Photo 4. Green weaver ant, Oecophylla smaragdina, pulling leaves together and fastening with larval silk.
Green weaver ant. It is also known as the green tree ant.
Restricted. East and Southeast Asia, Oceania. It is recorded from Australia, Papua New Guinea, and Solomon Islands. Similar species occur in Africa.
Oecophylla ants feed on small insects, and tend aphids, mealybugs and scale insects for their honeydew.
The ants vary in colour depending on locality (Photo 1); some pale yellow, others reddish to yellow-brown. The ants live in trees where they build nests (300-500 mm long), bringing leaves together and fasten them with silk from larvae – silk that is used for construction of cocoons before pupating (Photos 2-5). Even large nests are constructed within 24 hours. Colonies may have hundreds of nests in groups of trees containing more than half a million individuals. After a nuptial flight, queens lay eggs on a leaf and tend the larvae and pupae that develop into workers. A single queen inhabits one nest (in Australia it can be several) and its eggs are distributed to other nests in the colony. Worker ants are ‘major’ (8-10 mm) and ‘minor’ – half that length. Majors forage for food, defend and expand the colony (Photo 1); minors stay within the nests, caring for the young, and tend other insects for honeydew close by. Unfertilised eggs become males.
Oecophila ants do not sting, but can bite, and often spray formic acid on the wound causing irritation or pain.
Communication between these social insects uses pheromone trails, touching and even carrying individuals to where they are needed.
Oecophylla smaragdina is effective in controlling over 50 species of insect pests (e.g., sap-sucking bugs, beetles, caterpillars, thrips, and fruit flies) on many tropical tree crops (e.g., cashew, citrus, cocoa, coconut, lychee, mango, oil palm) and forest trees (e.g., African mahogany, eucalyptus, and hoop pine).
Look for nests of leaves fastened together with silken threads. Look for relatively large, light brown ants that move quickly, and fiercely defend their nests when disturbed.
Oecophila ants have few natural enemies. There are jumping spiders and a butterfly, Liphyra brassolis, in Australia, whose caterpillar eats Oecophylla larvae.
Although there are instances of the effectiveness of the weaver ants controlling pests in a number of economically important crops, these examples are not to suggest it should be introduced to countries where it is absent. A pest risk assessment that considers the economic and environmental consequences of such introductions should always be carried out and decisions made with utmost caution.
USES AS BIOCONTROL AGENTS
Examples of using Oecophylla smaragdina in integrated pest management programs:
ADVANTAGES & DISADVANTAGES
Advantages from using the ants are i) less damage to the trees, ii) higher quality fruit and iii) lower pesticide use. Disadvantages are that scale insects and their relatives are protected from natural enemies, the ants may reduce numbers of pollinating insects, and interrupt seed dispersal by mammals and birds. Minor blemishes on mango fruits have been reported from formic acid burns, and less noticeable anal spots. The major disadvantage from using Oecophylla smaragdine in IPM programs still seems to be their aggressiveness and willingness to bite when farmers are collecting colonies or harvesting fruit.
AUTHOR Grahame Jackson
Information from Antweb. (https://www.antweb.org/description.do?genus=oecophylla&species=smaragdina&rank=species); and Weaver ant. Wikipedia. (https://en.wikipedia.org/wiki/Weaver_ant); and Repelling fruit flies by weaver ants in oranges. Plantwise Factsheet for Farmers. Plantwise Knowledge Bank. (https://www.plantwise.org/KnowledgeBank/FactsheetForFarmers.aspx?pan=20157800369); and from Peng R, Christian K (2007).The effect of the weaver ant, Oecophylla smaragdina (Hymenoptera: Formicidae), on the mango seed weevil, Sternochetus mangiferae (Coleoptera: Curculionidae), in mango orchards in the Northern Territory of Australia. International Journal of Pest Management, 53(1), 15-24. Photo 1 Muhammad Mahdi Karim Red weaver ant, Oecopylla smaragdina in Bagalore, India. (https://en.wikipedia.org/wiki/Weaver_ant#/media/File:Red_Weaver_Ant,_Oecophylla_smaragdina.jpg). Photos 2&3 Howard Ensign Evans, Colorado State University, Bugwood.org. Photo 4 Basile Morin Nest of Oecophylla smaragdina (weaver ants), made of green leaves welded together. (https://commons.wikimedia.org/wiki/File:Nest_of_oecophylla_smaragdina_(weaver_ants).jpg).
Produced with support from the Australian Centre for International Agricultural Research under project HORT/2016/18: Responding to emerging pest and disease threats to horticulture in the Pacific islands, implemented by the University of Queensland and the Pacific Community.
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