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Diversified cropping systems to improve the natural regulation of mealybug vectors

CSSV is a vector-borne disease transmitted by small insects called mealybugs, which feed by piercing the cacao tree. These insects produce a sugar-rich honeydew that attracts ants, leading them to protect the colonies. Mealybugs are indeed preyed upon by a wide range of natural enemies, particularly ladybugs, parasitoid micro-Hymenoptera, and micro-dipterans known as gall midges. Our work has shown that cacao plantations managed in agroforestry systems have more species-rich and effective natural enemy communities in regulating CSSV-carrying mealybugs compared to less diverse plantations. Furthermore, natural mealybug regulation is most effective in the youngest and most diverse agroforestry systems. In aging systems, dominant and aggressive ant species establish themselves permanently and protect the scale insect vectors from their natural enemies, contributing to the proliferation of vectors.

Reducing the severity of CSSV symptoms through shading of crops

A study conducted in Ghana showed that agroforestry systems with a shade rate of approximately 50%, instead of the generally recommended 30-40%, could significantly reduce the severity of CSSV symptoms while maintaining satisfactory yields. Work carried out in Ghana and Côte d'Ivoire confirmed that disease outbreaks were more frequently observed in areas of plantations exposed to direct sunlight and that cocoa trees located under shade trees exhibited fewer symptoms. While the mechanisms involved in the effect of shade on CSSV are not fully understood, it is likely that the stress on cocoa trees exposed to high light levels and low soil moisture reduces their ability to resist the disease.

Limiting the spread of CSSV and protecting new plantings through barrier crops

The CSSV epidemic is primarily linked to the movement of the mealybug vectors. While the mealybugs' active dispersal is limited to a few meters, it is responsible for the disease spreading from an infected cocoa tree to its nearest neighbours, a phenomenon known as "oil staining." However, the mealybugs can also be carried by the wind over short distances, creating new outbreaks just a few dozen meters from the initial one. This observation quickly led researchers to consider the use of buffer strips or barrier crops to stop the spread of the disease by mealybugs. Within the BarCo and Cocoa4Future projects, we tested 10-meter-wide hedges of coffee and Acacia Mangium trees around new cocoa plantations established within disease hotspots. After six years of monitoring the health of the plots, our results show that barrier crops did not prevent mealybugs from entering the fields, but they did slow the disease's entry. Due to its rapid growth, Acacia Mangium formed an effective windbreak in less than a year, but it required significant maintenance. Coffee barriers also performed well, but they may require special attention because they host large colonies of ants that can transport infectious mealybugs over short distances. Consultations with producers revealed the crucial need to consider their socio-economic and cultural interests when selecting barrier plants.

Managing CSSV disease at the agricultural landscape scale

In Côte d’Ivoire and Ghana, entire regions appear to be resistant to CSSV, despite cocoa cultivation there for decades. In Côte d’Ivoire, in particular, these regions are often characterized by more diverse agricultural landscapes, where cocoa plantations are frequently isolated from one another by other crops. Based on this observation, we conducted an analysis of the effect of the landscape environment of the plantations on the incidence of the disease. Adapting an epidemiological model (Landsepi) based on incidence data from the ECAM cooperative in Méagui (southwest Côte d’Ivoire) allowed us to simulate the progression of CSSV in a more or less diverse landscape. Our results clearly show that the proportion of cocoa and the landscape configuration, via the degree of host aggregation, are potential levers for regulating the spread of the virus. Based on this observation, a prototype serious game was developed during a workshop at the cooperative, with the aim of exploring the feasibility of collective disease management based on crop diversification at the landscape scale. This approach, at the interface of modelling, agroecology, and participatory science, and designed to support more sustainable cocoa farming systems, was recognized with a nomination to the 2025 Octaave Key Challenge call for projects within the TACTIC project (Agroecological Transition through Territorial Coordination and Collective Innovation for the management of CSSV in Côte d'Ivoire). This nomination will allow for the continued development of the prototype, in order to assess the extent to which producers and cooperatives perceive the landscape as a relevant management unit and to test their capacity to develop coordinated strategies, under economic constraints, that reconcile individual profit with collective resilience.

Recommendations for combating CSSV disease

Whether deployed at the plantation level, in its immediate surroundings, or across the agricultural landscape, plant diversification must be the cornerstone of programs to combat cocoa mites. Developed collaboratively, taking into account the opinions of producers, and supported by strengthened technical capacities and financial incentives implemented by the government, these programs can be structured around the following recommendations:

1. Renew and diversify cocoa farming systems by incorporating trees to limit the proliferation of mealybug vectors by restoring the ecological balance necessary for the regulation of these insects by their natural enemies. It is also important to target certain prolific tree-roosting ant species, such as Crematogaster (black ants), which protect mealybugs, by manually destroying their nests in large trees and removing dead wood that may harbor them.
2. Adopt higher densities of shade trees (50% shade coverage) than usual recommendations in regions particularly affected by the disease to mitigate its impact on cocoa trees. Generally, even shade over cocoa trees helps protect them from water stress, which makes them more susceptible to the disease.
3. Integrate the use of barrier crops into plantation rehabilitation strategies for combating CSSV. Coffee trees and Acacia Mangium are good barrier crops, but the development of multi-species, multi-functional hedgerows adapted to the socio-economic and cultural needs of producers should be prioritized.
4. Develop collective CSSV management strategies based on crop diversification at the landscape level. To achieve this, it is essential to encourage cooperatives to strengthen monitoring of affected areas and to support producers in diversifying their crops and land management, by implementing strategies that balance individual profitability and collective resilience.