Wednesday, July 22, 2015

Sticky plant attraction, a new paper

I could not possibly do as good a job as the summary of this paper written by Elizabeth Preston, here, so I'll first tell the backstory - and when it comes out, I'll detail another cool part of it (which is not in the preprint version - instead buried in the appendices).

Heliothis phloxiphaga eating a flower bud of Aquilegia eximia, the serpentine or sticky columbine. Lake County, CA. 
Last summer, I spent most of my time studying Trichostema laxum, which I've written about in a previous post. I was trying to test my hypothesis that external chemical defenses are easily washed away by rain (which may be driving the pattern that few are found in wet areas) and spent countless hours doing various pieces of this - observing pollinators (the volatiles washed off might affect pollination positively or negatively), counting insects, leaves, buds, flowers and seeds (which can be done in situ!) and by July, the drought and some jackrabbits made this experiment look rather grim. I still haven't brought myself to analyze these data, as it ended so depressingly...

As the floral color polymorphism was not in the experimental population, I didn't notice it until the end of the summer, and gathered some, but not enough, data on it. Lake County, CA. 
I was getting a little frustrated and I wandered around, naturalizing, which is always a remedy for frustration (to me at least). I came upon a columbine - Aquilegia eximia - which I instantly knew held some potential for cool experiments. The first thing I noticed was that it was extremely sticky and covered in dead insects and the second was that it had a bunch of predators on it. I immediately thought to Billy Krimmel and Ian Pearse's cool paper on tarweeds (doi:10.1111/ele.12032) , in which they demonstrated that the dead insects provided food for predators, which protected the plant from herbivores. I figured initially that I would simply test this in another system.

A Hoplinus nymph (probably the most important predator in the system) approaches an
entrapped fly.
Because I was out in the field without access to a genetics lab to get dead flies, I couldn't replicate their design - where they added dead fruit flies to plants to supplement carrion - so instead I removed all the carrion from half of my 50 plants, hypothesizing that I would get a decrease in predators and an increase in herbivory (which we did!). I also thought hard about what else to test to add to Billy and Ian's work. I thought that, perhaps, it would be interesting to test whether the plants attracted the various entrapped insects (mostly small flies, wasps and beetles) somehow. Lots of plants attract insects - pollinators are the most obvious, but volatile signals attract predators, other herbivores and even birds (doi: 10.1111/ele.12177). Having petri dishes, plastic mesh and tanglefoot in my field kit - I made little sticky traps, with a sticky mesh top and a petri dish bottom and I put either columbine stems and leaves (a very small amount) or nothing in them. Collecting them 24 hours later, I found that the dishes with columbine had higher insects than the empty ones (which would demonstrate the ambient rate of insects landing on these traps). The trapped insects were also little flies, wasps and beetles, just like on the plants themselves.

Dead Hymenoptera on columbine (I may be giving up entomologist credentials, but I am not sure whether it is a wasp or an ant alate).

So this became a story - perhaps logically - that the plants were somehow attracting insects to kill and feed to the beneficial predators on their surfaces (retaining their services). I presented this work in a talk at a little student conference at Davis during recruitment weekend and played with several ways to frame the story. The first was to be rather dry - columbines attract insects and control a tritrophic defense (or something along those lines). Instead, I thought long and hard about trying to make a metaphor (socialism - a worker's paradise for the predatory bugs or a Roman bread and circus type thing, but they didn't really work) and while I don't remember how I came up with this - I settled on the sirens, figures of classical mythology who lured sailors to their deaths. I therefore framed it as these poor insects - innocent sailors of the California air - are somehow drawn to their deaths on the columbines. Of course, the columbines put the insects to good use in their defense, leaving open the question - which I am sure classical mythologists lose much sleep over - what did the sirens do with their collection of dead sailors?

Serpentine columbines in flower. Lake County, CA. 
Read more here!

Eric F. LoPresti, Ian Seth Pearse, and Grace K. Charles In press. The siren song of a sticky plant: columbines provision mutualist arthropods by attracting and killing passerby insects. Ecology.