The yellow crazy ant Anoplolepis gracilipes is thought to be indigenous to West Africa, but has invaded most of south and south-east Asia, as well as East Africa and the Indo-Pacific islands. It is also known as the long-legged ant, and often referred to as A. longipes. Its names reflect its yellowish colour, its habit of running around frantically in different directions – especially when disturbed – its slender (gracile) body, and long legs and antennae.
Promotes scale insects and sooty mould
The yellow crazy ant was primarily spread in cargo transported around the world, but in some cases it was deliberately introduced as a biocontrol agent of insect pests in coconut, cocoa and coffee plantations. It soon became an agricultural pest itself, however, because of its mutualistic relationship with Homoptera. In exchange for their sugary honeydew secretions, the ant protects scale insects from predators such as wasps and spiders, and also distributes the young to new host plants, so that they can form new populations. At high densities the sap-sucking bugs weaken plants, and this is compounded by sooty mould that colonises the honeydew, causing canopy dieback or even the death of the tree.
The ant may also undermine crops such as sugarcane and coffee by nesting at the base of plants. Furthermore, although it does not bite or sting, it defends itself by spraying formic acid, which causes skin burns and irritates the eyes of farm workers. The ant also subdues and kills prey in this way, aggressively attacking anything from small insects, spiders, crustaceans and molluscs to large land crabs, birds, mammals, and reptiles. By decimating endemic keystone species it can drastically alter community structure and species composition, and affect important ecosystem processes.
For example, on Christmas Island, an Australian territory bordering the Indonesian archipelago, yellow crazy ants have drastically reduced the population of the red land crab, killing an estimated three million crabs over an 18 month period. The crab is the dominant endemic consumer on the rainforest floor, foraging on seedlings and fallen leaves. Its decline is causing a fundamental shift in the structure of the forest ecosystem, due to increased seedling recruitment, enhanced species richness and slower breakdown of leaf litter. Furthermore, in parts of the island where the ants occur, more than 90% of trees and shrubs are infested with sooty mould, resulting in extensive canopy dieback. The ants also disrupt the reproduction of a variety of reptiles, birds and mammals on the forest floor and canopy.
The yellow crazy ant forms huge super-colonies, extending over several hectares and comprising as many as 300 queens and up to 36 000 workers. Control is best achieved by scattering protein bait pellets – made of fishmeal and laced with an appropriate insecticide – in the vicinity of the nest. Foraging ants take the bait back to the nest, where it is shared amongst the workers, larvae and queens. Since the queens are the only ants able to reproduce, killing them accelerates the eradication of the colony.
Reference: Sue Matthews Tropical Asia invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004