Whichever came first, the plant or the Pronuba (miller moth), the other must not have been far behind because each depends on the other for its existence, according to Botanist Dr. George Engelmann (1809-1884) and Missouri State Entomologist Dr. C. V. Biley.
The Spanish bayonet, as yucca is commonly called, grows in Europe but rarely bears fruit, while on our western plains, it produces abundant fruits and seeds. Both doctors studied the plant at night when the yucca was at its showy best.
They discovered that a certain insect visits the plant nightly, a moth different from any other that deposits its eggs deep into the flower’s heart.
The white, bell-shaped flower does not produce nectar but, when open, gives off a fragrance. Since the blossom is fully opened at night and is quite fragrant, it attracts the Pronuba.
It is interesting to observe this remarkable moth flying from flower to flower, working diligently at its specialized job.
Nocturnal Behavior of The Pronuba Moth
This symbiotic action begins during the day when the moth rests in the partially opened blossom.
It rests lengthwise along with one of the stamens, its white wings folded together over its back, blending perfectly with the white-walled blossom chamber.
The moth becomes active as dusk advances, and the flowers open and give off their fragrance.
It will climb one of the stamens and, with its specially constructed mouth parts, scrape all the pollen from the anther and roll it into a small pellet.
This is carried to a second stamen, and the pollen from it is added to the pellet—the process is repeated from stamen to stamen until a small ball is formed.
The Egg-Depositing Process
Leaving the stamen, the moth goes to the pistil, which it explores carefully. When satisfied with the locality, it turns about and faces the stigma or upper end of the pistil and then slowly backs down between two stamens, feeling the surface of the ovary carefully with the tip of its abdomen.
When a suitable location is found, usually just below the middle of the pistil, it stops the body movements and begins to drill with its ovipositor or egg-depositing device.
This tool is long, sharp-pointed, and specially adapted for drilling a small, deep hole into the heart of the pistil, where a single long, thread-like, transparent egg is deposited. This accomplished, the moth removes the ovipositor and proceeds up the pistil until its head reaches the stigma.
The stigma of the yucca is a triangular well or tube formed by the union of the three upright horn-shaped extensions of the ovary.
Securing a firm foothold on these extensions, the moth shaves off particles of the pollen pellet and, with its tongue, places them in the opening at the top of the style, forcing them down.
When done, it backs down the pistil and deposits another egg at a different point, returning to the stigma, inserts another pollen deposit. This may be repeated until the cavity is filled.
Fertilization of The Yucca Flower
Frequently, the moth will go to another blossom and repeat the interesting performance.
Each flower is receptive to fertilization only briefly since, after the first evening, the horn-like extensions close over the stigmatic chamber and thus preclude further pollination.
The yucca flower cannot pollinate itself, nor can the wind blow pollen from one plant to another because of the peculiar way it is stored. Consequently, the egg deposits from the Pronuba are the fertilizing agents.
The Timing Element
The phenomenal aspect of this exchange is the timing element. When the moth larva has matured, it bores its way out of the flower and into the ground, where it overwinters. There, it remains as a larva until the yucca blooms again in June.
By then, it has come of age and leaves its cocoon in the ground to become a tiny white moth that will spend much of its lifetime calling on yucca plants.
44659 by Mabel E. Shepherd