Plant Animal Associations

The Yucca Plant and Its Moth


Introduction

The yucca moth, otherwise known as the Tegeticula yuccasella, is an insect that exemplifies coevolution with its symbiont, the yucca plant, Yucca elata. These two organisms are most often found in dry regions within the eastern hemisphere, mainly the southwestern United States and Mexico (2). The unique situation that has developed over millions of years has genetically set this pair apart from any other genus. The moth is genetically coded to insert a pollen mass in the reproductive organ of the plant it cannot survive without (4). Due to this mutually beneficial symbiosis both organisms are able to survive and reproduce in a an extreme way.


The most common yucca found in cultivation is Yucca filamentosa.  http://www.zone10.com/yucca-plants-in-landscape.html
The most common yucca found in cultivation is Yucca filamentosa. http://www.zone10.com/yucca-plants-in-landscape.html



Following the Reproductive Cycle of the Yucca Plant and the Yucca Moth

Every spring both male and female yucca moths emerge from the ground from their cocoons that have been subterranean for the winter season. Once out of the ground, have two objectives that are worth noting. Firstly, they find mates that are also surfacing in order to reproduce. Once the female’s eggs are fertilized the second task of locating the nearby yucca plant must be accomplished.

As the moths are coming out of the ground and searching for mates the plant itself is also reaching maturity. Flower stalks that produce an inflorescence in which the flowers open at independent times are reaching their maturity. This results in the unique formation of pollinia, which are pollen grains that stick together and are soon collected by the female yucca moth.

Once the female moth has been impregnated and located a yucca plant it uses its specialized appendages, located near the mouth, called maxillary palpi, to ball together pollinia. Interestingly, males do not have these long, curved palpi (1).


A female yucca moth (Tegeticula maculata) in the upright pollination position on the pistil of Yucca whipplei. She forces pollen down into the central stigmatic depression, thus pollinating the flower. http://waynesword.palomar.edu/ww0902a.htm, W.P. Armstrong, 2000.
A female yucca moth (Tegeticula maculata) in the upright pollination position on the pistil of Yucca whipplei. She forces pollen down into the central stigmatic depression, thus pollinating the flower. http://waynesword.palomar.edu/ww0902a.htm, W.P. Armstrong, 2000.

With the pollen still being held in her palpi the female flies to a different yucca plant. Here she is ready for egg laying, using a specific body part, the ovipositor. She crawls down the flower to the yucca plant’s ovaries and finds the narrowest wall layer in which to deposit her egg. She lays the single egg in a cavity called the locule.

Once the egg laying process is complete it is now the plant’s turn to benefit from this symbiosis. The pollen mass that is still in

the female’s palpi is used for pollination. The plant has a very specialized placement of its stigma so that it only allows the yucca moth to pollinate it. It lies at the bottom of a tube surrounded by stigma lobes. The female crawls up to the opening and jams the pollen ball down to the receptive stigma here it rubs several times. The palpi is designed exactly to perform this task that allows for these life cycles to continue on (4).


The specialized palpi of the female yucca moth is designed so that it is able to push pollen into the receptive stigga of the yucca it pollinates. http://waynesword.palomar.edu/images/yucca12b.gif. Photograph by Armstrong. 2000.
The specialized palpi of the female yucca moth is designed so that it is able to push pollen into the receptive stigga of the yucca it pollinates. http://waynesword.palomar.edu/images/yucca12b.gif. Photograph by Armstrong. 2000.
Yucca larvea captured before it burrows down into the soil and creates a cocoon to protect it throughtout the cold season.http://www.ventanawild.org/news/se04/yuccamoth.html#credit11. Daniel Udovic. 2001.
Yucca larvea captured before it burrows down into the soil and creates a cocoon to protect it throughtout the cold season.http://www.ventanawild.org/news/se04/yuccamoth.html#credit11. Daniel Udovic. 2001.


Hundreds of pollen that contain sperm are carried along the pollen tubes to the seeds. These seeds are the food in which the larva that are now in the plant need in order to survive and flourish. The amazing part is that only a small percent of these seeds are eaten, keeping both the larvae and plant at a healthy balance.

The yucca moth larva hatches in the ovary where the female placed it (3). During early summer it feeds on the seeds within the plant. The larva then use silk to bundle seeds together inside the capsules and by late summer is able to bust out and either fall or produce a silk strand to reach the ground. At this point the larva is able to burrow in the ground and forma silk cocoon that gets covered in sand for protection during the cold seasons (6). This cycle is then able to initiate again the falling spring.





The Classic Case of Coevolution
Coevolution, in a biological sense, has multiple definitions that vary slightly from each other. One of the most commonly accepted definitions is based on the genetic change of an organism in response to a change of genetics of a related being. The Yucca Plant/Moth symbiosis is a classic example of coevolution because of its unique history and interesting back and forth changes in order to become such specifically suited partners (5). The most current estimates of this mutual evolution dates back to around 30 million years ago!

http://www.lvrj.com/multimedia/Yucca-moth-Joshua-tree-coevolution-fascinates-researchers-119987549.html

Slideshow of Research being done on Yuccas and Yucca Moth in Mojave Desert, Nevada. Pictures taken by Jessica Ebelher, 2007.

References

1. Armstrong, W.P. 1999. "The Yucca and Its Moth." Zoonooz 72 (4): 28-31.

2. Cox, G.W. 1981. "The Yucca With the Big Bang." Environment Southwest Number 493: 12-16.

3. Hogue, C. L. 1993. "Insects of the Los Angeles Basin." Natural History Museum of Los Angeles County.

4. Huth, O. Pellmyr, 2000. "Pollen-Mediated selective abortion promotes evolutionary stability of mutualism between yuccas and yucca moths." Ecology 81: 1100-1107.

5. Pellmyr, O., Thompson, J.N., Brown, J.M., & Harrison, R.G. (1996). Evolution of pollination and mutualism in the Yucca mother lineage. The American Naturalist 148(5), 827-847.

6. Powell, J.A. and R.A. Mackie. 1966. "Biological Interrelationships of Moths and Yucca Whipplei." University of California Publications in Entomology 42: 1-59.