Tree Spatial Structure, Host Composition and Resource Availability Influence Mirid Density or Black Pod Prevalence in Cacao Agroforests in Cameroon

Abstract

Combining crop plants with other plant species in agro-ecosystems is one way to enhance ecological pest and disease regulation mechanisms. Resource availability and microclimatic variation mechanisms affect processes related to pest and pathogen life cycles. These mechanisms are supported both by empirical research and by epidemiological models, yet their relative importance in a real complex agro-ecosystem is still not known. Our aim was thus to assess the independent effects and the relative importance of different variables related to resource availability and microclimatic variation that explain pest and disease occurrence at the plot scale in real complex agro-ecosystems. The study was conducted in cacao (Theobroma cacao) agroforests in Cameroon, where cocoa production is mainly impacted by the mirid bug, Sahlbergella singularis, and black pod disease, caused by Phytophthora megakarya. Vegetation composition and spatial structure, resource availability and pest and disease occurrence were characterized in 20 real agroforest plots. Hierarchical partitioning was used to identify the causal variables that explain mirid density and black pod prevalence. The results of this study show that cacao agroforests can be differentiated on the basis of vegetation composition and spatial structure. This original approach revealed that mirid density decreased when a minimum number of randomly distributed forest trees were present compared with the aggregated distribution of forest trees, or when forest tree density was low. Moreover, a decrease in mirid density was also related to decreased availability of sensitive tissue, independently of the effect of forest tree structure. Contrary to expectations, black pod prevalence decreased with increasing cacao tree abundance. By revealing the effects of vegetation composition and spatial structure on mirids and black pod, this study opens new perspectives for the joint agro-ecological management of cacao pests and diseases at the plot scale, through the optimization of the spatial structure and composition of the vegetation.

Tree spacing impacts the individual incidence of Moniliophthora roreri disease in cacao agroforests

“…This difference may have been attributable to the position of those trees in the vertical vegetation stratum which defines different shade intensity, and the scale on which the two plant populations had an impact on the disease. Forest trees are the highest vertical vegetation stratum . Moreover, they have a larger and more porous canopy than fruit trees, thus providing a poorer barrier at cacao level and more uniform shade than fruit trees.…”

“…Only a few studies have investigated how the spatial organization of diverse plants (hosts and non‐hosts combined) alters pest and disease incidence within agroforest plots …”

“…The spatial organization of the plant community also alters the microclimate (light, humidity, wind), which may influence the epidemiology of pests and diseases . These processes are particularly important in agroforestry systems in which the spatial structure may be involved in pest and disease regulation . Tropical agroforestry systems are often multi‐species and multi‐strata cropping systems, where the commercial crops are located in the understory, and associated with a diversity of companion trees presenting a set of spatial organizations .…”

“…18 These processes are particularly important in agroforestry systems in which the spatial structure may be involved in pest and disease regulation. 10,11,18 Tropical agroforestry systems are often multi-species and multi-strata cropping systems, where the commercial crops are located in the understory, and associated with a diversity of companion trees presenting a set of spatial organizations. 12 The composition and the spatial organization of trees in the higher vertical stratum (associated shade trees) modify microclimatic variables and the quantity and the quality of transmitted light available in the lower stratum.…”

Using natural areas and empowering women to buffer food security and nutrition from climate shocks: Evidence from Ghana, Zambia, and Bangladesh

“…Many forests and grasslands that are habitat for pollinator species are also habitat for species that provide pest regulation. Forests in particular have been found to contribute to pest regulation from Cameroon to Costa Rica (Gidoin et al, 2014;Karp et al, 2013). Additionally, forests and other natural areas contribute to water regulation, often retaining soil moisture during dry periods (De Groot et al, 2002;Wang et al, 2013), or facilitating water runoff and decreasing flooding during wet periods (Keesstra et al, 2018).…”