The white flowers of yuccas, jutting up from a whorl of sword-like green leaves, are a common site in eastern Kansas. Spikes with over 200 white cup-shaped flowers may extend as high as five or six feet. This is the eastern yucca: Yucca filamentosa.
Western Kansas has a smaller yucca species with about 20 larger flowers on a 2 foot high stalk, emerging from a smaller broad-leaf whorl: species Yucca smalliana. No bees visit these flowers. The yucca is pollinated by small white moths dependent on the yucca seeds for food for their larvae. In turn, the yucca depends on the moth for pollination to set seeds. Biology textbooks use this yucca-and-yucca-moth example to describe the concept of “mutualism”—the total dependency of two species upon each other. If one goes extinct, so does the other.
We tend to think that big concepts are discovered by scientists from elsewhere. But this relationship was first described by an entomologist working in eastern Kansas and Missouri in the late 1800s.
Charles Valentine Riley was the Missouri state entomologist. He often traveled west to work with colleagues in Manhattan, Kansas. The eastern yucca was brought by pioneers from the East Coast and continues to be found in towns and graveyards where it was planted. It branches by vegetative growth but can only produce seeds if pollinated by the yucca moth.
C.V. Riley experimented with our Kansas and Missouri yuccas to discover their beneficial relationship. In the daytime, our yucca moths hide inside the yucca flowers. The moth is also white and barely a half inch long. If you disturb it, it may fly out of one flower and into another.
After dark, the female flies from flower-to-flower. She “combs” the yucca flower anthers with her mouthparts, gathering a ball of pollen to fertilize other flowers. She then flies from flower to flower with this pollen ball under her chin, spreading the pollen on the female pistils. Before she leaves a flower she has pollinated, she lays several eggs in the base of the flower.
Fertilized flowers may then grow large seed pods. The petals drop away and the pods grow over an inch in length. These green pods can be broken apart into three sections with two rows of seeds in each section. The seeds are stacked like flat black coins. As the pod dries out and splits, the seeds will shake out on windy days over the next year. But in late June through July, there should also be two or three yucca moth caterpillars eating away on a few of those seeds—the yucca’s payment to the moth in return for fertilizing the plant.
Many homeowners do not like the ugly barren stalks and pods that are left after the white yucca flowers drop off. If they cut off the yucca stalks, the yucca moth larvae die as well. Those yucca plants will still flower each spring, but there will be no moths. And no pods.
C.V. Riley not only discovered this life cycle and mutual relationship, but he also described how our helpful species of moth, Pronuba yuccasella, probably evolved from distant relatives in New Mexico, where some other species of the moth were still pests feeding in the stem of yucca relatives.
Riley asked the questions and did the research that was possible at the time. It would take another century to develop the population biology question: what keeps a yucca moth from laying more eggs, producing enough young to eat up all of the seeds in the pod and therefore destroy the host plant’s seeds?
That question was asked in masters research by Marylee Ramsey in 1994 (now a biology teacher at Goddard High School). Her mark-and-recapture study showed that a few yucca moths did “cheat” and laid up to a dozen eggs per flower, and their larvae did occasionally eat all of the seeds. Her work is available as a Kansas School Naturalist, available free upon request from the Biology Department at Emporia State University, Emporia, KS 66801 (and found online at www.emporia.edu/ksn/).
A few years later, entomologists from Colorado confirmed how the yucca plant defends itself against such “cheater” moths. The yucca sheds a large number of its flowers at random, whether they are fertilized or not. If the “cheater” moth puts all of its eggs in a few flowers, there is a high risk of losing all of them. Like a roulette table in Las Vegas, the yucca plant forces the moth to place many small bets (lay a few eggs) in many flowers in order that some will survive.
If you have yucca plants on your property and they are producing pods, don’t cut the yucca spikes off. If there are 20 pods, there are probably 40 to 60 little yucca moth caterpillars inside that will develop into moths in future years. And if someone accuses you of being a yucca farmer, you can correct them: you are a yucca moth rancher—with about 40 to 60 head.