Stop and Smell the Evening Primroses, By Tania Jogesh, PhD Postdoctoral Researcher Chicago Botanic Garden

Plants are constantly conversing with the world around them; however instead of words they use a prolific array of chemical signals.  Perfumes and the pungent flavors in spices are all chemical compounds that have evolved to attract pollinators or ward of insects from eating them. In our research, we try to decipher why plants produce different floral scents and in particular we are interested in how interactions with insects have shaped the different scents produced by plants. 

We are a travelling circus of field biologists comprised of a postdoc (or two) and 2 to 3 research technicians. We live and work out of a research truck and travel throughout the western US to collect data on the floral scent and ecology of 16 species of evening primrose (family Onagraceae).

What brings us to the beautiful Tejon Ranch is the enigmatic Oenothera californica avita –the California evening primrose.

Our days at Tejon start at dusk, when the sunlight is almost gone. This is when buds of O. californica avita begin to open to reveal sweet scented, pale-white flowers. In a frenzy, we run from plant to plant to identify all buds which are likely to split. Before the flowers have opened and released their scent we enclose them within a plastic oven bags. We then attach a vacuum pump that will pull the scent away from the flower and into a filter that will trap the volatile scent compounds.

 Photo credit: K. Skogen

Many floral scents have primarily evolved to attract pollinators.  Scent is frequently coupled with sugary nectar so that pollinators associated the scent with a reward. The plant is rewarded in return by having its pollen dispersed to other mates– a mutualism that facilitates sexual reproduction in over 90% of all flowering plants.

So who are these visitors attracted to the scent of these evening primrose flowers? At dusk, after the scent apparatus is setup and turned on, we each find a patch of flowers to sit down next to and watch who visits.  Looking at the flowers’ long floral tube and pale white petals, we suspect that the most likely visitors hawkmoths. Hawkmoths are large moths in the family Sphingidae that hover like hummingbirds and have long tongues adapted for collecting nectar from long-tubed flowers.  Hawkmoths are large moths that make an attractive food source for birds, hence they like to fly in the relative safety of dusk.  For this reason many hawkmoth pollinated plants will typically open in the evening when moths are most active, hence the reason for the “evening” in the name evening primrose.

As we sit and wait, as if perfectly orchestrated, within five minutes of the flowers opening, we see 5 or more hawkmoths fly in and systematically visit every single flower in our view.

Photo credit: K. Skogen

After watching hawkmoths for an hour, we go back to our flowers to collect the filters and flowers and head back to camp. Late into the night, we elute the volatile compounds from the filters into little vials that will be shipped to Cornell University for analysis. This collection method allows us to examine each individual component that makes up the complex scent profile of the evening primrose scent. Using this information we can see how scent composition varies by species, by population and even within plants of one population.

While we predominantly think of floral scent for its role as a pollinator attractant, the signal can be picked up by others.  Much like a birdsong can attract mates and predators, floral scent that is meant to invite pollinators can also signal to potential herbivores that a tasty plant is close.  We think that scent might vary based not only on which pollinators are in the area, but also whether or not herbivores are present to eavesdrop on the flower’s invitation to pollinators.

In the Colorado evening primrose, Oenothera harringtonii, a few of populations produce scent without a compound called linalool. Remarkably, the presence of linalool was found to be correlated with the presence of a tiny caterpillar in the genus Mompha that was eating the seeds.  These caterpillars feed inside developing fruit and reduce the reproductive output of these plants. Thus populations historically associated with high numbers of these moths might be under selection to reduce their advertisement to pollinators to reduce herbivory.

However Mompha isn’t the only problem faced by these evening primroses plants. An interesting dilemma for these plants is that one of its best pollinators, a hawkmoth called Hyles lineata, also uses these plants to raise its young. The Hyles lineata caterpillars can chomp on the buds, flowers and fruits leaving behind a significant trail of damage!

Photo credit: R. Overson

The morning after scent collection we take an inventory of all herbivores on O. californica avita. We look for Hyles lineata eggs and caterpillars and check buds and fruit for Mompha. This year we found no herbivores on these plants. Is this population truly free from herbivory? Or could 2015 have been a bad year for Mompha and Hyles caterpillars in Tejon? Our methods only give us a snapshot into the ecology of O. californica avita at Tejon and we can only speculate on the absence of herbivory until we learn more.

After two nights of data collection, we head out on our next adventure – to find a new population, in a new landscape, to get a glimpse into the ecology and evolution of another evening primrose!

ABOUT THE AUTHOR

I am a biologist interested in the ecology and evolution of chemically-mediated plant-insect interactions. I am currently a post-doctoral researcher at the Chicago Botanic Garden in Glencoe, IL working with Krissa Skogen and Jeremie Fant on floral scent-mediated diversification of plants in the evening primrose family. I graduated with a PhD in entomology in Dr. May Berenbaum's lab where I worked on the ecology and evolution of wild parsnips and parsnip webworms in New Zealand. 

Twitter: @TaniaJogesh

Website: taniajogesh.com

Project twitter: @FlowerMoths

Project Flickr: LandscapesOfLinalool

Project website: onagmoth.org