Volume 41, Number 2,
June 1995:
The Yucca Plant and
the Yucca Moth

Text-only version

ISSUE HOME PAGE

ABOUT THIS ISSUE
- about KSN
- about the authors

IN THIS ISSUE
- introduction
- mutualism
- coevolution
- a "transparent" system
- yucca moth pollination
- male and female yucca moths
- mark and recapture
- C.V. Riley
- the yucca plant
- yucca flowers
- yucca products
- yucca pods and larval moths
- bailing out of the pods
- old pods
- what we do not know
- what prevents a cheater?
- how did the yucca and yucca moth relationship evolve?
- solving problems
- for additional information

SLIDESHOW
- View all images in this issue.

The Yucca Plant and the Yucca Moth
by Marylee Ramsay and John Richard Schrock

THE YUCCA PLANT
About 30 species in the genus Yucca are found in North America. Only Yucca glauca is common and native to the western Great Plains. It is found from South Dakota down through western Nebraska, Kansas and Oklahoma. Its stiff, broad and short sword-like leaves form green rosettes in the shortgrass prairies and sand hills. Because of the lack of moisture, the plants are spaced out across the dry semi-arid hills. Yucca glauca only produces from 20-60 white flowers per stalk. The common yucca found around towns and graveyards in the eastern Plains and across the Midwest is Yucca filamentosa. Also known in nursery catalogs as Yucca smalliana, Y. filamentosa is a cultivar that is native to the humid southeastern U.S. It occurs in the central U.S. only where planted and disperses only a short distance from where it was planted. It forms a rosette with longer narrow blades and produces several hundred smaller flowers.

The greatest variety of yuccas live in the Southwestern U.S. where they evolved as successful semi-desert plants. Arizona is home to 14 species, including the Spanish bayonet and the Joshua tree. There are also many yucca moth species in that area to pollinate the wide range of yucca species. Because of the great variety of yuccas and their moths in the southwestern U.S., biologists believe this is the region where most of the evolution of these groups occurred--a process called adaptive radiation. However, we are addressing just the Midwestern Yucca filamentosa and the yucca moth, Tegeticula yuccasella, where the of its coevolution was discovered.

YUCCA FLOWERS
Flowers are produced on a vertical stalk, called a raceme inflorescence. The flowers are similar to related lilies and have three sepals, three petals, six stamens (the male structures bearing pollen) and one pistil (the female structure containing the plant eggs in a ovary). The sepals and petals are greenish white and very similar in structure, so they are called "tepals."

The white yucca flower is a perfect place for a white yucca moth to hide in the daytime. Indeed, the yucca moths are so secure inside the flowers that a researcher has to batter a flower considerably to encourage the moth to leave. The moths are so cued to fly into white flowers that they readily enter a small white insect net made from the scoop from lemonade mix--they see it as another flower! Like many other moths, they are not attracted at all to lights at night.

The yucca pollen is on the anthers at the tips of the stamens; the pollen is sticky and remains so throughout the flowering season--a fact first observed by Riley. The yucca stamens bend away from the female stigma, and reach only two-thirds the length of the pistil. Through evolution, the flower is "doing" all it can to favor the moth and prevent self-fertilization.

YUCCA PRODUCTS
Indians used the yucca plant for many products. The strong fibers from the plant made cord, cloth, baskets and sandals. Raw flowers were eaten in salads, or boiled as vegetables. The immature pods were roasted and peeled before eating. Dried pods and seeds were ground into flour. The roots form a frothy soap(hence the name "soapweed) that was both a cleansing agent and a skin cream used for treating rashes. Leaves and roots were also used as a tea for treatment of headaches, arthritis and gonorrhea.


Figure 3. Yucca moth caterpillars found eating the seeds inside yucca pods during the summer. Each larva is over a centimeter long.