Yucca Pollination: a Story of a Synchronized Relationship

Honey bees get most of the press as far as pollinators are concerned. Sometimes butterflies and bats receive honorary mention. Usually only in certain circles do non-honey bees, essentially all bees save for the European brand that we raise in apiculture, take the stage. Yucca moths not so much.

From a botanical and evolutionary standpoint, yucca plants and yucca moths are an idol pair, celebrities among examples of obligate mutualism. As Olle Pelmyr included in the scientific article “Yuccas, Yucca Moths, and Coevoluation: a Review:”

The relationship between yuccas and yucca moths, characterized in an 1877 letter from Charles Darwin as “the most remarkable example of fertilisation ever published.”

Yuccas grow a habit resembling a porcupine or a fleshy mace—your pick—and sport a flowering spike easily several feet tall. Some yuccas resemble a succulent bush, but many are trees, including the infamous Joshua Tree of Joshua Tree National Park in California. Regardless of what a yucca looks like, each species pairs with a species of yucca moth that no other plant pairs with, even other yuccas.

Yuccas, consisting of two genera in the Agavaceae family called Yucca and Hesperoyucca, are native to the North American southwestern deserts and central plains. Two species living in the southernmost range actually grow in the rainforest and survive on the branches of other plants and rocks—essentially, the desert within the rainforest.

Yucca spike

Yucca spike of infloresences (and a complementary but irrelevant lady beetle) on the range in the Oklahoma panhandle.

The pollinating yucca moths consist of the two genera Parategeticula and Tegeticula and belong to the Prodoxidae family. Parategeticula larvae burrow into the developing yucca fruit that result in the placental tissue and immature seeds to form a cyst-like structure around the moth larvae, allowing them to eat from the inside. Tegeticula are sometimes referred to as “cheaters” because the moths may deposit their eggs after the yucca flowers are pollinated and transforming into fruit.

Female yucca moths have elongated mandibles that collect pollen from the yucca inflorescence, compact the thousands of pollen grains into a ball and carry it under her head to another flower, usually on another plant individual, to pollinate the site where she then decides to lay her eggs. Such sites are usually in the ovaries of flowers that just entered their first day of bloom.

Tegeticula

Female Tegeticula spp. deposits a pollen ball onto the yucca flower’s stigma (female fertilization site) after carrying it under her head. (photo courtesy of Sherwin Carlquist–awesome informative website on plant discoveries!)

A peculiar trait of the larvae is their tendency to remain masticating and residing in the yucca fruit capsules until the environmental conditions are just right: rainfall.

During a rainstorm the larvae make their way for the roots of the yucca, and there they prepare silken cocoons and enter a form of hibernation that may last for a year or many more. However many the years, they emerge when most of the plants have begun to flower.

This means the yucca moths have achieved an extremely accurate biological clock—to hatch from pupae stage in time to select first-night flowers during the few days an adult moth lives requires considerable local and species-specific adaptation. Moths can’t miss this flowering window and pollinate and oviposit (lay eggs) whenever the next yucca species blooms. Each yucca species blooms at disparate times of the year—the moths have to be virtually synchronized with the one host species in order to exist.

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