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Re-Wilding Route 1: Strange (bud) fellows

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Posted on: May 9, 2024


One of nature’s most intriguing and complex pollinator-plant interactions is playing out in yucca plants all across North America over the next month or two — including in our local yards and gardens. 

Most of us know yuccas on sight, even when they aren’t in bloom, by their stiff, pointed, swordlike leaves. They’re evergreen, so they stick out like a sore thumb, especially in winter (pun intended, since their sharp leaf tips can puncture skin easily). In summer, these spiky clumps throw up tall creamy spires of flowers, dozens on each stem, though they don’t all open at once. 

A few weeks after bloom, you’ll see large round seed pods forming on the stalk. Clearly something helped pollinate the flowers so they could set seeds. But if you had been looking for hummingbirds or butterflies or bees to visit these flowers, you’d have struck out. Who was the mystery pollinator?

When yuccas are blooming, if you fold open a few of the flowers, the overwhelming odds are you’ll find small moths the same shade of white as the petal clinging to the inside of the blossom, near the base. These are one of several species of yucca moths. All yucca moths are in the same taxonomic family — Prodoxidae — and in our area, they will be either Tegeticula yuccasella, the typical yucca moth, or the bogus yucca moth, Prodoxus decipiens. Both have the same complicated life history.

A mating frenzy of yucca moths inside the tube of a yucca.
Courtesy of Judy Gallagher

A yucca moth’s life revolves entirely around yucca plants. Adult moths appear as soon as the plants start to bloom. The males hunt for females among the flowers, and mating almost always takes place on or in a flower. They’re short-lived moths, and they don’t feed as adults. That’s why entomologists were really puzzled when they noticed female yucca moths gathering pollen as they flew from one yucca flower to another.

Turns out, those females are pollinating the yuccas, delivering pollen among the plants and fertilizing the flowers so they can produce seeds. What a kind, altruistic thing to do, one might think. 

Except nature is seldom kind or altruistic. The female moth is simply ensuring that her caterpillars will have food waiting for them when her eggs hatch.

After she fertilizes the flower, the moth lays her eggs inside the blossom where the seeds will develop. As they grow, the caterpillars feed on the immature seeds. The female moth lays only a few eggs in each flower, ensuring both that her caterpillars have enough seeds to see them to pupation and that the yucca has enough seeds left over, after the caterpillars eat their fill, to perpetuate its own population for future generations of moths. 

To be sure there aren’t so many caterpillars that they eat all the seeds, the female yucca moth scent-marks flowers on which she has laid eggs. Her scent warns other females that this pantry is already spoken for: Go find another yucca flower for your caterpillars. 

This finely tuned relationship between yuccas and both true and bogus yucca moths is called mutualism — both species depend on each other for existence. Wherever yuccas are planted, even well outside their normal southern and western ranges, yucca moths follow. I’ve even found them in a yucca plant in the middle of a salt marsh miles from any other yucca stand. 

Yuccas and yucca moths aren’t the only species sharing a relationship like this. If you’re fond of figs, you might be surprised that they, too, have pollinators that fertilize the flowers and lay their eggs inside maturing fig fruit. 

Fig fruits are actually inverted flowers — what we know as a fig is really the fleshy stem around many tiny fruits packed together, all facing inward toward each other. Think of it as an inside-out strawberry, with all the seeds on the inside. The delicate crunch of figs comes from those tiny seeds. In their ancestral lands, wild figs require the services of a tiny wasp that crawls inside the multi-flower through a special narrow entrance and fertilizes all the tiny ovaries. Then, just like the yucca moth, she lays some eggs to feed on the developing seeds, though not so many eggs that the resulting caterpillars will eat all of them. 

Fig wasps don’t even have a common name, but all of them are in the genus Pegoscapus. Each species of Pegoscapus fertilizes its own species of fig, and the fig flowers produce scents that only appeal to that species of wasp. A female wasp often breaks off her wings when she squeezes through the opening slit, but it doesn’t matter — she will die inside the fig after running around on all the fig flowers dropping pollen she brought with her from the fig that birthed her. 

Male wasps are wingless and never leave the fig; they mate with fresh females, who then fly off to find a new fig. If a female wasp develops quickly enough to emerge before the fig gets eaten by a farmers market shopper, she gathers pollen from her birth fig and carries it off to pollinate another. 

When we eat figs, we eat all of these things — the original female wasps, the dead males, and the developing grubs that haven’t yet made it to adulthood. For the squeamish, though, it may help to know that many cultivated figs are self-fertile and don’t rely on wasps to pollinate them.

The commonly grown Yucca filamentosa in bloom.
Courtesy of Creative Commons

Pollination can be as straightforward as a bee carrying pollen from one flower to another, or as complicated as the relationship between moths and yuccas or wasps and figs. The closer we look, the more we find that partnerships and enlightened self-interest between plants and insects seem to be the rule rather than the exception — partnerships we’re still working to discover.  

Have questions for Rick about the world of nature in and around the Maryland suburbs, or suggestions for future columns? Drop him a note at rb*******@gm***.com.



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