Many Antelope Valley residents are familiar with the Joshua Tree (Yucca brevifolia) and its presence in our Mojave Desert ecosystem, but are unaware of its pollinators. Here we will be discussing a pollinator of from the present: the Yucca Moth, and one from the past: Nothrotheriops shastensis, the giant ground sloth.
Pollinator of the Present: The Yucca Moth
Joshua Trees, like other species of yucca, rely on different strategies for pollination. Joshua Trees do not produce nectar and have small amounts of pollen (Joshua Tree Genome Project). Thus, they rely on different pollinators to assist them in reproduction, including the Yucca moths from the genus Pronuba (See Figures 1 and 2). Yucca moths are native to the Southwest of the United States and follow the range of Yucca plants, including the Joshua Tree (National Wildlife Federation). Yucca moths and Joshua Trees have an ecological relationship called mutualism, a relationship between two species in which each species benefits from a “service” provided by the other. The Yucca and the Yucca moths are linked by what ecologists call obligate pollination mutualism, which means that both the plant and the moth are benefiting from the pollination process which will be explained below (University of Wisconsin).
Figure 1: Photograph of a Yucca moth (University of Wisconsin UW Milwaukee).
There are many different species of Yucca moths, including the Tegeticula moth from the Prodoxidae family which we will be focusing on here (USDA). These moths are colored white, which helps the moths blend in with the creamy Yucca blossoms when they pollinate. Male and female Yucca moths mate in the spring, and shortly after this, the female moth must lay her eggs. She will visit the flowers of a yucca plant, including a Joshua Tree, and will remove pollen from the blossom’s anthers. To do this, the moth uses tentacles around their mouth to gather the pollen into a lump which she tucks under her chin. (National Wildlife Federation, University of Wisconsin). This pollen may make up to 10% of her weight (University of Wisconsin).
With her bundle of pollen, the female moth will search for a new Joshua Tree. Once she arrives at a plant, she will inspect its flowers and choose the ones that are at the right stage of development. She also checks if there are already eggs laid in the flower (USDA). She can detect the smell of other female moths with her antennae, and if other eggs have been laid inside a blossom, she will move on to another flower. If the flower is free, she will plant her eggs inside of the blossom and deposit the pollen onto the flower’s stigma which fertilizes it (National Wildlife Federation).
Figure 2: Photograph of Yucca moths (University of Wisconsin UW Milwaukee).
As the flower develops, it will form into a fruit and produce seeds. Inside the fruit, the moth’s eggs will hatch into larva that will eat some of the developing seeds (Joshua Tree Genome Project, National Wildlife Federation). Yucca months do not usually lay their eggs in already used flowers because that would produce too many caterpillars in one flower and there wouldn’t be enough seeds for them to feed off (National Wildlife Federation). This is also beneficial for the plant, in that there will be enough seeds leftover for the plant to reseed. After a few weeks, the larva is fully grown, and they will drop and make cocoons buried in the ground. There they will stay until next spring. Some larvae can remain dormant for more than a year if a yucca fails to bloom due to drought (USDA). Thanks to the Yucca moth, the Joshua Tree is able to spread its pollen to other Joshua Trees, which allows it to reproduce and create seeds. The Yucca moth in turn equally benefits from this process in that it uses the Joshua Tree flower and its seeds to house and feed its young.
The Coevolution of Yucca Moths and Joshua Tree Species
Recent studies on Yucca moth pollination in Joshua Trees has revealed that the trees are being pollinated by two similar species of moth. One of the two moths is bigger and is lighter grey in color. This discovery prompted further research into Joshua trees, which showed that trees associated with each of the two different species of moth are slightly different from one another (Joshua Tree Genome Project). Joshua trees growing in the western Mojave Desert are pollinated by the larger of the two moth species and tend to be taller, have a longer trunk, and longer leaves (Yucca brevifolia). Whereas Joshua trees from the eastern Mojave, which are pollinated by the smaller moth, are shorter and more bush-like, have more branches, and have shorter leaves (Yucca jaegeriana) (See Figure 3, Joshua Tree Genome Project). According to the Joshua Tree Genome Project, based on these differences, some botanists have argued that there may be two species of Joshua tree occurring in the western and eastern Mojave.
Figure 3: Examples of Yucca brevifolia and Yucca jaegeriana growing next to each other. Photograph by Jeremy Yoder. (Joshua Tree Genome Project).
In addition, researchers have discovered that the biggest difference between the trees pollinated by each species of moth is their flowers’ “styles” which is the portion where the eggs are deposited. Trees pollinated by the larger moths have longer styles and trees pollinated by the smaller moths have a shorter one. In addition, each moth is more successful at laying their eggs in their own corresponding trees. All these factors have led scientists to think that the two types of Joshua tree have adapted to the different species of moths. According to the Joshua Tree Genome Project, “evolutionary changes in the flowers may have occurred as a way to reduce the number of seeds that get eaten by the moths’ caterpillars. The moths, in turn, may have evolved differences in body size as a way to compensate for the changes in the flowers. This process – changes in one of the organisms causing changes in the other, and vice-versa– is known as ‘coevolution’”.
Pollinator of the Past: Nothrotheriops shastensis, the Giant Ground Sloth
During the Pleistocene epoch that lasted from about 2,580,000 to 11,700 years ago, Nothrotheriops shastensis, or the Shasta ground sloth, roamed North America in what is now central Mexico to the southern United States (Prehistoric Fauna). Nothrotheriops shastensis was one of the smallest species of ground sloth at 9 feet from snout to tail and weighing at one quarter ton (Prehistoric Fauna)! The animal had large hindlegs and a muscular tail which would be used to support the animal when it wanted to stand on its hind legs (See Figure 4). In addition, the sloth had long foreclaws which it would use to defend themselves from predators such as dire wolves.
Figure 4: Nothrotheriops shastensis (Prehistoric Fauna).
Nothrotheriops behaved like other typical ground sloths of North and South America, feeding on various plants like prickly pear cacti fruit, agave, desert globemallow, Mormon tea, saltbushes and catclaw acacia desert globemallow, cacti, and yuccas- including the Joshua Tree (Sharp, Prehistoric Fauna). Its claws would be used to reach past plant spines and grab at softer flowers and fruits (Prehistoric Fauna). It is thought that Shasta ground sloth played a big role in the dispersal of Joshua Tree seeds due to their consuming of the flowers. Preserved sloth dung has been found containing Joshua tree leaves and seeds, confirming that they fed on the trees (Prehistoric Fauna).
The Shasta ground sloth would go extinct as the last Ice Age ended for unknown reasons. It has been suggested that many animals may have died as the climate change altered their food sources. Predation by Paleoindians is also thought to be a cause to their demise. Widespread disease may have also affected various species (Sharp). It has been suggested that the loss of the Shasta ground sloths has caused the population of Joshua Trees to suffer, with the loss of their seed dispersal help (Prehistoric Fauna). MOAH has a portion of a fossilized Nothrotheriops arm bone in their Natural History collection (See Figure 5).
Figure 5: Nothrotheriops fossil from Florida, Catalog # 2022.FIC.600 (MOAH Collections).
Visit the Elyze Clifford Interpretive Center
To learn more about Joshua Trees, other plants, and wildlife native to our Mojave Desert, please visit the Elyze Clifford Interpretive Center located at 43201 35th St W, Lancaster, CA 93536 (See Figure 6). The Center is nestled within the Prime Desert Woodland Preserve, which spans more than 120 acres with almost four miles of trails. The Elyze Clifford Interpretive Center provides educational opportunities through its immersive location that includes special nature presentations and tours, free kid’s crafts, and community events. We are open Wednesday, Saturday, and Sunday from 10 am – 4pm (Closed Holidays). Visit our website for trail information, virtual tours, and for information regarding artist Tina Dille’s exhibition: The Muse which is currently on display: Elyze Clifford Interpretive Center | MOAH (lancastermoah.org).
Figure 6: The Elyze Clifford Interpretive Center (MOAH).
Works Cited National Wildlife Federation, “Yucca Moths”, Yucca Moths | National Wildlife Federation (nwf.org). United States Department of Agriculture, “Yucca Moths (Tegeticula sp.”, Yucca Moths (usda.gov). The Joshua Tree Genome Project, “What is the deal with Joshua trees and yucca moths?”, The Joshua Tree Genome Project. University of Wisconsin UW Milwaukee, “Yucca Moth (Family Prodoxidae”, Yucca Moth (Family Prodoxidae) - Field Station (uwm.edu). Prehistoric Fauna Roman Uchytel, “Shasta Ground Sloth”, Shasta ground sloth (prehistoric-fauna.com). Sharp, Jay, “Shasta Ground Sloth Nothroptheriops Shastensis The Story of its Discovery at the Aden Crater in New Mexico”, Shasta Ground Sloth (Nothrotheriops shastensis) - DesertUSA.