Written by Nancy Miorelli

Chocolate and Flies?
Yes, my Facebook is in Spanish because I’m trying to learn it.

Short answer – Yes,  but it’s impressive the system works at all.

Where Does Chocolate Grow?

Chocolate in Ecuador. From the pod to the store.

Chocolate, or the Cacao plant (Theobroma cacao) grows all over the world but mainly in the tropical regions. Chocolate is grown in West Africa, Indonesia, Central and South America, and in Hawaii. This presents a challenge for chocolate production, because the pollinators also have to be found in these regions which are very different from one another.

How does the Cacao plant get pollinated? Let’s take a look at the involved parties.

Getting to Know the Players:

Buds, a flower, and the fruiting pod growing directly off the branch of the cocoa tree.

The Cacao Flowers: First of all, cacao flowers are weird. They’re small, little white flowers that dangle vertically from the trunk of the cacao tree and are nearly odorless. At first glance it looks like they don’t provide any nectar, but microscopic nectaries are found on the stems and base of the flowers. Basically, it’s not particularly attractive to anything …

…except to a little fly.

The Pollinators:  The little midge (Diptera: Certatopogonidae: Forcipomyia) involved in this relationship  is distributed worldwide, and only this particular genus of fly pollinates the chocolate flowers. It’s tiny. Like really tiny. Like 1-3mm tiny. However, only flies between 2-3mm can pollinate the flowers because the others are too small to reach the important bits.

The little midges. Mating pair.
Christophe Quintin (CC BY-NC 2.0)

Like all flies that are closely related to mosquitoes, their egg and larval stages are tied closely to moist areas and it primarily breeds in the rainy season. Depending on the species, some of these midges lay their eggs in the rotting cocoa husks in plantations while others find animal dung the perfect baby food. Regardless of the menu, the dish has to be wet and rotting.

The adults are also fluid feeders, some feed on nectar, while other feed on blood as their family common name, the biting midges, might suggest.

The Cacao Flower Plays “Hard to Get”

H. Zell Wikimedia Commons
Edited by Nancy Miorelli

Now that we’ve got the introductions out of the way, I want to highlight confusing shape of the flower.

I want to direct your attention to the structure known as the staminode. It’s like the stamen, the male part of the flower, but these don’t produce pollen and are therefore sterile. The pollen producing anthers are hidden by hoods and can only be accessed by tiny insects near the base of the flower.

The style, the female part of the flower, is located in the middle of all the sterile male bits. The insects must bypass the staminodes to deposit pollen on the style.

And it gets more complicated, because each tree produces three variants of the flower.

1. Converging: The staminodes all point inwards at the tip
2. Parallel: The staminodes all point straight
3. Splayed: The staminodes curve outwards at the tip.

(Frimpong-Anin et al. 2014)

This difference might seem trivial to us, but these small measurements make a difference. Only the converging and parallel types can be successfully pollinated by the flies. This is okay, because the trees make the most of these types anyway (56% and 37.5% respectively). However, apparently the flies are very picky and like landing on the parallel flowers the best.

The Relationship is “Complicated”

By Tatters (CC BY 2.0)

And it gets even *more* complicated because the tree cannot pollinate itself. Therefore the flies have to root around in some other tree’s flower bits, then make its way over to another tree to have a chance at pollinating it.

Oh, and the flies are bad at flying.
Oh, and they can barely carry enough pollen to pollinate one flower.
Oh, and each flower only typically lasts between 24-48 hours.

No, It’s Not Efficient

This process is about as efficient as you might expect it to be. Some estimates suggest that only 1 out of every 400–500 flowers produce fruit.  The cocoa plant can produce upwards 50,000-100,000 flowers in it’s 25 year lifespan.  On average, a tree can produce 100-250 fruiting pods over its life.

During my time in Ecuador I went to a chocolate factor and was able to nibble on the flesh that surrounds the beans.

Only 10-30% of the pods will make it to maturity. Each of the pods produces between 20-50 beans which are covered by a sweet flesh that is edible. Approximately 400 beans are required to make one pound of chocolate.

So let’s max this all out and say you have a tree that produces all 250 pods in its life, 30% of the pods make it to maturity and all those pods produce 50 beans. That’s 3,750 beans over its 25 year lifetime,  which means one tree can make about a whole whopping 9lbs of chocolate.

Makes you think twice about those 5lb Hershey bars.

References:

1. Frimpong-Anin, Adjaloo MK, Kwapong PK, and Oduro W. 2014. Structure and stability of cocoa flowers and their response to pollination. Journal of Botany 2014(513623) 6pps.

2. Kaufmann T. 1974. Behavioral biology of a cocoa pollinator, Forcipomyia inornatipennis (Diptera: Ceratopogonidae) in Ghana. Journal of the Kansas Entomology Socieity 47(4): 541-548.

3. Kaufmann T. 1975. Studies on the ecology and biology of a cocoa pollinator, Forcipomyia squamipennis I & M. (Diptera: Ceratopogonidae), in Ghana. Bulletin of Entomological Research 65(2): 263-268.

4. Pound FJ. 1931. Studies of fruitfulness in cacao. I. A note on the abscission of the flowers. II. Evidence for partial sterility. In the First Annual Report on Cacao Research for 1931-1932. pp 24-28.

5. Stephenson AG. 1981. Flower and fruit abortion: Proximate causes and ultimate functions. Annual Review of Ecology, Evolution, and Systematics 12:253-279.

6. Wirth WW & Howarth FG. 1982. The “Forcipomyia ingrami” complex in Hawaii (Diptera: Ceratopogonidae). Proceedings Hawaiian Entomological Society 24(1): 127-151.

7. Young AM, Schaller M, Strand M. 1984. Floral nectaries and trichomes in relation to pollination in some species of Theobroma and Herrania (Sterculiaceae). American Journal of Botany 71(4): 466-480.