Yuccas and Yucca Moths

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Yuccas and Yucca Moths

Marfa Public Radio

Broadcast on March 20, 2008

Yucca in bloom

Yucca - brought to you by yucca moths. (CDRI staff photo)

By Megan Wilde

The next time you come across the creamy white bouquet of a yucca plant, peek inside a blossom. The flower might be the daytime hide-out of a little white yucca moth. Without this moth, yuccas wouldn’t exist. And without yuccas, neither would yucca moths. What’s so special about this relationship? And is it unusual in nature?

Dr. Chris Ritzi, an entomologist at Sul Ross State University, explains that many plants are pollinated by insects, and some insects specialize in pollinating certain kinds of plants. But yucca moths are exceptionally specialized pollinators. Their entire lives revolve around yuccas, to the point that most of these moth species pollinate only one species of plant. In exchange for pollination, the yucca provides a safe hide-out and plenty of food for developing moth larvae.

This elegant dependency is an ancient one, and it’s especially important in the Chihuahuan Desert, where yuccas are a dominant plant in the landscape. The familiar spiky green towers of Spanish Daggers and Soaptree Yuccas dot many a hillside here, and every one of these plants depend on yucca moths for survival.

The relationship between yuccas and yucca moths is truly a win-win situation, Dr. Ritzi says. It begins with how the moths pollinate the plants. A female yucca moth uses special tentacles on her jaw to scoop up sticky yucca pollen, roll it into a tiny ball and stuff it under her chin. In the process, she punches a hole in the base of a flower—where the plant’s ovaries are—and deposits a few of her eggs inside. Then, she climbs the flower pistil, grabs a pollen ball from under her chin, and packs it into the stigma, deliberately pollinating the flower.

Dr. Ritzi says this is really unusual. Most insects brush pollen onto a plant’s stigma while looking for nectar, so pollination happens accidentally. But yucca moths literally put all their eggs in one basket when they lay their eggs in a yucca plant’s blossoms. They don’t want to leave anything to chance. So by forcefully pollinating a blossom, a yucca moth is making sure that the flower will continue to develop and that it produces plenty of plump seeds, which the moth’s offspring will need to survive.

After being pollinated, the yucca flower’s white petals eventually fall off, usually in late summer. Left behind are big green pods, inside which the moth’s eggs and the yucca’s seeds are growing and maturing. When the moth egg hatches, the nutrient-rich yucca seeds provide a smorgasbord for the newborn caterpillar. Fortunately, the tiny caterpillar has a modest appetite and only eats a portion of the seeds. This is the trade-off for the yucca – some of its seeds get eaten by the moth’s offspring, but without the moth’s forceful pollination, it wouldn’t have had any seeds to begin with.

Once the caterpillar has grown a bit, it chews a little hole in the yucca pod wall, grabs one last bite of seed, and then crawls out its escape hatch and either drops to earth or descends on a thread of silk. Scientists aren’t sure which. Then, the caterpillar burrows several inches underground and goes into a cocoon stage, hopefully before winter hits. The larva usually surface the following spring, though if it’s a bad year, they might wait longer. Either way, when warm temperatures and summer rains come, and yuccas begin to bloom again, the pupated yucca moths emerge from underground, and the cycle begins again.

Now, occasionally a pod will end up with a whole bunch of caterpillars inside, all the seeds are gobbled up, and obviously some moths have cheated. But amazingly, this rarely happens. There are a few theories that explain why.

One is that yucca moths leave chemical clues inside the flower when they’re pollinating it. Other moths detect these signals and know someone else’s eggs have already been deposited there.

Another theory is that the plants somehow know when the system’s being abused. Yuccas are known to shed pods, even ones that have been pollinated. So, it’s thought, if too many eggs are weighing down a pod, the yucca plant may drop it. The moth’s future offspring inside are also dropped and die, so those moths that cheat don’t end up getting to reproduce. Because of this, yucca moths have evolved to spread their eggs over multiple pods.

This relationship between yuccas and yucca moths is obviously mutually beneficial. It also seems like it might be precarious to be so tightly dependent on another species for survival. As a pollinator, wouldn’t it be safer to keep your options open, in case something happened to yuccas? But, Dr. Ritzi says this is simply an evolutionary choice. Many insect species opt for the security of being a generalist—they pollinate a variety of plants, or even a few different similar species. Specialists though, like the yucca moth, choose to get extremely good at what they do, in the hope that they’ll always have the option to do it. Both choices have their risks—a generalist can be out competed and a specialist can lose its niche.

The yucca moths’ niche is secure for now. Yuccas are plentiful not only in the Chihuahuan Desert, but also throughout the desert southwest and even up through the Great Plains into southern Canada. As yuccas have spread across these habitats over time, the moths have gone with them. But, in some places, such as Europe, yuccas grow only because humans transported them there, and the moths weren’t invited along for the ride. Yuccas in those distant places never produce seed.

Megan Wilde is the Nature Notes coordinator. Dr. Chris Ritzi is chair of the Biology Department at Sul Ross State University.

Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde@cdri.org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on March 20, 2008.

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