NATURE NOTES:
The Evening Primrose is the Center of an Aromatic Drama in the Desert Night

photograph courtesy Krissa Skogen. The white-lined sphinx moth, or hawk moth, is the primary pollinator for many evening primrose species. The plant uses light-colored flowers to increase its visibility in the dark, but scent – sweet and aromatic – is a critical way it attracts moths in the Western night.

Poet Rita Dove wrote that the plants “wait until the world’s tucked in and the sky’s one ceaseless shimmer – then lift their saturated eyelids and blaze, blaze all night long for no one.” A century and a half earlier, the English Romantic John Clare claimed that, “hermit-like, shunning the light, it wastes its fair bloom upon the night.”

Evening primroses bloom at dusk – and it might seem their beauty is squandered on the night. But in truth there’s no “waste” in the primrose’s blooming – or in the often sweet aroma of its white, yellow or pink flowers. A group of botanists have been probing the plant’s mysteries in our region, and across the West. They’ve discovered a nocturnal drama of attraction and danger, in which scent plays a central role.

Krissa Skogen, of Clemson University, and Jeremie Fant, of the Chicago Botanic Garden, are members of a botanical team that, with funding from the National Science Foundation, is studying evening primroses – plants of the genus Oenethera.

“It’s the bias of when humans are active,” Skogen said. “And it can be tricky, and tiring, to try and collect these data at night.”

“We know very little about what goes on at night,” Fant said. “It’s crazy to think, but there must be so much activity at night. What goes on at night is just beyond us, and even if we tried to watch we wouldn’t see everything.”

Pollination – in which plants use the activity of other creatures or forces to reproduce – is a foundation of life on Earth. And in recent years, it’s been the focus of increased public awareness. Attention has centered especially on the stark decline of honeybees, on which many of our own staple crops rely.  

But there’s another, shadow world of pollination, beyond the daytime business of the birds and bees. In West Texas, bats are critical pollinators for agaves. And, despite Rita Dove’s poetic phrase, evening primroses don’t “blaze for no one.” Their blooms are meant for one nighttime rambler in particular. 

You’ve likely seen it making its midnight creep, perhaps near a porch light. The white-lined sphinx month has a wingspan of 2 to 3 inches. They’re known as “hawk moths” or “hummingbird moths.” And they’re the primary pollinators for most evening primrose species.

The plants’ light-colored flowers increase its visibility at night. And it produces abundant nectar, making it an attractive food source for the moths. But a central way it advertises its presence in the dark is through scent. Many evening primroses produce a compound called linalool – which has a sweet odor, akin to the smell of Fruit Loops.

Skogen and Fant began to study these plants more than a decade ago, focused on a Colorado species known as the Arkansas Valley evening primrose. They found a complex nocturnal dance.

Hawk moths appear to be thoroughgoing vagrants, that wander long distances across the landscape. That’s a boon for the evening primrose – hawk moths pick up pollen as they nectar, and spread the plant’s genetic material far and wide. But there’s a price: the hawk moths lay their eggs on the plants, and the emerging larvae feed on their host. 

And they learned that the plants’ intoxicating scent was attracting unwanted visitors: “micro-moths,” of the genus Mompha. These moths, a centimeter or less long, don’t pollinate the plant, but they do lay eggs that later predate upon it.

For some plants, the downside of their perfume appears to be too great. The botanists found that some evening primrose populations were not producing linalool – and had thus reduced Mompha infestations.

“The whole interaction is, these plants smell great,” Fant said. “They’re clearly attracting pollinators, but they’re also attracting predators. So this plant has to play the game of, make myself attractive, but it also in doing so makes itself vulnerable. The scent that we love is also what allows them to be attacked – it’s a double-edged sword.”  

Skogen and Fant recruited other experts – and expanded their focus.

“A lot of the work we ended up doing and the species that we focused on took us to New Mexico, West Texas, Arizona and California,” Skogen said, “and we ended up collecting data on six different species in West Texas and adjacent New Mexico.”

They’ve collected data on evening primrose scent and pollination in Big Bend, Guadalupe Mountains and Carlsbad Caverns national parks. It could shed light on the role aroma plays in the push and pull of attraction and antagonism. For even as we sleep, the drama of the natural world continues.