Another handicap is added to the world supply of chocolate. It is the cocoa swollen shoot virus disease (CSSVD) and it is “a real threat,” says Benito Chen-Charpentier, professor of mathematics at the University of Texas at Arlington (USA), in the study “Cacao sustainability: The case of cocoa swollen-shoot virus co-infection”, which has just been published in the journal PLOS ONE.  

According to this report, around 50% of the world’s chocolate comes from cocoa trees in the Ivory Coast and Ghana. The harmful virus is attacking Ghana’s cocoa trees, causing crop losses of between 15% and 50%. Spread by small insects called mealybugs that eat the leaves, buds, and flowers of trees, this virus is very harmful to the base ingredient of chocolate.

“Pesticides do not work well against mealybug, forcing farmers to try to prevent the spread of the disease by cutting down infected trees and planting resistant specimens. But, despite these efforts, Ghana has already lost more than 254 million cocoa trees in recent years,” he adds.

Farmers can combat mealybugs by administering vaccines to trees to inoculate them with the virus. But these vaccines are expensive and vaccinated trees produce a smaller cocoa crop. So Chen-Charpentier and her colleagues at the University of Kansas, Prairie View A&M, the University of South Florida and the Ghana Cocoa Research Institute have developed a strategy: use mathematical data to determine how far apart vaccinated trees can be planted to prevent the insects from jumping from one tree to another, spreading the virus.

“Mealybugs move in different ways: from canopy to canopy, carried by ants, or carried by the wind. What we needed was to create a model for cocoa farmers, so they could know how far away they could safely plant the vaccinated trees from unvaccinated ones, in order to prevent the spread of the virus while maintaining affordable costs.”

Experimenting with mathematical pattern-making techniques, the team created two different types of models that are still experimental. Models that allow farmers to create a protective layer of vaccinated cocoa trees around unvaccinated trees.