Last month I wrote about wasps as generalist pollinators (The Buzz on Pollination is Not All About Bees: Wasps Are Pollinators Too!). Today, let’s look at a very specialized type of wasp and its role in pollinating fruit.
Do you love dried figs as much as I do? Well then, thanks to tiny wasps are in the order! In fact, through their evolutionary history, fig trees established a pollination system that is among the most fascinating that exist. Even though we eat only a very small number of them, there are hundreds of fig species, and each one is pollinated by one or a few species of tiny wasps from a family called Agaonids.
These wasps are specialized in pollinating fig flowers, with a type of pollination interaction called ‘nursery pollination’. In this type of pollination, the wasps offer the service of pollination in exchange for a place to raise (‘nurse’) their offspring. But how does this work?
Let’s start by saying that a fig is like a sunflower that has been turned into itself, with all their tiny flowers facing to the inside of the fig. Some of these flowers are only female (those will make seeds), and some are only male (they offer pollen).
When visiting figs, the female wasps enter and find the female flowers. Using their long ovipositor (their organ for laying eggs), they lay one egg in it. After doing so, they take some of the pollen they previously collected on the male flowers of a different fig and they actively push pollen into the flower. While doing so, they make sure that the female flower is pollinated, and that the flower tissues grow and mature, which is what their offspring will eat while they develop inside the flower. When the offspring emerges from what was the flower, they reach to the male flowers and collect pollen, after which they leave the fig and start searching for a different one to start the cycle again.
Now, you may be thinking that this system can’t work. If the offspring eats all the female flowers (where seeds are!), then the plant has none left for their reproduction! You are right!
Well, it turns out that the plants have found a way around. Among all the female flowers available to the wasps for their egg-laying, some are short and sterile, and some are long and fertile. While the short ones are the ones that wasps can develop in, the long ones will not lead to any larval development.
The female wasps can’t know which one is long or short, so they just try to lay eggs and pollinate all the female flowers. Only some of their offspring will survive. By doing this, the plant sacrifices some flower tissue in exchange for the pollination of all the fertile flowers.
If nobody cheats, both the plant and the wasps benefit from the interaction, with the wasps having a safe place for some of their larvae to develop, and the plants having a safe means for pollen transfer and seed production. Isn’t pollination fascinating?
And to end this blog today, let me tell you a story. It turns out that most fig species are tropical and subtropical, and thus there are not many fig species native to the continental US. However, we love figs, and thus farmers from California decided to try to produce figs there since the climate is very appropriate for fig tree growth. These fig trees that we eat are from varieties of the species Ficus carica, originally from the Mediterranean/Middle East.
At the end of the XVIIIth Century, farmers in California were trying to produce figs, but with little to no success. Eventually, researchers realized that those cultivated figs did not produce fruit if the flowers had not been pollinated. Wasp pollinators were needed to accomplish that. By the end of the XIXth century, the wasps associated with Ficus carica in their native Mediterranean range were imported and introduced into the USA. Figs containing larvae of the wasp species were then left in baskets close to the planted fig trees, letting the females emerge from the figs and pollinate the cultivated Californian trees. After doing this, the fig production picked up, and today a similar technique is still used in those varieties of figs that need pollination for fruit set (for example, the Smyrna and Calimyrna varieties). Today, California is the world’s third top fig producer. Talk about the economic value of pollinator wasps!
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park