On warm spring nights in the foothills of Southern California, what appear to be tall ghosts can be spied in the distance. The Hesperoyucca whipplei, also known as chaparral yucca, are in bloom...and they don’t want to be alone. Their white flowers seem to glow in the moonlight, a beacon to attract tiny nocturnal moths that will ensure the flowers’ on-going existence.
Photo credit: National Park Service
The pollinator the flowers are attempting to lure are yucca moths, Tegeticula maculata. The yucca and moths have an amazing relationship, a form of symbiosis called mutualism, that is beneficial to both organisms. The yucca and the yucca moth are so interdependent that one cannot survive without the other. And this relationship goes back over 40 million years.
Photo credit: Jonatahn Numer
Yucca Moth Tegeticula yuccasella
Let’s start with the yucca.
The chaparral yucca takes 7-10 years to reach maturity. Until then, it is a formidable plant with long sword-shaped leaves with very sharp points protecting the main plant body. In the spring, when it has matured and there has been sufficient rain, the yucca will send up a flower spike from the center of the plant. During the next two weeks, the spike will grow as much as a foot per day. As the spike grows it starts to resemble one of its relatives, asparagus, another member of the Asparagaceae family.
The pictures below are of the same Hesperoyucca whipplei in Eaton Canyon from May 1, 2019 to November 19, 2019
When the spike reaches its maximum height of 10-15 feet, it will begin to bud out and produce hundreds of white, bell-shaped flowers. This is when things get interesting.
While the flowers are opening, white ¼-inch yucca moths emerge from subterranean cocoons. The conditions that signaled to the yucca that this is the right year to flower also signal the moths that it is time to appear.
The floral spike in full bloom on June 10, 2019
So here comes the female moth, attracted to the ghostly candle that is the yucca. Unlike other moths, she has no tongue. In her short life she has many duties, but eating is not one of them. Instead of a tongue to suck nectar, she has evolved two tentacles at the sides of her mouth. After mating, she uses these to collect pollen from the anthers of yucca flowers from a single plant, rolling the pollen into a ball.
Photo credit: WP Armstrong
To assure cross pollination, she is off to another yucca plant. Her goal is to find a flower that has yet to be pollinated by another moth. Her night flight is perilous, as bats are out too, and although small, the moth is white and very visible.
After arriving at a new yucca, and after detecting no telltale pheromones of other female moths on a flower, she proceeds to use her ovipositor to pierce a small hole in the flower’s superior ovary. Then she takes care to only lay a few eggs within the flower. Too many eggs could trigger the flowers to abort, her offspring to die, and the yucca would fail to reproduce. It is a careful balancing act that guarantees their mutual success.
Photo credit: WP Armstrong
Now the yucca moth packs some of her pollen ball on the stigma of the flower’s ovary, ensuring pollination, seed production and food for her offspring. Then she is off to another flower to repeat the process. In a few days, with her eggs safely deposited, her short adult life will be over.
Photo credit: Symbioticworld
Soon the yucca flowers wither and fall, leaving behind the pollinated ovaries. The ovary will grow and swell with seeds. Meanwhile, the little moth egg is still in the seed pod. As the pod produces seeds, the yucca moth larva hatches and is ready to eat. By laying only one or two eggs in the ovary, the moth has ensured plenty of food for the larva and more than enough seeds remain to spawn future yuccas.
A yucca flower impaled on its pointed leaf on September 20, 2019
In late summer and fall, when yuccas are covered with mature seed pods, a red larva is inside all of them. Soon the pods will begin to split open, raining black seeds to the ground. The larva will also drop to the ground, burying itself and forming an underground cocoon that will be its home until it reappears as a moth when conditions are ideal in the spring.
In fall the yucca plant also nears the end of its lifecycle, though the stalk will remain upright for some time.
The dried seed pods on November 19, 2019
The yucca moth is the chaparral yucca’s only pollinator. And different species of yucca have species of moths that are their sole pollinators. When you see a yucca in full bloom in Eaton Canyon you will never seen another pollinator approach the flowers. No bees, no flies, no butterflies, no birds. This is because the yucca is the only species in its family that does not produce nectar, only pollen.
The same cannot be said for the yucca’s cousin, the century plant that blooms up by the main trail in Eaton Canyon. Its flowers are alive with hummingbirds, hooded orioles, and all the traditional pollinators. One reason for this is that the yucca is the only species in its family that does not produce nectar, only pollen.
It is only the yucca moth and a curious but long-established mutual relationship that keeps the iconic yucca thriving in the California foothills.
References:
Forty million years of mutualism: Evidence for Eocene origin of the yucca-yucca moth association By Olle Pellmyr and James Leebens-Mack www.pnas.org 1999
U.S. Forest Service, Yucca Moths by Beatriz Moisset
W.P. Armstrong, www.palomar.edu
Coevolution of Yuccas ans Yucca Moths, Caleb Tackey and Patricia M. Gray, Western Undergraduate Research Journal 2018
Edited by Robin and Phil Hopkins
Credits:
All photos by Susan Hopkins unless otherwise credited