The Ancient Romance of the Yucca and the Yucca Moth
Each June Nebraska’s Sandhills and western plains are illuminated for a few short weeks by the stately flowering spikes of Great Plains yuccas, whose spires of twenty to sixty ivory-white flowers emerge from a radiating array of needle-sharp leaves and rise above the rest of the vegetation like Roman candles freeze-framed in flight. Sometimes whole hillsides are transformed by this sudden flower display, which is often variegated with sprinklings of blue spiderworts, golden-yellow hoary puccoons, and white daisy fleabanes. Then, after a few weeks, the spectacular show is over, but for the yucca the most interesting part of the story has barely begun.
Probably most residents of Nebraska can easily recognize a yucca; it is part of a group of about thirty mostly western plants in the lily family having succulent, bayonet-like leaves and huge spikes of large whitish or ivory-colored flowers. The largest of the American yuccas occur in our hottest southwestern deserts. There they may grow to as tall as twenty-five to thirty feet and develop branched, tree-like configurations. Large or small, all yuccas produce spikes of large cup-shaped flowers that open during the night and may remain conspicuous for several weeks.
Yuccas are part of a fairly ancient branch of the plant family tree, perhaps dating back more than fifty million years, when seed plants were first evolving. They have a relatively simple and primitive flower structure, with six conspicuous petal-like structures (three petals, three sepals) of the same color, size, and shape. There is a central, three-parted ovary with a protruding spindle-shaped pistil that extends out from the center of the blossom and ends in a small, indented tip adapted to receiving pollen. Six white stamens surround the ovary, producing packets of sticky pollen at their tips. The stamens are much shorter than the pistil and curve outwardly from it, reducing chances for accidental self-fertilization. The nocturnal flowering, fragrant flowers, and conspicuous white blossoms of yuccas all provide strong clues to the identity of every yucca’s pollinator, night-flying moths.
Yucca moths likewise represent one of the oldest and most primitive subfamilies of moths. There are fewer than one hundred known species, and some of the earliest were probably the first moths to develop a nocturnal activity pattern. Most existing species are highly specific as to their food habits, and the single species that pollinates our native Great Plains yucca is especially adapted to feeding upon and reproducing inside this species of yucca.
Some close relatives of yucca moths eat the vegetative parts of these plants, thus injuring them. Some of these very closely resemble true yucca moths and are called “bogus yucca moths.” It is possible that ancestral yucca moths operated in this same exploitive way, their larvae feeding on developing yucca seeds, which are highly nutritious and rich in proteins. But, by also eventually assuming pollinating responsibilities for the yucca, the moths assured themselves that there would be abundant seeds available for their larvae to consume. A biologically interesting question obviously follows: How and why did the moth modify its egg-laying behavior in such a way as to also assure the yucca’s pollination?
Likewise, how did the yucca transform from a generalized reproductive strategy that probably was originally dependent on wind pollination to one highly adapted for the attraction of and cross-fertilization by a highly specific insect pollinator? By adopting such a risky reproductive mode, the yucca has gained the benefits of precise and efficient cross-pollination, but it also risks losing its entire seed crop to an efficient seed predator. The yucca thereby has not only tied its future survival to that of the moth but also has limited its potential geographic range to that of its single insect pollinator.
As the moth developed new mouth structures and abilities to extract pollen from the stamens, the yucca evidently also modified its pollen characteristics to be more readily gathered by the moth. Equally importantly, the yucca has developed protective adaptations that increase the probability of efficient cross-pollination but reduce the chances of a moth laying so many eggs per flower that all the developing seeds will be consumed by hungry moth larvae.
It is still unknown as to why each moth lays so few eggs per blossom. Typically, each fertilized yucca seedpod chamber holds only one to three moth larvae, leaving the majority of seeds in the pod to mature and eventually be disseminated. It has been suggested that, although each yucca produces several dozen flowers, many blossoms are prematurely dropped, including some that might have already been pollinated. Such apparently random blossom-shedding might force the moth to lay only a few eggs in each of many flowers if it is to increase its chances that at least some of the flowers it pollinates will persist and develop seeds. Not all marriages are made in heaven!
With all this complicated biological background information, it is finally possible to understand the complex story of the moth and the yucca much better. Yucca moths of the Great Plains are so tiny—up to about a half inch long—that they nearly impossible to find. After many long hours of fruitlessly watching yuccas near sunset in hopes of seeing a yucca moth flying in, I finally discovered that by vigorously shaking a flowering stalk I could often cause a moth or two to drop out. In fact, yucca moths often spend the entire day hiding in the half-open blossoms, where their white color renders them almost impossible to see. However, similarly white-colored crab spiders also often hide in these blossoms, waiting patiently for insect prey, which makes yucca blossoms not the safest place imaginable for moths to spend the day!
Male yucca moths are smaller than females, and they lack both an ovipositor and specialized mouthparts that are adapted for gathering and transporting pollen. The moths mate within yucca blossoms as they open, but thereafter the male plays no part in the process. Female moths visit and pollinate flowers between dusk and midnight. She inserts her knifelike ovipositor into the side of the ovary and typically lays one egg in each of the ovary’s three main subdivisions or chambers. Before flying to another flower, she scoops up some of the stamens’ sticky pollen with her specially modified appendages. She then forms the sticky pollen mass into a ball, which she carries to each of the flowers that she later visits. There she pushes some pollen into the central opening of the flower’s stigma and inserts additional eggs, often repollinating the flower between egg-laying bouts. It is not yet known whether a female moth can recognize which flowers have not yet already been pollinated or how many total eggs she might lay at one time. It is known that individual females remain near their “home plant” and are active for less than a week. During this time they presumably survive by eating yucca pollen.
After fertilization the yucca’s flowering stalks elongate and enlarge, while large and elongated three-chambered seedpods are formed, each of which is subdivided into two smaller units. During this period the moth eggs hatch and the larvae begin to consume the developing seeds. After it matures over the next month or two, each larva chews its way out of the seed capsule, falling to the ground and pupating in the soil. The adult moths emerge the following spring as ground temperatures warm and the yucca-blooming season arrives.
If too many moth larvae are developing in any single ovary the yucca may abort its seed production. However, in most cases many seeds survive and mature, even in seedpod chambers occupied by two or three larvae, the probable result of egg laying by multiple females. Mature yucca seeds are rounded, flattened, and tightly stacked atop one another, so that several hundred may be present in a single pod. The winged seeds are released when the dried pod splits open and may be carried by wind some distance from the adult plant. Probably only a very few seeds are lucky enough to germinate and survive in the arid environment of western Nebraska, just as very few moth larvae are likely to survive long enough to complete their own life cycle.
This highly unlikely scenario, in which each of two very diverse species depends entirely on the other to achieve its own survival and reproductive success, is sometimes called obligatory mutualism by ecologists. Less technically oriented observers might simply call it amazing.