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Crops > Fruits & nuts > Mango > Oecophylla smaragdina, Pacific & SE Asia

Crops Fruits & nuts Mango Oecophylla smaragdina, Pacific & SE Asia

Oecophylla smaragdina

November 2004. After the report of a project involving the use of the green ant, Oecophylla smaragdina, in Australia, Vietnam and Thailand, members asked whether this ant was to be promoted elsewhere. A 3-year study funded by the Australian Centre for International Agricultural Research has found the ferocious predators is a cost-efficient way to keep most pests away from the fruit. It was reported to be as cost–effective as chemical control in the mango orchards where it has been trialed in Australia.

For images of the ant, see: for photos.

It was doubted that the project was actually promoting the use of this ant where it was not already established. it was pointed out that it is common in parts of Vietnam, and native to tropical Queensland, Australia (and Asia). Its distribution in Australia is coastal tropical, with some records of it occurring 200-300 km inland. However, if it were to be introduced elsewhere, it should be subjected to ISPM #3 Code of conduct for the release of biological control organisms.

The ant is predatory on insects such as caterpillars, and as such it is beneficial. However, it farms or protects honeydew producers such as aphids, scale insects and mealy bugs. This has an effect on the efficiency of predators and parasites as well as promoting the growth of sooty mould when honeydew production becomes excessive. For example, the insect is a problem on coffee in the highlands of Papua New Guinea, increasing harvesting cost. They also harrass the predators of green scales, Coccus viridis and C celatus.

Oecophylla is a nuisance to orchard works especially at harvest time, and to anyone working or walking near trees, including bushwalkers. These ants aggressively defend “nests” and trees and, whilst they do not sting, chemicals in the saliva can irriatate the skin. They occur in large numbers, and are very good at working themselves under clothing to bite. Nestscolonies occupy relatively large areas. This may be 20 trees or more in an orchard. A single cluster of leaves silked together is not an independant nest, but simply a part of a nest. These are somewhat dynamic in that they may grow, or move when leaves die. They also often house honeydew producers such as scale insects that also affect the dynamics.

Hawaii has already considered this ant as an unwelcome introduction, should it occur. This is what was said:

The Asian arboreal weaver ant [Oecophylla smaragdina (Fab.)] is widely distributed from Asia to Australia, where it occupies a wide range of forest habitats from savanna and monsoon dry forests to more mesic habitats and rain forests (Hölldobler and Wilson, 1994). A closely related species lives in Africa. Weaver ants use their larvae as spindles to weave nests in the canopy, and their ability to select an optimal environment within the canopy for their nests gives the group a wide tolerance for different forest types. Given the Asian weaver ant’s known distribution and preferred environments, it would be able to invade all forested habitats in Hawai’i except perhaps the wettest and coldest rain forests.

The ant is a voracious arboreal predator, which can exclude all sensitive animals from its nest tree as well as closely neighboring trees. Colonies can contain 500,000 or more workers, and control a territory of a dozen or more large trees (Hölldobler and Wilson, 1994). They control the entire tree surface from the ground up and kill virtually all animals found within their territory (Hölldobler and Wilson, 1994). Native forest birds would be naïve to such as competitor and probably would be unable to nest or forage near an active ant nest. Both native invertebrates and several native forest bird species, as well as the endangered tree roosting native bat, would be severely affected, and the extinction of many currently listed species as well as many currently non-endangered species would be expected if this species established in Hawai’i.

The Asian weaver ant is often considered beneficial by farmers, who have lionized the ants and introduced them to their orchards for pest control for centuries (Way and Khoo, 1992). The species has been introduced to south Pacific islands for biocontrol of palm pests (Greenslade, 1965). However, its effects on either the intended target or potential nontargets have not been recorded. It could be introduced into Hawai’i illegally by well intentioned gardeners returning from Asia. Less likely is the possibility that fertile queens could arrive as stowaways in aircraft or in shipments of cut flowers or other plant material. Their exceptionally complex behavior makes weaver ants popular research animals. The related African species is established in entomological laboratories in the continental U.S. (Hölldobler and Wilson, 1994) and could be moved to Hawai’i. Hölldobler and Wilson (1994) describe a method to transport small colonies within hand luggage on aircraft.

Greenslade PJM (1965) Promecotheca opacicollis Gestro (Coleoptera: Chrysomelidae) on the island of Tikopia. Pacific Insects 7:661 664.
Hölldobler B, EO Wilson (1994) Journey to the ants. Harvard University Press, Cambridge, MA.
Way MJ, Khoo KC (1992) Role of ants in pest management. Annual Review of Entomology 37:479 503.

See also: Paul Van Mele (former CABI) and Nguyen Thi Thu Cuc, Professor of Entomology, Can Tho University, Vietnam, Ants as Friends, Improving Your Tree Crops with Weaver Ants. This is a basic book intended for extensionists and farmers useful for background information.

Actually, Oecophylla smaragdina was not “introduced to south Pacific islands for biocontrol of palm pests (Greenslade 1965). What Greenslade and several others did was to try to exploit an existing situation. That is, Oecophylla smaragdina, is endemic to Solomon Islands and several other Pacific countries, but researchers had noted in the 1930s that where it was present, coconuts yielded well. Research focussed on trying to replace some species of ant already on a crop with Oecophylla smaragdina so that it could perform its “beneficial acts”. More recently (c. late 1960’s) a new ant was accidentally introduced, Wasmania auropunctata, which has spread throughout the country and has replaced Oecophylla smaragdina and several other ant species. Unfortunately, this is a mixed blessing: while it does control some pests, it is also a fire ant and has an irritating sting. It may also have an effect on other beneficial and non-pest organisms.