Yucca & Yucca Moth

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WAYNE'S WORD Volume 9 (Number 2) Summer 2000

The Yucca
and Its Moth

Pollination That
That Depends On
A Small Moth

Modified From: Zoonooz Volume 72 (4): 28-31.
April 1999 by Original Author W. P. Armstrong.

The genus Yucca is one of the most remarkable groups of flowering plants native to the New World. It includes about 40 species, most of which occur in the southwestern United States and Mexico. Although they are often associated with arid desert regions, some species are native to the southeastern United States and the Caribbean islands. What truly sets this genus apart from other flowering plants is their unique method of pollination: A specific moth that is genetically programmed for stuffing a little ball of pollen into the cup-shaped stigma of each flower. Like fig wasps and acacia ants, the relationship is mutually beneficial to both partners, and is vital for the survival of both plant and insect. In fact, yuccas cultivated in the Old World, where yucca moths are absent, will not produce seeds unless they are hand pollinated.

Chaparral yucca (Yucca whipplei) on a remote ridge in the rugged San Gabriel Mountains of southern California.

Blossom of chaparral yucca (Yucca whipplei) showing a male and a female yucca moth (Tegeticula maculata).

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.

A stately Joshua tree (Yucca brevifolia) in the northern Mojave Desert of San Bernardino County, California.

Mojave yucca (Yucca schidigera), a native species in coastal and desert areas of San Diego County. Unlike the chaparral yucca (Y. whipplei), it produces a compact flower cluster, a distinct basal trunk on old specimens, and leaves with conspicuous marginal fibers. Although its range overlaps that of Y. whipplei, it requires a different species of yucca moth (Tegeticula yuccasella).

See More Photographs Of Yucca Plants
Yuca (Merremia aurea) in 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.

Yucca elata in southeastern Arizona. The dried leaves of this species are used in Indian basketry.

Photos Of Papago Yucca Baskets
See Article About Devil's Claws

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.

Stigma lobes and central stigmatic orifice of the Mojave yucca (Yucca schidigera). The female yucca moth (Tegeticula yuccasella) forces a little mass of pollen down into the orifice so that it makes contact with the recessed receptive area, thus pollinating the plant.

Feathery stigma lobes and central stigmatic depression of the chaparral yucca (Yucca whipplei). The female yucca moth (Tegeticula maculata) presses a pollen mass into the central stigmatic orifice, thus pollinating the plant and insuring seed production and food for her larva.

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."

A female yucca moth (Tegeticula maculata) in the upright pollination position on the pistil of Yucca whipplei. She forces pollen down into the central stigmatic depression, thus pollinating the flower.

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.

Close-up view of the head of a female yucca moth (Tegeticula maculata) showing the prominent, coiled, maxillary palpi. She uses her prehensile palpi to collect, compact and carry a pollen ball (pollinium). These enlarged, specially adapted palpi are absent in the male yucca moth.

Several pollinia (pollen masses) and a stamen from the flower of chaparral yucca (Yucca whipplei). The 2 anther sacs at the top of the thick stalk contain little pollen masses called pollinia (2 inside each anther sac). The female yucca moth (Tegeticula maculata) collects up to a dozen pollinia and forms them into a golden ball of pollen.

More Photographs Of Yucca Moths & Yucca Weevil

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.

Close-up view of the abdomen of a female yucca moth (Tegeticula maculata) showing the slender ovipositor used to insert an egg into the ovary of a Yucca whipplei flower.

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.

Cross section of the ripened ovary of Yucca whipplei showing 6 columns of flattened black seeds (2 columns per carpel).

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.

See The WAYNE'S WORD Jumping Bean Article

Longitudinal section of the seed capsule of Yucca whipplei in October showing the larva of a yucca moth (Tegeticula maculata) inside its feeding cavity in one of the seed chambers.

See Close-Up View Of Larva Inside Joined Seeds

Cross section of the seed capsule of Yucca whipplei in October showing the robust larva of a yucca moth (Tegeticula maculata).

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.

The sand-covered, silken cocoon of the chaparral yucca moth (Tegeticula maculata). The preputial larva remains inside this subterranean cocoon during the winter months. In spring, when yucca plants are once again in their blooming cycle, the larva undergoes pupation and soon an adult moth emerges from the ground.

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.

A dead, basal leaf rosette and flower stalk of chaparral yucca (Yucca whipplei) at the end of its life cycle. This species takes at least 6 or 7 years to bloom and then it dies.

The ashy remains of a yucca plant (Yucca whipplei). During the heat of a smoldering fire, the leaf rosette and caudex has been converted into white ash.

References About Yucca Pollination:

  1. Armstrong, W.P. 1999. "The Yucca and Its Moth." Zoonooz 72 (4): 28-31.

  2. Cox, G.W. 1981. "The Yucca With the Big Bang." Environment Southwest Number 493: 12-16.

  3. Hogue, C. L. 1993. Insects of the Los Angeles Basin. Natural History Museum of Los Angeles County.

  4. Powell, J.A. and R.A. Mackie. 1966. "Biological Interrelationships of Moths and Yucca Whipplei." University of California Publications in Entomology 42: 1-59.

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