Cacao pollination services are driven by pollen deposition, shade management and forest proximity

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Justine Vansynghel, Emily A. Martin-Poppenborg, Nils-Christian Schumacher and Ingolf Steffan-Dewenter, Department of Animal Ecology and Tropical Biology, University of Würzburg, Würzburg, Germany, Carolina Ocampo-Ariza, Bea Maas and Teja Tscharntke, Agroecology, University of Göttingen, Göttingen, Germany, Bea Maas, Department of Botany and Biodiversity Research, University of Vienna, Vienna, Austria, Emily A. Martin-Poppenborg, Zoological biodiversity, Leibniz University Hannover, Hannover, Evert Thomas, Bioversity International, Carlos Ulloque-Samatelo, Universidad de Piura, Piura, Peru

Background/Question/Methods
Pollination ecology of the Amazonian chocolate tree (Theobroma cacao) is poorly understood, even though the crop relies on pollination services for its production. Across the tropics, livelihoods of over 6 million smallholder farmers are sustained by its production, both in- and outside of its native range. Several drivers of pollination services, such as the contribution of different arthropod groups, the role of pollen quantity as well as the effects of landscape and management factors on flower visitation are understudied, particularly in regions of origin of cacao. With this three-part study, we aim to improve understanding of these interrelated drivers of cacao pollination services in its center of origin. We sampled flower visitors with insect sticky glue in 20 cacao agroforest in Northern and Southern Peru, across canopy closure and proximity to forest gradients. We also related pollen quantities to fruit set success and compared fruit set rates between manually and naturally pollinated flowers.
Results/Conclusions
The most abundant visitors were aphids (38%), ants (13%) and thrips (10%) in Northern and thrips (49%), Cecidomyiid and Ceratopogonid midges (11%) and parasitoid wasps (8%) in Southern Peru. This large diversity of flower visitors and dominance of herbivores raises questions about the main pollinators of cacao. Visitation rates ranged between 0.18 to 0.31 visitors/flower in the North and 0.19 to 0.70 visitors/flower in the South with regionally different effects of canopy closure and distance from forest. This indicates a need for climate-zone-related landscape management strategies to maximize flower visitation. A threshold of > 115 pollen grains was necessary for fruit set, and hand-pollination of flowers increased fruit production. This highlights a potential source of low natural fruit set, if flower visitors are unable to transport such a pollen load. Therefore, we propose hand pollination as a promising alternative to overcome current cacao productivity limitations until an improved management of native pollination services is achieved.