In the U.S. southwest, Joshua trees evolved a rare, fussy pollination scheme
C. I. Smith
A year when vandals trashed a Joshua tree in a national park during a U.S. government shutdown is a good time to talk about what’s so unusual about these iconic plants.
The trees’ chubby branches ending in rosettes of pointy green leaves add a touch of Dr. Seuss to the Mojave Desert in the U.S. Southwest. Its two species belong to the same family as agave and, believe it or not, asparagus. And the trees bloom with masses of pale flowers erupting from a branch tip.
“To me [the flowers] smell kind of like mushrooms or ripe cantaloupe,” says evolutionary ecologist Christopher Irwin Smith of Willamette University in Salem, Ore. His lab has found a form of alcohol in the scent that actually occurs in mushrooms, too.
It’s tough to tell how old a Joshua tree is. Their trunks don’t show annual growth rings the way many other trees do. The desert trees became headline news in January when vandals trashed at least one of them at Joshua Tree National Park (SN Online: 1/12/19).
What gets biologists really excited about Joshua trees is their pollination, with each of the two tree species relying on its own single species of Tegeticula moth. That could make Joshua tree reproduction highly vulnerable to climate change and other environmental disruptions.
Typically, insects pollinate a flower “just by blundering around in there” as they grope for pollen and nectar for food, Smith says. But for the female moths that service the Joshua trees, pollination “does not look like an accident.”
The moth isn’t sipping nectar. Joshua trees’ glands no longer work. And moth offspring don’t eat pollen. However, the moth will lay eggs that hatch into caterpillars that will need to eat the seeds that form inside the pollinated flowers. So the moth climbs into a Joshua tree blossom, unfurls long, semi-translucent tentacles from her mouthparts and collects tree pollen into a heavy, yellow wad that she tucks under her head. When she reaches another flower, her tentacles deliver some of the pollen load to fertilize that flower’s ovules.
The moth pollinating the Yucca brevifolia species of Joshua tree, which occupies the western part of the Mojave Desert range, is considered a different species from the moth pollinating the Y. jaegeriana trees toward the east.
Smith spent some 10 years trying to observe the moths in action, but even catching a glimpse was tough. Pollination happens when flowers have opened just enough for a tiny moth to slip inside.
A breakthrough came from undergrad William Cole, who was “exceptionally patient and very slow and very quiet,” Smith says. After a couple of days of watching and snapping pictures on a 2015 field trip, Cole returned to camp one evening with some video.
It showed a small, eastern moth positioning herself inside a flower. Her hypodermic-like, egg-laying organ snakes downward to where the flower’s ovules lie. Smith had been wanting this evidence for his studies of how the trees and moths evolved codependent lives. “I was rolling around on the ground and cackling and yelling,” he says. “Finally, finally, finally we had … video.”
FERTILIZING A FLOWER Wedged into a Joshua tree flower, a female Tegeticula antithetica moth uses long, skinny brownish tentacles to scoop yellow pollen from a wad she has gathered under chin. She then spreads the pollen onto the receptive female surface of the bloom.
W.S. Cole, Jr. A.S. James and C.I. Smith/Annals of the Entomological Society of America 2017
W.S. Cole, Jr. A.S. James and C.I. Smith. First recorded observations of pollination and oviposition behavior in Tegeticula antithetica (Lepidoptera: Prodoxidae) suggest a functional basis for coevolution with Joshua tree (Yucca) hosts. Annals of the Entomological Society of America. Vol. 110, July 2017, p. 390. doi: 10.1093/aesa/sax037.
L. Hamers. Here’s how the record-breaking government shutdown is disrupting science. Science News Online. Published January 12, 2019.
C. I. Smith. Coevolution of Joshua trees and their pollinators. Mojave National Preserve Science Newsletter. April 2010, p. 4