The Riddle of the Cocoa Tree — ScienceDaily
Cocoa has long been a sought-after raw material for the global food industry. At first glance, therefore, it seems surprising that little biology knows about the pollination of the cocoa tree – although it is precisely this process that is the basis of fruit set and ultimately yield.
At second glance, however, we quickly understand why the pollination of this tropical crop conceals so many secrets: the cocoa flowers are very small and are generally found in the thousands on a tree. The insects that congregate on the flowers are also tiny and very diverse in terms of species. All of these factors make systematic observations difficult.
Study in Northern and Southern Peru
A new study now provides more clarity. It was conducted in Peru by an international research team from the Chair of Animal Ecology and Tropical Biology at Julius-Maximilians-Universität (JMU) Würzburg, Germany. The lead organization for the project was Bioversity International, the Deutsche Gesellschaft für internationale Zusammenarbeit (GIZ) funded the project.
South America is the region of origin of the cocoa tree, which is present there in the undergrowth of the humid tropical forests. In agriculture too, the cocoa tree is planted in the shade of taller trees, in so-called agroforestry systems. Researchers applied glue to cocoa flowers in 20 such systems in northern and southern Peru to determine which animals visit the flowers. They also analyzed the influence of the degree of shading and the distance to the nearest forest on the activity of visitors to the flowers.
The results were published in the journal Ecological solutions and proofs. The first author is biologist Justine Vansynghel, who has been a PhD student at JMU under the supervision of Professor Ingolf Steffan-Dewenter since 2018.
A wide variety of insects frolic on the flowers
In the cocoa agroforests of dry northern Peru, aphids (38%), ants (13%) and thrips (10%) were the most frequent flower visitors. In the more humid south, on the other hand, thrips (65%), midges (14%) and parasitic wasps (10%) are the most frequent.
In the north, the researchers counted all the more insects on the cocoa tree flowers as the plantations were more shaded. In the south, on the other hand, the insects preferred to stay in plantations with less shade, at least during the rainy season in which the study took place. Distance to the nearest forest played no role in the extent of flower attendance in either region.
Pollen transfer and fruit set remain poor
Justine Vansynghel’s team also observed that only 2% of pollinated cocoa flowers bear fruit. Hand transfer of pollen tripled fruit set to seven percent, which is still very low.
The doctoral student can only speculate on the factors that limit fruit set. One of the reasons could be that there are simply no effective pollinators for cocoa in Peru. This is suggested by the fact that only very few pollen grains were counted on most cocoa flowers, an average of 30. Four times that amount would be needed for successful pollination, according to the literature. Another reason for low cocoa fruit set could be that individual cocoa plants are genetically incompatible with each other.
Many questions remain to be clarified
There are therefore still significant gaps in our knowledge of cocoa biology. “Among other things, it would be important to identify the main pollinators,” explains Justine Vansynghel. Then, it would also be possible to develop higher-yielding agroforestry systems and improved management strategies in Peru’s cocoa-producing regions.
Why are cocoa yields in Peru much worse than in Africa or Asia? “In Indonesia, you can get about 50% fruiting with hand pollination. This is probably because the plantations there are not using the indigenous South American cocoa clones, but higher yielding clones,” explains the JMU researcher.
On the other hand, cocoa plantations in Africa and Asia are threatened by so many diseases and pests that large monocultures can be wiped out all at once. Another problem with high-yielding non-native clones is that they only produce good crops for five to ten years. After that, the old plantations are abandoned and the existing forest is cut for new plantations.
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Material provided by University of Würzburg. Original written by Robert Emmerich. Note: Content may be edited for style and length.