The Macabre World of Figs and Parasitic Pollinating Wasps

Credit: Bon Appetit (no pun intended!)

The lifecycle of the fig wasp reads like a macabre horror tale, yet it is often cited as a premier example of mutualism (the scientific name for plants that trade insect development for seed production).

BBC Video by David Attenborough (not for the faint-hearted)

Sometime back, I came across the intriguing snippet from the second episode of the BBC TV Natural History series Perfect Plant by the eminent British biologist and natural historian, Sir David Frederick Attenborough detailing the unique mutualism of figs and wasps - this prompted me to take a closer look at this unique relationship.

Fig trees (Ficus, Moraceae) are tropical plants with numerous species around the world - so it might come as a surprise that all fig trees are pollinated by minuscule host-specific parasitic wasps of the family Agaonidae, each uniquely attached to one and only one type of fig, yet so small and insignificant that they go unnoticed.

Let's start with the basics: A fig you eat is not a fruit; it is an inflorescence or syconium (the scientific name for a cluster of many flowers and seeds contained inside a very enlarged fleshy bulbous stem that surrounds the tiny flowers inside). Furthermore, the crunchy little things that you eat in a fig are the seeds, each corresponding to one fertilized flower. 

Here's how pollination works in monoecious (the scientific name for flowers containing both male and female parts) figs.

Credit: USDA

Notes: 

1. The fertilization of dioecious (The scientific name when male and female reproductive organs in separate fruit) figs is far more complex and is not covered here.
2. Ficus carica, commonly called the common fig, is parthenocarpic (a scientific name that refers to the development of fruit without fertilization) and produces a sterile fruit that lacks seeds. 
3. This is an intermediate type of fig that has two crops annually. Like the common fig, the first crop (breba) is completely parthenocarpic and does not require pollination and fertilization of flowers, but the second crop develops only when the flowers are pollinated, as in the case of Smyrna fig. San Pedro, King, and Gentile are some of the common cultivars of this type.

1. When the female flowers inside the immature monoecious fig are ready for pollination, the fig emits an enticing aroma made up of volatiles (consisting of different esters, ketones, and alcohols) that attract only female wasps of the host-specific type that have adapted to that cultivar. 

2. The only link the fig florets have to the outside world is through a tiny bract-lined opening at the apex of the fig, called the ostiole, through which the pollinating female fig wasp pushes its way inside inexorably lured by the scent. Studies have shown the ostiole seals off when the female enters preventing any escape, and, in some cases, reopens when the fertilization is complete to allow the next batch of females to exit and repeat the process.

3. As it struggles to get inside through the small opening of the ostiole, the female wasp usually loses its wings and pieces of antennae. It does not matter because the female wasp, now called a foundress, will never need them again, doomed to die soon. 

4. Once inside, it crawls around the interior of the fruit, not only pollinating many florets but also laying its eggs inside the future seeds that will nourish its progeny and also spreading the pollen collected from the previous fig where it was born. 

5. Having fulfilled her life's mission the female wasp dies inside the fig and is digested by the fig, providing nourishment and possibly a bit of unique flavour 😅!

6. In a matter of 3 to 20 weeks, depending on the type of wasp, the eggs become grubs that grow inside the seeds. 

7. After completing their full development in a few weeks they emerge from inside the seeds. The males emerge first and start looking for unhatched females to mate with. 

8. There is extreme sexual dimorphism in adult fig wasps, with the wingless males, far smaller than the females completing their life cycle within their natal fig. They don’t have wings as they will never leave the fig they were born in. 

9. After mating with the unhatched females, most males like their mother are doomed to die inside the fig, which was their whole world for all their short lives. A few males live long enough to tunnel a way out of the fig which the females use to escape. 

10. Females emerge from eggs already fertilized. In addition, the male fig flowers are ripe and loaded with pollen, which the females load up in their pollen sacs.

11. Instinct tells the females to hastily exit the fig, ready to find another fig by the scent to repeat the process - but it is a desperate race against time - her life span is now only 48 hours.

12. But wait, there is one more twist. What if the female wasp lays her eggs but does not keep her end of the bargain to pollinate the fig? Studies by Cornell University showed that in such cases, the fig tree is ruthless. If the pouched female wasp did not deliver the pollen, the fig tree unceremoniously drops the fruit — essentially killing the wasp eggs inside. There are no free lunches it would seem.

FAQ: Now you must be wondering, “Do we eat dead wasps when we eat figs?” The short answer is that it depends — that is, if you eat parthenocarpic figs aka common figs (Ficus carica), meaning they are seedless then there is no chance of it happening. If however there are seeds, then there is a little or no chance of ingesting residual half-digested wasps this is because figs produce a very effective chemical called “ficin” that breaks down and digests the wasp bodies. Ficin is so effective at breaking down and digesting, animal proteins that natives of Central America eat fig sap to treat intestinal worm infections.

Strangler fig (Credi: WorldAtlas)

Footnote: The aptly named Strangler Fig (Ficus aurea) is pollinated by the host-specific wasp (Pegoscapus mexicanus), it's a parasitic nightmare that starts its life innocuously as a tiny epiphyte on an arboreal branch of host tropical trees, sends down tendril-like roots, steals their soil nutrients, water, and even sunlight through a canopy of dense leaves all the while the tendril-like roots twist around the host plant eventually encasing it completely, strangling its trunk and cutting off nutrient flow until it dies and rots away, leaving behind just the hollow, draped with the murderous fig. 

References 

1. Moisset, B. (n.d.). Fig wasps (By U.S. Forestry Services). US Department of Agriculture - Pollinator-of-the-month Fig Wasps. Retrieved February 5, 2024, from https://www.fs.usda.gov/wildflowers/pollinators/pollinator-of-the-month/fig_wasp.shtml
2. Public Affairs. (2011, May 20). The story of the fig and its wasp. Ecological Society of America. Retrieved February 5, 2024, from https://www.esa.org/esablog/2011/05/20/the-story-of-the-fig-and-its-wasp/
3. Borges, R. M. & Centre for Ecological Sciences, Indian Institute of Science, Bengaluru, India. (2021). Interactions between FIgs and Gall-Inducing Fig wasps: adaptations, constraints, and unanswered questions. Frontiers in Ecology and Evolution, 9. https://doi.org/10.3389/fevo.2021.685542
4. Khatib, S., & Vaya, J. (2010). Fig, carob, pistachio, and health. In Elsevier eBooks (pp. 245–263). Science Direct. https://doi.org/10.1016/b978-0-12-374628-3.00017-7