Discover the Yucca Moth: The Sole Pollinator of the Joshua Tree

Today, we’ll discuss the unique relationship between the yucca moth and the Joshua tree. Flora and fauna often rely on one another. Fauna, especially, relies on flora as a source of nutrition, shelter, and safety. In some cases, the relationship is completely symbiotic. A symbiotic relationship is one where a pair of species forms a close association with each other. These associations aren’t always positive. There are three main types of symbiosis – mutualism, parasitism, and commensalism. Two other relationships – predation and competition – are sometimes seen as symbiosis but this is incorrect. In the case of the Joshua tree and the Yucca moth, we will be exploring an incredibly unique obligate mutualistic relationship.

First, let’s look at the different kinds of symbiosis.

Types of Symbiosis

As we noted in our intro, there are three main types of symbiotic relationships. We’re going to check those out and also quickly explore two more relationships that are commonly thought to be symbiotic but are not. We’ll describe them and then explain why they are not forms of symbiosis.

• Mutualism is a symbiotic relationship in which a pair of species forms a close association and both species benefit from this relationship. We’ll explore this form of symbiosis more in this article.
• Parasitism is a relationship between two species in which one species benefits from the other, which is harmed but does not immediately die. Viruses, tapeworms, and leeches all share parasitic relationships with humans.
• Commensalism is an interaction between species in which one of the organisms benefits, while the other organism isn’t really affected. Commensalism can be as simple as a bird making a nest in a tree.

Now, let’s look at the other two relationships we mentioned before:

• Predation is a relationship between two animals in which one kills and consumes the other. This cannot be considered a symbiotic relationship because it does not occur over a long period of time. An example of predation is a pack of hyenas killing and eating a wildebeest.
• Competition is a relationship between organisms that must compete for the same resources at the same time in the same place. It is not symbiotic because it is an indirect interaction over resources, not a direct interaction. An example of competition is the relationship between cheetahs and lions, who both feed on similar prey and are negatively impacted by one another’s presence.

Now, let’s take a look at the Joshua tree.

What is a Joshua Tree?

A Joshua tree (Yucca brevifolia), despite its name, is not actually a tree. In fact, this unique plant is actually a succulent. A yucca, to be exact. A succulent is any plant that has formed thick, fleshy tissue in order to store water and be drought-resistant. Examples of succulents include aloe, all varieties of cactus, and Joshua trees. Yucca is a genus of succulents that exists in the agave family. About 40 species of succulents are yucca, including the Spanish dagger, Adam’s needle, the spineless yucca, and the Joshua tree.

The National Wildlife Federation reveals that Joshua trees were named for Joshua – a biblical figure. 19th-century Mormon settlers discovered and named the Joshua tree while journeying west, and they believed the outstretched limbs of the succulent were meant to guide them.

Joshua trees reach heights of up to 70 feet and grow from a singular trunk. Most Joshua trees do not exceed 40 feet. This succulent reaches heights of three to nine feet before branching out and forming clusters of beautiful white flowers and spiky leaves. Joshua trees can only be found in the Mojave Desert. In fact, this is a major indicator species for the Mojave Desert, and Joshua trees stop growing around the same place where the boundary for the desert ends. Joshua trees are slow-growing and can live for a long time. One example is a plant in California that is thought to be over 1,000 years old!

What is a Yucca Moth?

The Yucca moth is a small, off-white, or brown, moth that has a brief adult lifespan. Unlike other moths and butterflies, this moth does not have a long tongue. Instead, tentacles extend from its mouth to help gather pollen. Yucca moths are the only pollinators of all Yucca plants, which includes the Joshua tree. Yucca moths do not feed in their adult lives, only collect pollen. The life cycle of a yucca moth lasts about a year, and most of this time is spent in the pupal stage.

Yucca moths live where yucca plants live, which means they have a variety of habitats but can most commonly be found in the southwest United States and Mexico. The Sonoran Desert alone hosts ten species of yucca. Yuccas also occur in the southeastern United States and in North Dakota. Because of their unique relationship, you will not find a yucca plant without the presence of yucca moths in the area – and vice versa.

Like many other species of animals, spring is the time for yucca moth mating. This calls the adult male to the end of his life, while the female has to lay eggs. This is where yucca plants – and today, specifically, Joshua trees – come into the picture.

A Perfect Pair

When the yucca moth is ready to lay her eggs, she visits the flowers of a yucca plant. The special tentacles around her mouth are used to remove pollen from the plant and transport it to another plant. Yucca moths are picky about their flowers – a yucca moth will not work with a flower that has already been visited or fertilized by another female moth. This is to reduce the risk of too many caterpillars hatching in one spot, resulting in food shortages. When she finds a suitable flower, she deposits her collected pollen on its stigma, resulting in fertilization. Now that her gift is given, she lays her eggs in that flower.

By the time the eggs hatch, the flower is rich with seeds and fruit. The tiny caterpillars eat their fill and retreat underground to form their cocoons over the winter. The plant seeds left behind by the caterpillars will be moved about by rodents, offering them a chance to root and grow on their own. The cocoons of the yucca moth will hatch early in the spring and the cycle will repeat itself. The yucca plant will be pollinated and the moths will guarantee another season for their species.

What is Obligate Mutualism?

There is a definite difference between mutualism and obligate mutualism. Perhaps another way to describe this is “codependence” or “interdependence”. Humans have a negative view of the word “codependent”, but in this case, we use it not as a descriptor of a toxic relationship cycle, but rather, as a description of two species that absolutely need each other to survive.

This is distinctly different from another form of mutualism – facultative mutualism. Facultative mutualism is a form of symbiosis in which two species benefit from one another but they do not need each other to survive. Examples of this include the fascinating relationship between ants and aphids, in which ants protect aphids and are rewarded with food. Neither of these species depends on the other for survival. Instead, they simply enhance the quality of life for one another.

Since the yucca and the yucca moth can’t live without each other, it is imperative to make sure they don’t perish. A helpful educational video by the National Park Service better highlights the obligate mutualism that the yucca moth and yucca plant share.

Will Climate Change Impact This Relationship?

Yucca moths are stable and do not face a current or impending risk of extinction – unless something happens to the yucca plant. Since yucca moths are a specialist species, they are adept at performing their life cycle and thrive in any place yuccas are healthy and growing. Unfortunately, Joshua trees and other yuccas may face challenges that would extend to their sole pollinators.

Joshua trees and other yucca plants require a cold period in order to flower. Compare this to the maple tree’s sap production. A maple tree produces the most sap in a climate with above-freezing days and below-freezing nights. Without the overnight freezes, the sap run is not as prodigious and lasts much less time. Similarly, one cold frost in autumn makes apples sweeter. Because of the necessity for cold weather, climate change is a threat to the yucca plant. If temperatures do not allow a cold period for these plants to bloom, there will be nowhere for yucca moths to go.

One study by Jennifer Harrower and Gregory S. Gilbert explores changing climate patterns and their potential effect on the yucca moth and Joshua tree. The study explores climate change and conditional outcomes that may shape the future of these species. It seems that the key to ensuring the survival of yucca moths is to work to conserve yucca plants in their native ranges.

Where can I see a Joshua Tree?

One of the best places to visit a Joshua tree is at Joshua Tree National Park. This beautiful national park combines two deserts – the Mojave Desert and the Colorado Desert. Plan a visit to Joshua Tree National Park to see some of the most incredible desert plants and animals, including a wide variety of yucca plants.