How a Tiny Relative of the Mosquito Helps Make Chocolate a Reality

Feb 13, 2018

In this month’s episode of Field Notes, Susan Knight explains how a tiny relative of our mosquito holds the key to all that chocolate you plan to eat tomorrow on Valentine’s Day.

Valentine’s Day is coming up, and with it, a celebration of chocolate. Almost everyone loves chocolate, but you may not realize that the hero of the chocolate story is a tiny biting midge, a relative of our friends the mosquito and black fly.

Chocolate comes from the cacao tree, a small evergreen tree native to tropical parts of Central and South America, though it is mostly cultivated in Africa.  The cacao tree should not to be confused with another iconic South American plant, the coca plant, that brings us the mind-altering cocaine.  All the same, the cacao tree itself is fascinating because of its history, its elaborate botanical features and of course, its delicious product.

The first part of the scientific name, or genus, of the cacao plant, is Theobroma which comes from the Greek meaning “food of the gods”, named by Carl Linnaeus, the originator of the system by which every plant and animal has a two-part name. Christopher Columbus was reportedly the first European to encounter this plant in the early 1500s, and the Kew Botanic Gardens in London still has the first specimen sent to Linnaeus.  However, the plant was highly valued by the native peoples living in this area long before Columbus arrived, with archeological evidence going back almost 2000 years.

The plant itself is rather odd looking.  The cacao fruits or pods pop right out of the trunk of the tree. The fruit looks kind of like a papaya, or maybe a skinny acorn squash.  Inside the pod is pulpy white flesh that is evidently quite delicious on its own. The cacao seeds, often called beans, that get turned into chocolate, are embedded in the pulpy flesh. Each pod contains about 30 to 50 seeds and it takes about 400 seeds to make a pound of chocolate. Over its lifetime of about 25 years, one cacao tree may make less than 10 pounds of chocolate.

Like the fruits, the flowers that must be pollinated before there can be any pods also pop right out of the trunk of the tree.  The flowers can only be pollinated by one genus of a tiny biting midge, Forcipomyia. The males do most of the pollinating; the females are usually off biting someone.   The flowers are small, but have an intricate structure. Like many plants, the cacao flowers have both the male and female parts within the same flower. However, the stamens, which carry the pollen, are hidden by little hoods, and only the tiny midges are small enough to make their way through this obstacle course to get to the pollen. On top of that, the flowers also have staminodes, or stamen imposters, that stick out from the flower, right in the middle of the female parts, making it even more difficult for the flies to bring pollen to the female part of the flower. Complicating matters even more, the pollen must come from a different tree than the tree that is going to bear the fruit. And, each flower needs to receive a large clump of pollen if the flower is to be successfully pollinated. The flies can barely carry this big wad of pollen, because the flies are actually pretty lousy fliers.  On top of that, each flower is only open for 1 to 2 days. Even though the tree produces lots of flowers, it is no wonder that very few of them get pollinated, let alone set fruit.

So, it is not surprising that cacao farmers plant huge plantations of cacao trees to try to bump up production. However, as often happens when you try to mess with Mother Nature, many cacao plantations experienced even worse chocolate production than expected.  Why? Well, you can’t just plant the trees.  You need that specialist pollinator, that little midge, that can deal with the complicated flowers with the tricky staminodes, the short flowering time, the need for lots of pollen for pollination, and you need lots of them. In many agricultural settings, pollinators are taken for granted, but often, the pollinator is the limiting factor in fruit set.  Just as honey bees are essential, but declining in US agricultural settings, the cacao midge was not abundant enough in the giant cacao plantations.  Researchers found that the farmers needed to improve the habitat for the little midge, who, like their mosquito and black fly relatives, needs a nice moist environment, and needs to live right next to the trees.  Through experiments, they found that the midges were much more abundant and able to successfully pollinate the flowers when there was a damp, messy pile of cacao husks left on the ground near the trees. The increased numbers of pollinators resulted in an increase in the number of pods, and an increase in the yield of cacao beans.

So, when you are cursing mosquitoes and black flies next summer, remember their cousins the cacao midges that deliver all that delicious chocolate on Valentine’s Day.

For Field Notes, this is Susan Knight, of UW-Madison’s Trout Lake Station.