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Yucca Moths

December 19, 2007

After a rainy winter, the Mojave Desert blooms. Billions of waiting seeds, scattered a year ago or fifty by the desert’s annual plants, respond to a good wet season by sprouting, covering the earth with a pale green fuzz. To eyes accustomed to the red and brown of desert varnish on rock this can be a bit disconcerting, like a layer of mold on an orange. But leaves emerge from the mass of shoots, and within a few weeks the green is covered in blossoms. Evening primroses push out pale, paper-thin petals, impossibly delicate against the desert landscape. Globemallow hangs bright orange blossoms on its gray-green leaves. Indigo bush bears spikes of electric blue beanflowers, and the prostrate sand verbena holds disks of pink florets less than an inch wide. In the washes, thin arcing leaves — one per plant — emerge slowly from the sand. They feed the bulbs of the desert mariposa, Calochortus kennedyi, which send up vivid orange, three-petaled flowers in July.

Some of the desert bloom is not as colorful. Buds on a few scattered Joshua trees stir strangely, driven by climate, the time of year, and some unknown internal clock. Stalks emerge from the whorls of sharp leaves at the end of the trees’ branches. The stalks look vaguely like large asparagus spears. They extend about a foot and a half from their bases; some a little longer, some shorter. Short subsidiary stems emerge from their sides, some of them branched. Each stem holds flowers about two inches long. At first glance the flowers resemble orchids, three petals and three sepals — six “tepals” — opening only at the ends of the flower. When they reach full size, the flowers make a solid mass the length of the inflorescence. Their cream-colored flesh is thick and waxy, and if you pinch a blossom and eat it, the slight soapy flavor isn’t much of a hindrance to edibility. You can batter and fry the blooms.

Stick your face into a mass of Joshua tree flowers during the day, and the effect is less than overwhelming. The flowers have a pallid scent, somewhat floral, somewhat fungal; a bit like stepping into your grandmother’s closet. At night the flowers open, and the scent becomes more pronounced. Even then it is not particularly remarkable. That’s OK. The flowers aren’t talking to you. A conversation is taking place to which you are not privy. Your flashlight is far too bright to illuminate it, but if you have a circle of red cellophane handy — and who doesn’t? — slip that over the lens, let your eyes adjust, and look again.

Brown moths flit back and forth from each open flower, their wingspans a bit more than half an inch, a fringe of fine hairs on their wings. They crawl inside the flowers, engaged in some sort of work you cannot quite see. Sometimes they leave one flower, crawl into an adjacent one. Sometimes they come to the lip of a flower, then fly just out of range of your red light, and you cannot guess where they land.

They are yucca moths, Tegeticula, the Joshua tree’s chosen life partner. Yucca moths and Joshua trees have co-evolved a relationship from which neither can escape. They depend on one another. The yucca moth can’t reproduce without the Joshua tree. The yucca can’t reproduce without the moth.

On any given day over the last decade or so, I have had at least one woody-looking oblong pod in my home, often more than one, rattly tan husks of dubious appearance adorning desks and bookshelves. Now and then I will take one, pry it open with my fingers. It resists until it snaps open, scattering little black seeds all over the room. Those that don’t scatter stay in tight little rows within the husk, usually six of them arranged around the core, separated by dry, spongy cellulose. They fit together as if machined, and it almost seems a shame to break them out of their divisions.

One or more of those rows is almost always different, chaotic dull, seeds glued together with dark, congealed sawdust. Those seeds are perforated as if by a one-eighth inch drill bit, perfect circles gouged away on the broad sides of each seed in the row. Sometimes the drill bit is still there, a dingy gray-brown, segmented caterpillar eating its way through the row: a larval yucca moth.

When Joshua trees are ready to bloom, yucca moths emerge from the ground. They mate, presumably somewhere in the vicinity of the flower stalk, though no one knows for sure. The males are a little smaller than the females. When they have finished mating, the males go off to become food for night lizards and western pipistrelle bats. The females get to work.

Female yucca moths have a set of tentacles growing from their mouthparts. These tentacles are unique in the world of insect anatomy. They are unarticulated: they have no joints. The female moth walks to the anther of the Joshua tree flower, scrapes some sticky pollen from the anther with her tentacles, molds the pollen into a ball, then sticks it into a little cavity on the underside of her head, which would be just below her chin, had she a chin.

She walks further into the flower, to a swollen mass of waxy white tissue at the base: the ovary. She stabs it with her ovipositor, a sharp-tipped protrusion from her abdomen built like a hypodermic needle. She lays a number of eggs within the ovary in a set of structures called ovules.

She then climbs a short stalk, the style, to the stigma, the flower’s female reproductive surface. She takes a bit of her pollen and packs it into the stigma, fertilizing the flower. This act sets her apart from most other pollinating animals. Bees and bats and beetles pollinate by accident, transferring pollen that they collect inadvertently. The yucca moth pollinates Joshua trees as an act of volition, if moths can be said to have volition. She departs and finds another flower to fertilize and lay eggs in, and then another. After a few nights of this labor, she dies.

Joshua trees will shed flowers into which yucca moths have deposited too many eggs. The trees shed flowers if the moth has not packed enough pollen into the stigma, or if too much of that pollen came from the same flower, raising the risk of inbred seedlings. (The moths use less pollen in each flower than they collect. They will often tote a mix of pollen from many trees, reducing the risk of self-pollination.) If the moth has left a proper balance of eggs and pollen, the flower sets seed, each of the ovules becoming one of those tight-packed rows of black flakes. The eggs hatch. The larvae begin to eat.

Larvae usually destroy fewer than half the seeds in a ripe fruit. At the end of a row, they emerge, drop to the ground, and if they find themselves on desert soil rather than on my hardwood floor, they burrow into the ground. No one knows how deep they go. In the lab, they routinely get to the bottom of the soil container provided them, up to eight inches down. Wherever they stop burrowing, they spin a silken cocoon and go to sleep.

Though that sleep may last for years, there comes a time when something — soil moisture, temperature — spurs the larva to pupate. Shortly thereafter, adult moths emerge from the soil, look for their own kind and for flowering Joshua trees to pollinate. They don’t always find the trees in flower. Some years no Joshua trees in a given stand will bloom at all. Some years the Joshua trees bloom massively, and then a few lean years of no flowers follow.

Each species in this partnership pays a price. The moth totes up to a tenth its weight in pollen, and sends occasional entire generations out into the world without reproducing. The Joshua tree offers up a few dozen seeds in each fruit as a sacrifice. The individual moth, the individual seed matter little.

It seems a strange strategy, to tie your species’ future so wholly to the fate of another. If your timing is off, you could come out of the ground to a forest of non-blooming trees. There is an admirable blind confidence at work within the yucca moth, flinging entire generations into the sky and trusting that the trees will be there to catch them.

by Chris Clarke

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  1. December 19, 2007 at 10:19 pm

    Fascinating & lovely – look forward to the Joshua Tree book.

  2. December 20, 2007 at 11:41 am

    Very glad to see your work here, Chris, and the reading adds a lot to it. I’m looking forward to the book a great deal.

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