By Andrew Stuart
Tall ‘Spanish daggers’ stand in the Big Bend desert, their limbs extended in expressive gestures. Another variety – the Great Plains or “soapweed’ species – dots the Llano Estacado.
Yucca plants are found across much of the Americas. But they flourish in West Texas, and are among the most striking of plants in our plains and deserts.
They’re also part of one of nature’s truly remarkable pairs. Yuccas and yucca moths are entirely dependent on one another. In the weeks to come, the drama of that bond will play out in the West Texas night.
Charles Valentine Riley was the first to study it deeply, in the 1870s. He was awed.
“She works with a vigor that would indicate combined pleasure and purpose,” Riley wrote. “There is between [her] and [her] food-plant a mutual interdependence which excites our wonder, and is fraught with interesting suggestions.”
Pollination often has a sidelong quality, as when pollen clings to the body of a bee, and is transported among flowers as the bee feeds on nectar. What Riley observed was very different. Female moths, using mouthparts or “tentacles” uniquely suited to the task, gather yucca pollen into balls. Then, “full of apparent purpose and deliberation,” he wrote, they thrust that pollen into the tubes of other yucca flowers – performing the work of pollination.
David Althoff leads a lab at Syracuse University. Biologists, he said, are still puzzling out the “interesting suggestions” of this relationship.
“It’s called an obligate pollination mutualism,” Althoff said. “What really makes people excited about it is the idea that two different species – we have plants and insects here – seem to be at some level cooperating, in that they’re actually getting mutual benefits out of the relationship.”
For yuccas, it’s pollination – and seeds for a new generation. And the moth? She’s not feeding – yucca flowers produce no nectar. Instead, she’s providing for offspring she’ll never see.
After pollination, the moth lays her eggs in the yucca flower. Her larvae will feed on some, but not all, of the developing yucca seeds.
The moth’s egg-layer, or ovipositor, is uniquely adapted to its task.
“I usually think of them as having like a long hypodermic needle,” Althoff said. “They have this needle thing that pushes through all that tissue, and they have to be long to get past the fruit wall and next to the seeds.”
Weeks after the parent moth has died, yucca-moth caterpillars – often bright orange – chew through the fruit, and fall to the ground. They burrow – up to a foot deep. There, they’ll spin cocoons. They may emerge the following spring. But in arid regions like ours, where yuccas may not bloom every year, they can remain underground for a dozen years. What cues moths to emerge at just the time when yuccas are blooming is a mystery, Althoff said.
Yucca moths are the sole pollinators for almost all yuccas. But the relationship has its antagonism – and yuccas have developed mechanisms to ensure the arrangement isn’t abused. If too many eggs are laid in a yucca flower, the plant “abscises,” or drops off, that flower.
That defense, in turn, has triggered an evolutionary end-run. In West Texas, there are two “cheater” moths. They don’t pollinate, and they lay their eggs late – when the yucca can no longer abscise its fruit.
It’s an evolutionary interplay that boggles the mind. West Texas has half a dozen yucca species. Each could have its own yucca moth.
The relationship is a window into how species develop. In his research, Althoff has found that moths can pollinate yuccas other than their natural host species. But their larvae don’t survive.
“What it suggests is that – you have the behaviors and tentacles to be a pollinator, and you recognize any yucca species and you can pollinate it, but it might be at the level of being a seed-feeder, that there are chemical defenses across different species of yuccas that prevent them from actually using lots of different species,” Althoff said.
Yuccas are famously filled with saponins, soapy compounds, which are toxic to insects. It could be that each moth species has evolved to tolerate the specific saponins in its host plant.
Yuccas’ creamy white flowers are a sign of spring in West Texas. If you see the blooms this year, think of the ancient dance that will unfold when the sun sets.