THE YUCCA MOTH AND THE FLOWER

BY VESTA M. AND RALPH D. CORNELL PHOTOGRAPHS BY RALPH D. CORNELL In the far-flung distances of the American Southwest there are many natural phenomena that are unique and of intriguing interest to casual observer and experienced naturalist alike. Let the passerby but stop to observe almost anywhere and he will find some aspect of the intricate processes by which life in those areas can survive.

Conspicuous in the panoramic scenes of the Southwest are the yuccas, always arresting in form, especially beautiful during their blooming season which extends from early March to late autumn. Equally prominent in interest is the life cycle of the yuccas, for therein lies a tale of wondrous cooperation between the plant world and the insect world.

There are some thirty-odd varieties of yucca native to the United States and over half of that number occur naturally in the Southwest. They vary greatly in form, size and appearance. In the high deserts are the grotesque figures of the Joshua Trees, with their "gaunt, stiff arms propping the star-sagged sky." In the low, dry deserts are less conspicuous species which are little more than rosettes of harsh needle-like leaves with short-stemmedflower clusters. Lighting the hills and canyons of the coastal regions is the shimmering beauty of fifteen foot flower spikes of the Candles of God.

A tourist might well hazard the guess that the yucca is some kind of cactus. They are often found growing side by side and, to the non-botanist, there might seem to be a certain similarity in appearance. Certain it is that the rapier-like leaves of the yucca are as protective in their sharpness as are the thorns of the cactus. And yet, strangely, the naturalist knows the yucca as a lily, a member of the great order of Liliaceae; a brother under the skin to the more tender and succulent garden forms of lilies with which we all are familiar. This kinship is recognizable in the individual flowers of the yucca and it is doubtless to this kinship that we owe the ethereal quality of the tall blooming tapers which reach toward heaven out of a harsh and forbidding background. Yucca flowers are white or shaded with cream, red or violet, usually cup shaped or somewhat flattened into saucer-like form. Their petals are thick and of an almost alabaster translucency. They grow, tightly clustered, along a heavy flower stalk which is generally erect, with the individual blossoms pendulous. However, in some varieties, just the opposite occurs for both stem and blossoms. The flowers open at night, like white chalices, but do not close during the day as do so many of the nocturnal blooms. They have a sweet and penetrating fragrance which is sometimes too strong for human pleas-

DRAWINGS OF PRONUBA YUCCASELLA ARE COPIES FROM PLATES IN THE MISSOURI BOTANICAL GARDEN'S 3RD ANNUAL REPORT, PUBLISHED IN 1892. ORIGINAL DRAWINGS MADE BY C. V. RILEY, PH.D.

ure but which seems to be attractive to certain kinds of moths.

When these plants of the Western Hemisphere are transplanted to European gardens or to other sections of the world where they do not occur naturally, they may flourish and flower profusely but they do not form fruit. To obtain fertile seeds it is necessary to hand-polli-nate the individual blossoms. Why should this be? In solving the riddle, scientific observation has established that the key to the yucca's fecundity is a tiny white moth with the formidable name of Promuba Yuccasella. This diminutive moth, not present in other lands to which the yucca has been introduced, is the missing factor necessary to the life cycle of the plant. Reciprocally, the yucca is equally necessary to the life cycle of the moth, as we shall see. Each is dependent, one upon the other, for the perpetuation of its kind. And we wonder, has it always been so, or, in the dim past, was each able to exist independently, and then, as changes came, did each find its life needs met by the other and give up a precarious independence to this perfect adapta-tion? Suffice it to say, that, at present, the yucca would not produce seeds were it not for the moth, because the flower-parts of the yucca blossoms are so constructed that it is physically impossible for the pollen of the flower to reach its stigma without special help. It must depend upon external means, such as that provided by the moth, for the transfer of its pollen. Coincidentally, the moth finds in the yucca blooms, the perfect incubation medium for its eggs and food supply for its larvae. (Until very recently it has been believed that all yuccas were depend-ent exclusively upon this moth for fertilization. Recent scientific research has disclosed, however, that some spe-cies of yucca apparently can be fertilized without the moth but even these seem to "employ" the moth in addi-tion to any other insect aid.) Reciprocally, the yucca is equally necessary to the life cycle of the moth, as we shall see. Each is dependent, one upon the other, for the perpetuation of its kind. And we wonder, has it always been so, or, in the dim past, was each able to exist independently, and then, as changes came, did each find its life needs met by the other and give up a precarious independence to this perfect adapta-tion? Suffice it to say, that, at present, the yucca would not produce seeds were it not for the moth, because the flower-parts of the yucca blossoms are so constructed that it is physically impossible for the pollen of the flower to reach its stigma without special help. It must depend upon external means, such as that provided by the moth, for the transfer of its pollen. Coincidentally, the moth finds in the yucca blooms, the perfect incubation medium for its eggs and food supply for its larvae. (Until very recently it has been believed that all yuccas were depend-ent exclusively upon this moth for fertilization. Recent scientific research has disclosed, however, that some spe-cies of yucca apparently can be fertilized without the moth but even these seem to "employ" the moth in addi-tion to any other insect aid.) Flowers usually make themselves alluring to insects by the secretion of nectars, by the development of odors pleasing to insects, by the formation of showy and attractive parts, and by the development of flower clusters which make it easy for insects to sample the blossoms without traveling too far from flower to flower. Although the yucca blossoms provide all these lures, apparently not all are needed to entice the moth. It is believed that the moth does not take food of any kind during this phase of its life cycle, so that nectar would not appeal as food. The significant attraction may be only that of sight or odor. The chief function of white in flowers seems to be to make them more easily apparent to insects, particularly to those which fly by night. All yucca blossoms being white, or nearly so, it is not surprising that the yucca moth is active only at night. Nor is it surprising that it is the female moth which is attracted, since it is her role to assure life for her progeny. A part of the remarkable synchronization is the timing. The mother moth appears at the proper season, when the yucca blossoms have opened and are receptive to pollen. This is particularly impressive when it is realized that each variety of yucca blossoms at its own special time, from as early as March to as late as autumn. Perhaps the timing is a matter of temperature control which affects both the plants and the insects. But, whenever the blooming season begins, there is the moth ready for its part in the wonderful process of nature. If the moths should not appear, no fruit would set from that year's flowers.

Arriving on the scene with this unfailing timeliness, the mother moth goes busily from flower to flower. She gathers pollen with her front legs and tentacles, carefully rolling it into a tiny ball until it attains about three times the size of her own head. She holds this pollen ball firmly against her breast as she flies from flower to flower. Arriving at a different blossom than that from which the pollen was gathered, she pierces the new flower ovary with her oviposter and deposits four or five tiny eggs at the base of the pistil. Then she thrusts the ball of pollen which she has been carefully protecting, down into the stigmatic tube of the flower in such a manner as to brush its sides with the pollen, thus insuring its fertilization. So it is that the mother moth lays her eggs in the ovary of the flower in order that her eggs may be sustained and protected from the heat and the rains. There they stay until the time comes for them to hatch into the tiny larvae which constitute the second stage of the life cycle of the insect. Again nature's rhythms of growth are well synchronized, for the flower ovary is small and undeveloped at the time the eggs are deposited in it, too small to contain more than the tiny eggs. But the fertilized yucca parts grow rapidly. A large seedpod is developed for the hungry little worms by the time the larvae emerge from their egg case and are ready to go to work.

Once they are hatched, the fruit maggots start to eat within the cool, green seed receptacles of the yucca and their growth keeps pace with that of the plant. The delicate balance of life never falters. The larvae of the moth consume the seeds of the yucca, but never to the point of depletion. When the busy little grubs have satiated their appetites and reached their growth limit, there still are uneaten seeds. Since each grub consumes an average of about twenty of them and since there are only 4 or 5 worms per pod, the balance always is in favor of a seed surplus for yucca reproduction.

One cannot find a mature pod of yucca without encountering and observing either the larvae or their telltale excavations and castings, for there would be no yucca seedpods unless the flower had first been pollinated by the mother moth.

When the larvae have eaten their way to maturity and through the seed vessel, and when the insides of the pod are becoming dry and unpalatable anyway, that inner urge which baffles all human understanding guides the tiny yucca dwellers to the periphery of the fruit. There they cut their way through the outer membrane and drop lightly to the ground, curling into tight little balls as they fall. They then go into the soil at the base of the plant and pupate. That is, they enter into another stage of life which intervenes between the larvae form and the winged or perfected insect form. Most of us are familiar with some of the different types of cocoons and pupae within which the marvel of metamorphosis takes place to change a repugnant little grub into a shimmering insect capable of flight.

The pupae of the moths lie protected in the earth until the seasons have rolled around and again the yuccas are ready to send forth their heraldry of bloom. Then the fragile, gossamer-winged moths break through their chrysalises to seek the receptive blossoms within which to start anew another life cycle of progeny; and the miracle of life goes on.

One wonders to what purpose this interesting dual life cycle is lived from year to year. Is it a closed cycle, unrelated to other lives that impinge upon it, or is it an instance which typifies the interdependence of all life forms? There seems to be more of reason in nature's plan than mere self-sufficiency. Perhaps it is an attestation to the fact that all forms of life are under the control of one basic principle of life and by that principle are brought into cooperation.