The yellow crazy ant is a global invader that uses formic acid to blind crabs.

Nature · Published 3 October 2026 Swipe more like this →

In short: Named for their erratic movements, yellow crazy ants are among the world's most destructive invasive species. By forming massive supercolonies, they have devastated ecosystems from Hawaii to the Indian Ocean, where they blind and kill millions of land crabs, fundamentally altering the structure of entire rainforest landscapes.

The yellow crazy ant, Anoplolepis gracilipes, is a large, long-legged species native to Southeast Asia. While they do not bite or sting, they defend themselves and subdue prey by spraying formic acid. On Christmas Island, this chemical warfare has caused an ecological meltdown, killing an estimated 15 to 20 million land crabs. These crabs are keystone species that turn over soil and control plant growth; their removal has allowed weeds and stinging trees to flourish, while scale insects—which the ants farm for honeydew—have multiplied and killed mature forest trees.

The ants' success as a 'tramp species' stems from their aggressive nature toward other ants and their ability to form supercolonies. A 2023 study revealed a bizarre biological mechanism: male ants are obligate chimeras. When an egg is fertilized by a specific type of sperm, the parental nuclei divide separately within the same egg, creating a male with two distinct sets of genetic material. This unique reproductive strategy, combined with generalized nesting habits that allow them to hitchhike on human transport, makes them incredibly difficult to eradicate.

Control efforts have proven complex. On Christmas Island, aerial baiting with fipronil successfully reduced ant densities by 99%, but the effects are temporary. Researchers have turned to biological controls, such as introducing a wasp that preys on the scale insects the ants rely on for sugar. However, such methods may not work on mainland Australia, where the ants have access to a wider variety of nectar and honeydew sources. With potential economic costs estimated at over A$3 billion, the search for long-term solutions remains a critical priority.

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