oceanic island". Ecology Letters 6 (9): 812–817. doi:10.1046/j.1461-0248.2003.00512.x. 
  15. ^ Green, Peter T., Green P.T. et al.; Lake, P. S.; O'Dowd, Dennis J. (1999). "Monopolization of litter processing by a dominant land crab on a tropical oceanic island". Oecologia 119 (3): 435–444. doi:10.1007/s004420050805. 
  16. ^ Meek, Paul D. (2000). "The decline and current status of the Christmas Island shrew Crocidura attenuata trichura on Christmas Island, Indian Ocean". Australian Mammalogy 22 (1): 43–49. 
  17. ^ "Yellow crazy ants invade Northern Australia". 
  18. ^ Hill, Mike, Hill M. et al.; Holm, Katy; Vel, Terence; Shah, Nirmal Jivan; Matyot, Pat (2003). "Impact of the introduced yellow crazy ant Anoplolepis gracilipes on Bird Island, Seychelles". Biodiversity Conservation 12 (9): 1969–1984. doi:10.1023/A:1024151630204. 
  19. ^ "New South Wales Government: Invasion of the yellow crazy ant - key threatening process listing". 

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