Yellow crazy ants (Anoplolepis gracilipes) are a highly destructive invasive species. Named for their erratic, quick movements when disturbed, they are a recognized environmental threat, often listed among the planet’s most problematic invasive species.
Origin and Global Dispersal
Their precise native range is not definitively established, though evidence suggests origins in Southeast Asia or the Indian Ocean region. Their global spread has largely been facilitated by human activities. They are frequently transported accidentally through international trade and shipping, often hitchhiking in sea cargo, timber, or potted plants. This human-assisted dispersal has allowed them to establish populations far beyond their original habitat.
Major Invasive Hotspots
Yellow crazy ants are a significant invasive problem across numerous tropical and subtropical regions. Christmas Island, an Australian territory, represents a severe hotspot where they form extensive “supercolonies” leading to widespread ecological disruption.
In mainland Australia, infestations are prevalent in Queensland, especially within the Wet Tropics World Heritage Area (Cairns and Townsville), and other coastal and industrial areas (Maryborough and Brisbane). They are also present in north-eastern Arnhem Land, Northern Territory. A single infestation in New South Wales has since been eradicated.
Pacific Islands have experienced widespread invasions, including Hawaii, French Polynesia, and New Caledonia. Other affected island groups include:
Micronesia: Caroline, Gilbert, Mariana, Marshall, Palau, Rotuma, Santa Cruz Islands.
Melanesia: Fiji, Solomon Islands, Tokelau, Vanuatu.
Polynesia: Austral, Cook, Line, Marquesas, Niue, Samoa, Society, Tonga, Tuvalu, Wallis and Futuna Islands, and Johnston Atoll.
In the Indian Ocean, invasions include:
Seychelles, Madagascar, Mauritius, Réunion, Cocos (Keeling) Islands, Rodrigues, Agalega.
Africa: Tanzania (Dar es Salaam and Zanzibar), South Africa.
Asia: Japan (Amama Oshima, Bonin, Okinawa, Minami-Daito islands), Papua New Guinea, Indonesia, Malaysia (Borneo), India.
Caribbean islands and parts of Central and South America also report their presence.
Consequences of Invasion
Yellow crazy ants cause significant ecological and economic impacts in invaded regions. Ecologically, they displace native ant species and form massive “supercolonies” (millions of ants, hundreds of queens), altering ecosystem dynamics.
On Christmas Island, they have drastically reduced red land crab populations, killing millions by spraying formic acid into their eyes and leg joints. This loss of a keystone species disrupts the forest floor, leading to an accumulation of leaf litter and changes in vegetation composition.
They prey on native wildlife, including invertebrates, reptiles, small mammals, and bird chicks. Their mutualistic relationship with sap-sucking scale insects, which they “farm” for honeydew, causes further damage. By protecting these insects, ants lead to outbreaks that stress plants, promote sooty mold growth, and can cause forest dieback.
Economically, they threaten agriculture by tending sap-sucking pests, reducing crop productivity in sugar cane, coffee, coconut, and fruit trees. They can also harm livestock, such as chickens and pigs. Beyond environmental and agricultural damage, they become a nuisance to human communities, invading homes and electrical equipment. Their defensive formic acid spray can cause skin and eye irritation in humans and temporary blindness in animals.
Controlling Invasive Populations
Controlling yellow crazy ant populations involves ongoing efforts and considerable challenges. Baiting programs are a primary strategy, utilizing insecticides like fipronil in granular baits. Research also explores other baits, such as those containing boric acid. Habitat management techniques are also employed to make areas less hospitable.
Biological control methods are investigated and implemented; for instance, a micro-wasp has been introduced on Christmas Island to manage populations. Despite these efforts, eradication is difficult and often a long-term endeavor, particularly in large or remote infested areas. Consistent funding and collaborative efforts among governments, communities, and researchers are important for mitigating impacts and preventing further spread.