The cacao flower is about the size of your fingertip and grows directly from the trunk and main branches of the tree from multiple delicate flowering cushions. If successfully pollinated, they grow into cacao pods which range in size from roughly 10-30cm (4-12 inches), that is anything from an avocado up to an American football.
The flowers are so small and delicate, the pollen so difficult to get to, that insects more commonly associated with pollination, like butterflies and bees, are far too large to interact effectively with the flower. Cacao flowers are pollinated by tiny midges, both the biting and non-biting kinds from the Ceratopogonidae family. Yes, those same pesky biting no-see-ums that are about the size of a poppy seed help to pollinate cacao. Amongst these aptly named ‘chocolate midges’ Forcipomyia squamipennis is the most widely documented.
These midges are hard to spot, not just because of their size, but they are also most active at dawn and dusk. They work in harmony with the cacao flowers which open just before the sun rises, for no more than a couple of days. Not all that much is known about them and their relationship with cacao, scientists are not even entirely sure what the cacao flower does to attract the midges in the first place, as it has no discernible scent and does not produce a nectar. The cacao flower is also not the only flower these midges visit, with a lack of alternative pollinators they just happen to be the best adapted to the job, so it is no wonder then that successful pollination rates are so low.
Although under the right conditions, cacao trees produce thousands of flowers a year the average pollination rate, depending on your source, is measured at anything from 0.3%-30%.
Assuming a 10% success rate, that’s 90% of potential cacao fruits never making it to fruition, literally.
The way cacao is cultivated could make all the difference.
In monocultures there is only cacao so nothing else to distract the midges from the cacao flowers however, this is where the advantages end. Monocultures have much higher sunlight levels, as there are no overstory shade trees, making them hotter and drier. Pesticide and fertilizer use also tends to be higher creating an inhospitable environment for midges to thrive in large numbers.
Agroforestry systems like those on our farm, offer the ideal shady, damp environment for midges to live and reproduce. Deep, humid leaf mulch, decomposing cacao pods and plants material create the perfect habitat. Although there are many more food sources from many different species to compete with the cacao flowers, the overall chances of pollination are higher.
Successful pollination of the cacao flower requires roughly 250 grains of pollen to produce the 20-60 seeds within a cacao pod. Visiting midges are only big enough to transfer around 30 pollen grains at a time.
To further complicate matters, most, not all, cacao varieties are auto-incompatible meaning they only accept pollen from flowers on different trees. Chocolate midges are thought to range about 6m (20ft), cacao trees are usually planted 2.5-4m apart (8-13ft), so the number trees within a midge’s territory is low.
This built-in requirement for cross-pollination between different trees plus the amount of pollen needed for the successful growth of a single fruit means a lot of midges need to do a lot of travel between a lot of flowers.
Flowers that do not receive any or enough pollen are aborted by the tree and drop to the ground within a couple of days after opening. Viewed like this it seems somewhat miraculous that it ever works at all.