An ancient cultured citrus ant, the Oecophylla smaragdina, also known as the yellow fear ant or yellow citrus ant, has been used in China for almost 1700 years to protect citrus fruit against damage by insect pests. Use of ants as biological control of insects was widespread in the citrus orchards of Sihui, Qingyuan, and Panyu districts around Canton, as well as in Caoching and Gaozhou.
The citrus ant is one member of an ant family characterized by its habit of binding leaves and twigs together with silk to form tight nests in the tree. They feed on various insects that attack the orange, tangerine, lemon, and pomelo trees and their fruit. They do not, however, affect the scale pests or mealy bugs. The mealy bugs were often heavily infested by parasitic wasps.
Whole orchards can be colonized by securing a nest on one tree and then connecting it to adjacent trees with bamboo strips. The strips serve as bridges for the ants to build new nests in nearby trees.
In the 1950s and 1960s, with the advent of powerful organic insecticides, Chinese growers temporarily abandoned the "old-fashioned ant" for modern chemicals. Ant technology was resumed, however, when application costs rose steadily and pests resistant to the chemicals became more common.
A major problem with citrus ants in some areas is that they canot survive winter, unless nests are collected and the ants fed. Nonetheless, it has been reported that in the Jianmei region of Huaan district, ants were able to survive the winter without any intervention from growers, having become a permanent and integral part of the orchard ecosystem.
Interestingly, the ant nests that survived best were those located in the pomelo trees, which gave better protection than orange trees because of their large size and thick foliage. The nests that survived cold weather then provided the critical mass of ants to multiply and build new nests in the orange trees in the spring. Researchers suggested moving ant nests from orange trees to pomelo trees before harvesting oranges, a procedure that has worked well in establishing the citrus ant as the biological control of insect pests in the mixed orange-pomelo orchards in Huaan district.
Chinese researchers conclude that perhaps the "intercropping" of orange trees with another tree that can shelter the ants in winter can be applied to other areas in China as well.