Without pollinator insects, many of our crops cannot reproduce and bear fruit. However, it is often not clear which insects these are. This is also the case with cocoa plants. Now researchers have found that ants and flies seem to play a decisive role in their pollination – and not, as previously assumed, bearded flies. It is also beneficial for pollination if the cocoa plantations are surrounded by trees that provide shade.
In order for plants to bear fruit, they must be pollinated. Some plant species can pollinate themselves or the wind carries their pollen to the next plant. But often it is also insects that take on this function. Many of the crops produced worldwide depend on such pollinator insects. That makes the animals essential for our food supply. However, it is not always clear which insect species pollinate which plant.
Who is pollinating the cocoa plants?
Even with cocoa plants (Theobroma cacao), an important global supplier of raw materials, it was not yet certain which insects are responsible for their pollination. That is why scientists around Manuel Toledo-Hernández from the University of Göttingen have now got to the bottom of the question. Based on their research, the research team wanted to find out how the number of cocoa plant pollinators and their performance can be increased. To this end, the researchers carried out two experiments with 42 small-scale cocoa agroforestry operations in central Sulawesi, Indonesia. First they applied a special glue to more than 11,000 blossoms on more than 500 trees over a period of eight months. The flower visitors were temporarily stuck on this and so they could determine their number and type.
In their first experiment, the team investigated on 18 farms what influence the distance between the forest, the agro-frost trees and the farm has on the occurrence of pollinators and what effect the shadows cast by trees. In their second experiment, Toledo-Hernández and his colleagues measured the effect of the leaves lying on the ground on pollinators on 24 other cocoa farms. Your assumption: Large amounts of leaf litter could lead to increased biodiversity on the plantations and thus to a larger number of flower visitors. In their experiment, the leaf litter of the cocoa plants was therefore increased on some areas and removed on others or left in its natural state in order to test this hypothesis.
Ants and flies instead of bearded mosquitoes
The result: The most common pollinators of the examined cocoa plants were not the midges (Ceratopogonidae), blood-sucking bearded gnats, as previously assumed. Instead, Toledo-Hernández and his team caught many two-winged birds such as flies and, most commonly, ants on the flowers of the cocoa plants. “We were surprised that we did not catch any midges, even though they are considered to be the most important pollinators of cocoa,” said Toledo-Hernández. “This shows that the pollinators are more diverse than previously known, but also that there is still a lot to learn.” According to the researchers, the potential of ants as pollinators is not only expressed directly through the transport of pollen. But also indirectly by forcing other pollinators to change flowers quickly when they occupy the flowers.
The experiments also showed which factors influence the occurrence of the pollinator insects. Accordingly, a larger canopy and increased shadows increase the frequency of ants and dipteras, while the distance to the forest had no effect. It was also shown that trees near the plantations can increase the frequency of flies and ants, while the amount of leaf litter under the cocoa trees is irrelevant. “In summary, it can be said that maintaining forests and agroforestries in the vicinity of the farm, maintaining the canopy and a minimum of leaf litter encourage visitors to cocoa blossoms,” concluded the research team. “Therefore, farms with shade trees that are embedded in a biodiversity-friendly landscape are important for the conservation of potential pollinators and pollination services and thus for the promotion of sustainable cocoa cultivation.”
The findings could be helpful for other plantations in order to increase the number and performance of pollinator insects there. “Current global cocoa initiatives should take into account the role of biodiversity-friendly habitats in the conservation of pollinators, because the promotion of pollinators can offer an ecological alternative to combine high yields with nature conservation,” the scientists concluded.
Source: Georg-August-Universität Göttingen, specialist article: Biological Conservation, doi: 10.1016 / j.biocon.2021.109106