'Eastern' Joshua trees and their sole pollinators, 'eastern' Yucca moths Active
What drives the synchronized timing of Joshua Tree flowers and their sole pollinator moth?
Did you know that moths can be pollinators? Even more amazing is that Joshua trees, the scraggly icons of the Mojave Desert, require a single moth species to ensure fertile seed production. The association between the Joshua tree and its pollinating moth is a rare example of an obligate mutualism, which means that the species are entirely dependent upon each other. Joshua trees depend on the moth for pollination and the moth requires Joshua trees to complete its own life cycle.
Joshua trees – and the single species of Yucca moth that pollinates their blooms & feeds on their seeds – rely upon timed emergence for their mutual survival
Joshua trees are long-lived monocots, a group of flowering plants that includes lilies, orchids, irises, grasses, and palms. They occur in two taxonomic forms, or species: the 'western' Joshua tree (Yucca brevifolia) and the 'eastern' Joshua tree (Yucca jaegeriana), and are easily recognizable by their twisting branches and clusters of spiky leaves atop shaggy tree trunks. They are the icons of the Mojave Desert in the southwest, including having a park – Joshua Tree National Park – bearing their name. Across the Mojave in California, Arizona, Nevada, and Utah, they cover over nine and a half million acres in high desert country. From as early as February to late April, Joshua trees produce large waxy inflorescences – stalks of fragrant white flowers.
Joshua tree reproduction depends entirely on pollination by a tiny moth the size of a rice grain, of the genus Tegeticula. Much like bees, these moths visit Joshua tree flowers, and in turn, get something out of the deal. After pollen gathering, Tegeticula lay eggs on Joshua tree flower parts which develop into seeds. The caterpillars then eat some, but not all of the seeds, and the life cycle thus continues for both species.
SBSC research on the Tegeticula species that is the sole pollinator of 'eastern' Joshua trees
Studies on Joshua tree pollination have usually focused on 'western' Joshua trees, or on how Joshua trees and Tegeticula moths have co-evolved into their mutually beneficial relationship.
The moth Tegeticula antithetica, a related species to other Tegeticula moths, was discovered in 2013. However, its pollination of the 'eastern' Joshua tree, (Yucca jaegeriana) was only discovered in 2017. Little is known about the drivers that determine the timing of visitation and populations of this ‘eastern moth,’ and Joshua tree pollination and seed production.
Researchers at SBSC began a study in 2022 with the intent to unravel T. antithetica and 'eastern' Joshua tree interaction, and how climate change might affect both pollination of the Joshua tree and the survival of the moth. During the first year, SBSC selected 16 flowering Joshua trees among four sites at the Arizona Joshua Tree Forest in northwest Arizona. These sites were chosen for their range in elevation, a scientific method that is used to study changes in temperature and biodiversity at different sites where elevation is a surrogate for different climate conditions. In 2023, researchers added two additional sites west of Wikieup, Arizona, and with the addition of more sample trees at both sites, made observations of 36 flowering trees.
What are the factors that influence blooming and pollination, and how might a changing climate affect pollination and thus life cycles for each species?
Joshua trees have multiple threats across their range. These threats include climate change, wildfire, invasive grasses and forbs, and urban and renewable energy development. The big question is if changing climate affects Joshua tree pollination.
One threat, which is not well understood, is the potential for decline or loss of pollination, the precursor to Joshua tree population growth. Because climate change may threaten both the Joshua tree and its pollinating Yucca moth in ways yet unknown, understanding is needed about the population dynamics and phenology of the Yucca moth-Joshua tree relationship.
As climate changes, will the moth’s activities and Joshua tree flowering adapt together or separately, or will they move into more favorable locations at the same time and speed – in synchrony? Climate change projections have shown the climate window for Joshua trees is changing, but managers and policymakers do not know if the climate window for the moth will also change.
SBSC began with these research queries:
- Does the visitation of the pollinating moth on a Joshua tree flower vary by seasonal climatic variables (temperature and precipitation), the number of blooms on the tree, or the tree density of an area?
- How do the above factors influence the production of viable seeds?
- Can an effective method to assess moth abundance in a site be developed? (Currently, no such method had yet been established).
SBSC measured moth and other invertebrate visitation of individual inflorescences using sticky traps collected at monthly intervals in the spring of 2022 and 2023. They estimated the number of flowers and seed pods that developed on each of the sample trees, and collected seed pods to evaluate the number of fertile seeds developed. They also measured conditions that may influence pollination, such as the density of other Joshua trees around the sample tree, other associated vegetation, and the temperature and precipitation during the flowering period.
Tegeticula antithetica, the pollinating moth, is miniscule – less than a quarter inch in length – and its behaviors are mysterious. Although researchers found sticky traps could successfully capture visiting moths, they noticed the relation between production of seed pods and the number of moths visiting them was not consistent. Sticky traps can tell us if moths are visiting an inflorescence, but are probably not a good tool for monitoring, in this case.
In the first year of sampling, Joshua trees had prolific flowering, but in the second year flowering was less abundant, and some of the sampled trees that flowered the first year did not flower the second year. SBSC researchers are busy analyzing the data from both years to compare how environmental variables factored in seed production between the two years and among the sites at different elevations.
SBSC will provide an update on this study as our ecologists untangle the results of the research.
Did you know that moths can be pollinators? Even more amazing is that Joshua trees, the scraggly icons of the Mojave Desert, require a single moth species to ensure fertile seed production. The association between the Joshua tree and its pollinating moth is a rare example of an obligate mutualism, which means that the species are entirely dependent upon each other. Joshua trees depend on the moth for pollination and the moth requires Joshua trees to complete its own life cycle.
Joshua trees – and the single species of Yucca moth that pollinates their blooms & feeds on their seeds – rely upon timed emergence for their mutual survival
Joshua trees are long-lived monocots, a group of flowering plants that includes lilies, orchids, irises, grasses, and palms. They occur in two taxonomic forms, or species: the 'western' Joshua tree (Yucca brevifolia) and the 'eastern' Joshua tree (Yucca jaegeriana), and are easily recognizable by their twisting branches and clusters of spiky leaves atop shaggy tree trunks. They are the icons of the Mojave Desert in the southwest, including having a park – Joshua Tree National Park – bearing their name. Across the Mojave in California, Arizona, Nevada, and Utah, they cover over nine and a half million acres in high desert country. From as early as February to late April, Joshua trees produce large waxy inflorescences – stalks of fragrant white flowers.
Joshua tree reproduction depends entirely on pollination by a tiny moth the size of a rice grain, of the genus Tegeticula. Much like bees, these moths visit Joshua tree flowers, and in turn, get something out of the deal. After pollen gathering, Tegeticula lay eggs on Joshua tree flower parts which develop into seeds. The caterpillars then eat some, but not all of the seeds, and the life cycle thus continues for both species.
SBSC research on the Tegeticula species that is the sole pollinator of 'eastern' Joshua trees
Studies on Joshua tree pollination have usually focused on 'western' Joshua trees, or on how Joshua trees and Tegeticula moths have co-evolved into their mutually beneficial relationship.
The moth Tegeticula antithetica, a related species to other Tegeticula moths, was discovered in 2013. However, its pollination of the 'eastern' Joshua tree, (Yucca jaegeriana) was only discovered in 2017. Little is known about the drivers that determine the timing of visitation and populations of this ‘eastern moth,’ and Joshua tree pollination and seed production.
Researchers at SBSC began a study in 2022 with the intent to unravel T. antithetica and 'eastern' Joshua tree interaction, and how climate change might affect both pollination of the Joshua tree and the survival of the moth. During the first year, SBSC selected 16 flowering Joshua trees among four sites at the Arizona Joshua Tree Forest in northwest Arizona. These sites were chosen for their range in elevation, a scientific method that is used to study changes in temperature and biodiversity at different sites where elevation is a surrogate for different climate conditions. In 2023, researchers added two additional sites west of Wikieup, Arizona, and with the addition of more sample trees at both sites, made observations of 36 flowering trees.
What are the factors that influence blooming and pollination, and how might a changing climate affect pollination and thus life cycles for each species?
Joshua trees have multiple threats across their range. These threats include climate change, wildfire, invasive grasses and forbs, and urban and renewable energy development. The big question is if changing climate affects Joshua tree pollination.
One threat, which is not well understood, is the potential for decline or loss of pollination, the precursor to Joshua tree population growth. Because climate change may threaten both the Joshua tree and its pollinating Yucca moth in ways yet unknown, understanding is needed about the population dynamics and phenology of the Yucca moth-Joshua tree relationship.
As climate changes, will the moth’s activities and Joshua tree flowering adapt together or separately, or will they move into more favorable locations at the same time and speed – in synchrony? Climate change projections have shown the climate window for Joshua trees is changing, but managers and policymakers do not know if the climate window for the moth will also change.
SBSC began with these research queries:
- Does the visitation of the pollinating moth on a Joshua tree flower vary by seasonal climatic variables (temperature and precipitation), the number of blooms on the tree, or the tree density of an area?
- How do the above factors influence the production of viable seeds?
- Can an effective method to assess moth abundance in a site be developed? (Currently, no such method had yet been established).
SBSC measured moth and other invertebrate visitation of individual inflorescences using sticky traps collected at monthly intervals in the spring of 2022 and 2023. They estimated the number of flowers and seed pods that developed on each of the sample trees, and collected seed pods to evaluate the number of fertile seeds developed. They also measured conditions that may influence pollination, such as the density of other Joshua trees around the sample tree, other associated vegetation, and the temperature and precipitation during the flowering period.
Tegeticula antithetica, the pollinating moth, is miniscule – less than a quarter inch in length – and its behaviors are mysterious. Although researchers found sticky traps could successfully capture visiting moths, they noticed the relation between production of seed pods and the number of moths visiting them was not consistent. Sticky traps can tell us if moths are visiting an inflorescence, but are probably not a good tool for monitoring, in this case.
In the first year of sampling, Joshua trees had prolific flowering, but in the second year flowering was less abundant, and some of the sampled trees that flowered the first year did not flower the second year. SBSC researchers are busy analyzing the data from both years to compare how environmental variables factored in seed production between the two years and among the sites at different elevations.
SBSC will provide an update on this study as our ecologists untangle the results of the research.