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Guardians of the mango May 3rd, 2015 by

I have read about weaver ants, but was not quite prepared to see the gleaming, reddish-gold insects nesting in their large mango trees in Benin, West Africa. A colony can have millions of ants, and they are highly territorial, taking over the canopy of a towering tree, or several, if they can find a bridge between them.

Part of the beauty of a mango tree is its young leaves, which emerge a different color from the older ones. These trees in Benin were splashed with patches of sea green leaves that had just emerged. And here and there on the lower branches, Paul pointed out to me where the weaver ants had pulled several of these fresh leaves together and clothed them with silk, making little purse-like chambers, which housed a troop of ants.

Other types of ants dig tunnels underground from one chamber to the next. The leafy chambers of the weaver ants are connected by invisible paths along the tree branches. I’ve seen ants in trees, but never in such perfect adaptation.

Like other ants, the weaver ant workers are all females, a family of sisters that wards off all invaders, biting humans that climb the trees, but also eating or frightening away any fruit flies—which can devastate the mango crop. Fruit flies lay eggs just under the fruit’s skin. The maggots hatch there and the tree aborts the damaged fruit, littering the ground below it with rotting mangos.

Simply tolerating the ants is a free way of managing fruit flies. Farmers can do simple tricks to help the ants, e.g. making “ant bridges” between trees with ropes or sticks that connect ant trees to neighboring ones, allowing the creatures to spread.

Florence Anato and her colleagues, experts on ants and fruit flies, had written a fact sheet for farmers on weaver ants. I stood in the grove while Florence asked mango grower Pierre Denjamin to read the draft fact sheet and comment on it. He read the whole page through and said that here people say that ants are the guardians of the mango tree. He already knew a lot about the ants, but after reading about them said that now he would do more to protect them.

The three authors of the fact sheet each invited three farmers to review it. They mentioned some intriguing local knowledge: when different colonies of weaver ants meet in the tree tops, the ants fight, and their battles leave black spots on the fruit, which farmers do not like. Florence and her colleagues knew about the black spots, but did not realize that farmers found them so annoying. They edited the fact sheet to say that if neighboring colonies were meeting and fighting, farmers could cut the branches that connected the rivals, to keep them from fighting. Without fights between colonies the problem of the black spots caused by formic acid would by and large be resolved.

It always helps the authors of fact sheets or other training materials to meet more often with their audience.

Further reading: Ants as Friends. By Paul Van Mele and Nguyen Thi Thu Cuc.

Peng, R. and Christian, K. (2013) Do weaver ant (Hymenoptera: Formicidae) marks affect mango internal quality and storage life? Journal of Economic Entomology 106(1), 299-304.

Van Mele, P., Camara, K. and Vayssières, J.F. 2009. Thieves, bats and fruit flies: Local ecological knowledge on the weaver ant Oecophylla longinoda in relation to three ‘invisible’ intruders in orchards in Guinea. International Journal of Pest Management, 55(1), 57-61. Download here.

Related blog story: Sugar sweet ants

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