Depending on the authority, yuccas are usually placed in the lily family (Liliaceae) or the agave family (Agavaceae). The name Yucca is derived from "yuca," a Carib Indian name for the cassava or tapioca plant (Manihot esculenta) of the euphorbia family (Euphorbiaceae). Yucca is also the creole word for cassava. "Yuca" is not to be confused with the lovely yellow-flowered morning glory (Merremia aurea) of the Cape Region of Baja California. Apparently the connection with starchy cassava roots is that yucca buds and young flower stalks of are also roasted for food. Yuccas are trunkless shrubs with rosettes of stiff, sword-shaped leaves arising at ground level, or tree-like with distinct trunks and limbs. Examples of the rosette forms include Spanish bayonet (Y. baccata) and chaparral yucca (Y. whipplei). Tree-like forms include the Joshua tree (Y. brevifolia) of the California and Arizona desert region, and tree yucca or "datillo" (Y. vallida) endemic to southern Baja California.

Yucca leaves contain bundles of elongate fiber cells which can easily be pulled out of the leaf blade like strands of thread. The strong leaf fibers are used for cordage and rope. [Sisal rope is actually made from the leaf fibers of the related genus Agave, specifically the Mexican species A. sisalina.] Bleached, sun-dried yucca leaves from the Arizona species Y. elata are used for white coils in Papago Indian baskets. Greenish-yellow designs come from unbleached yucca leaves, while red patterns are produced by a natural dye from yucca roots. The black designs come from the long, curved pods of Proboscidea parviflora ssp. parviflora, better known as devil's claws or unicorn plants.

Creamy white blossoms are produced in large erect flower clusters (panicles) during late spring and summer. In the Mexican species (Y. filifera), the panicles are up to six feet long and are pendent rather than erect. Individual yucca flowers have six fleshy petaloid segments which are called tepals by some botanists since the petals and sepals are indistinguishable. The pistil of each flower terminates in a three-lobed stigma, the lobes in some species with glistening, feathery branches. The stigma lobes surround a central orifice that leads to a recessed receptive stigma. In order for pollination to occur, masses of pollen must be forced down into this central stigmatic depression. Herein lies the adaptive advantage and marvelous genetic programming of a little moth that is absolutely vital for the survival and perpetuation of yucca plants.

Several species of Yucca are cultivated in southern California, including the Baja California endemic Y. vallida, the Mojave Desert yucca (Y. schidigera), and the chaparral yucca (Y. whipplei). The latter species grows wild throughout the coastal mountains of southern California, decorating the chaparral each spring with huge, candle-like flower clusters that may reach 12 feet. In their native habitats, all these yucca species require pollination by a female moth of the genus Tegeticula (Pronuba). For example, the pollinator of Mojave yucca (Y. schidigera) in the Mojave Desert and Y. filamentosa in Missouri is a white moth named T. yuccasella, while the pollinator of joshua trees (Y. brevifolia) is a dark gray moth named T. paradoxa. According to J. Powell and R. Mackie (University of California Publications in Entomology Volume 42, 1966), yucca moths are not all host specific because T. yuccasella was collected from 19 different species of Yucca.

The moth that pollinates Yucca whipplei in the chaparral of San Diego County is Tegeticula maculata. Details of yucca pollination in San Diego County are described in a fascinating article by George Cox (Environment Southwest No. 493, 1981). Our local female yucca moth is a small black moth about 8-10 mm long. In the Transverse Range of Los Angeles and San Bernardino Counties and northward this moth is white with black specks. The chaparral yucca moth is easy to spot in San Diego County among all the moths that emerge if you shake a flower stalk during the spring months. Some of these other moths include a smaller, whitish species in the genus Prodoxus that lives on the yucca plant but does not pollinate the flowers. In fact, Y. whipplei contains three species of these non-pollinator "bogus yucca moths."

Each spring, male and female yucca moths emerge from their subterranean cocoons. They crawl to the surface and fly to nearby yucca plants. During this moth emergence period, male and female moths presumably rendezvous with each other and mate. At this time the yucca plants have developed erect flower stalks and the flowers open one-by-one into a magnificent inflorescence. At maturity, yucca pollen grains adhere into sticky masses called pollinia, two inside each chamber of the anther. Unlike most other flowering plants, the pollen is not dispersed as individual grains. The gravid (pregnant) female yucca moth collects up to a dozen pollinia within the yucca flower and forms them into a golden mass. She uses a pair of long, curved, prehensile appendages in the mouth region (called maxillary palpi) to collect, form and carry the pollen ball. Male yucca moths (and most other moth species) do not have these greatly enlarged, specially adapted palpi.

At this point the female is ready for egg laying. It is presumed that the moths fly to another plant, as in the well-documented behavior of another yucca moth Tegeticula yuccasella. The female moth crawls into a flower and positions herself on the side of the ovary, head outward, and inserts her egg-laying device (called an ovipositor) into the ovary wall near the partition between adjacent ovary sections (carpels). The ovary wall is thinnest near the partition between carpels. A single, slender egg is inserted into the ovule chamber (locule). Now she is ready for pollination--the crucial event that enables the perpetuation of all yuccas in the wild.

After inserting her egg into the flower ovary, the female moth (still carrying a pollen mass in her coiled palpi) climbs to the top of the ovary. Uncoiling her palpi from the pollen mass, she draws them back and forth over the stigma, pressing pollen into the central stigmatic depression. This insures pollination of the flower in which she has deposited an egg. Germinating pollen grains send hundreds of sperm-bearing pollen tubes into the ovary, resulting in the fertilization of hundreds of ovules (immature seeds) inside, some of which provide food for the hungry moth larva.

The yucca moth larva hatches inside the green developing ovary of the flower during late spring and summer and begins to feed on the maturing seeds. It remains inside the ovary (seed capsule) through the summer and fall, high on a branch of the flower stalk. The seed capsule is composed of three sections or carpels, each with two columns of seeds. At maturity during the fall, up to 38 flattened black seeds lie in tightly packed tiers within each column, resembling coins stacked in a dispenser. In the column containing the moth larva, six to 14 of the seeds in the lower portion of the tier are fastened together with silk, and a robust, pinkish larva occupies a cylindrical feeding cavity within these joined seeds. According to Powell and Mackie (1966), yucca capsules may be occupied by more than one larva, but the average number is usually one or two. Although the larva is a seed predator, it only consumes a small percentage of the hundreds of seeds within the capsule. Since the larva develops into a moth that pollinates the yucca plant, the relationship is clearly beneficial to both partners. By comparison, the relationship between the Mexican jumping bean (Sebastiana pavoniana) and its symbiotic moth (Laspeyresia saltitans) is clearly one-sided. The moth is a seed predator but plays no role in the pollination of its host shrub.

By late fall, dark brown yucca seed capsules split open between the seams of the carpels, releasing hundreds of black seeds. The pinkish moth larva remains inside its little feeding cavity of fused seeds within the capsule until the first autumn rains. Then it emerges from the capsule and drops to the ground. Some authors report that the larva lowers itself on a strand of silk, but I have never observed this on Yucca whipplei. Upon reaching the ground the larva burrows into the soil and constructs a silken cocoon covered with grains of sand. The cocoon may be spherical or elongate, about 6-8 millimeters long. The larva remains in its cocoon during the winter months until spring rains and warming temperatures presumably stimulate pupation and the emergence of an adult moth. Cocoons observed in captivity did not contain a pupa until shortly before the emergence of a moth in spring. It is imperative that the adult moths emerge when yucca plants are once again in bloom so that this remarkable cycle between a moth and a plant can be renewed.

As winter approaches in San Diego County, chaparral yuccas release their seeds as the basal rosettes of leaves die and turn gray. This marks the completion of a life cycle that began with a small black seed at least six or seven years earlier. The following spring new yuccas will sprout from seeds, and a continuous succession of beautiful yucca flower stalks will appear as long as there are undeveloped areas of natural vegetation and yucca moths to pollinate the flowers.