Ed Yong in Aeon Magazine:
A few degrees north of the equator, in the hot, humid rainforests of Ghana, two groups of farmers are vying for dominance over the world’s most productive chocolate-growing region. Chocolate is made from cocoa beans found in the large, rugby-ball-shaped pods of cocoa trees. People have been planting these trees in Ghana since the late 19th century and the crop is a mainstay of the country’s economy. But the trees have recently proved an inviting target for a wily group of rival agriculturalists, whose practices threaten the long-term survival of cocoa in Ghana. The trouble is, these competitors aren’t humans. They’re ants.
Ants have been farming for millions of years longer than humans. These particular ants herd mealybugs — small, sap-sucking insects that look like woodlice dipped in flour. The ants shepherd and protect the mealybugs so they can ‘milk’ the sugary nutritious fluids in their waste. The bugs used to drink primarily from local rainforest trees, but when humans started clearing the forest to make way for cocoa, the ants adapted, by driving their livestock into the fresh cocoa pastures.
This strategy shift entangled the cocoa trees in a web of pests and pestilence. When mealybugs drink from trees, they inject them with a pathogen called cacao swollen shoot virus (CSSV). In local rainforest trees, the effects of CSSV are mild, but cocoa — a newcomer to these forests — hasn’t had a chance to evolve countermeasures. As a result, the virus pummels the trees, swelling their shoots and roots well beyond their usual size while draining the colour from their leaves. Before long, often only a few years, the trees die.