Anoplolepis gracilipes, known most commonly as the Yellow Crazy Ant (YCA), is a relatively large, yellow to orange colored ant with long legs, large eyes and extremely long antennal scapes. Considered by the IUCN/SSC Invasive Species Specialist Group (ISSG) to be among the 100 worst invasive species in the world, Anoplolepis 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 (O'Dowd et al. 2003). Although widespread across the Pacific, Anoplolepis gracilipes can cause significant damage to native biological diversity, and strong quarantine measures are encouraged to keep it from spreading to new localities.
Diagnosis of worker among Antkey species. Worker caste monomorphic. Antenna 11-segmented. Antennal club indistinct. Antennal scape length greater than 1.5x head length. Eyes large; break outline of head. Antennal sockets and posterior clypeal margin separated by a distance equal to or greater than the minimum width of antennal scape. Dorsum of mesosoma with metanotal impression, but never with a deep and broad concavity. Metapleuron with a distinct gland orifice. Propodeum and petiolar node both lacking a pair of short teeth. Propodeum lacking posteriorly projecting protrusion. Propodeal declivity less than twice length of propodeal dorsum. Waist 1-segmented. Petiole upright and not appearing flattened. Gaster armed with acidopore. Distinct constriction not visible between abdominal segments 3+4. Hairs not long thick and produced in pairs. Yellowish-brown to reddish-brown.
Anoplolepis gracilipes is separated from Anoplolepis custodiens by the longer antennal scapes and by the eyes that break the outline of the head. The only other species included here with such long antennal scapes is Paratrechina longicornis, but Anoplolepis gracilipes is easily distinguished by its yellowish to reddish brown color (versus dark brown to black).
Anoplolepis custodiens, Paratrechina longicornis
From Wetterer (2005)
In tropical Asia and tropical islands of the Indian and Pacific Oceans, A. gracilipes occurs throughout the moist lowlands, but is not commonly found in arid regions and sites above 1200 m elevation. In tropical Africa, it is known only from Dar es Salaam and nearby Zanzibar. In tropical Australia, A. gracilipes has been recorded primarily from moist monsoon rainforests along perennial springs and streams in the northern region and in a few towns on the north and east coasts. In the Neotropics, there are records of A. gracilipes from western Mexico.
In subtropical Asia, A. gracilipes ranges up to 26-27oN in northern India, southern China, and southern islands of Japan. I found only six records from latitudes >27o, two from exterminated urban populations (Auckland, New Zealand; Brisbane, Australia) and three from probably temporary populations (Valparaíso, Chile; Durban, South Africa; Zayul, Tibet). The sixth population, on Amami-Oshima Island, Japan, may or may not be temporary.
Anoplolepis gracilipes is not yet known from many moist lowland tropical areas where it would probably thrive, including west-central Africa and much of the Neotropics. Populations in western Mexico are prevented from expanding eastward by a central mountain range, but may be able to spread south, around the mountains, to the Caribbean, Central America, and South America. Records from arid Baja California, Mexico indicate that A. gracilipes can invade and persist in areas with arid climates, perhaps due to moderating effects of irrigation.