This Plant Covers Itself in Insect Corpses as a Grim Defense Mechanism
Unpleasant, sure, but also completely genius.
Image: LoPresti et al
It's a shrewd but certainly dark adaptive strategy. The serpentine columbine, a pretty herbaceous plant endemic to California's wet coastal regions, doesn't defend against or attack its enemies directly. Instead, it sends out a chemical signal, which attracts random nearby bugs who then detour to the columbine to check things out but suddenly find themselves ensnared on the plant's "sticky" surfaces—which are leaves and appendages coated with layers of hairlike barbs. These insect passerby, known to biologists as "tourists," are trapped and eventually die,
The result is that the columbine winds up with a beneficial coating of death. This sheen of corpses is more properly referred to as carrion and it serves to attract carnivorous bugs and spiders, who then unwittingly protect the plant by attacking and-or repelling herbivores that would otherwise pose a threat to the columbine. This strategy, described in the current issue of Ecology, is the only indirect defensive mechanism of its type that's so-far been observed, though the researchers behind the report note that it may be quite common.
The general idea is known as carrion provisioning. "Predatory arthropods may seek out this carrion ephemerally or remain on plants with a reliable carrion food source," the UC Davis-based researchers write. "Predators then either repel (a nonconsumptive effect) or kill (a consumptive effect) herbivores."
These poor insects—innocent sailors of the California air—are somehow drawn to their deaths on the columbines.
The UCD biologists, led by Eric LoPresti, conducted two field experiments. The first was to test out whether the columbine indeed attract tourist insects through some chemical mechanism, and the second to determine whether or not the carrion actually has a beneficial effect. The first portion consisted of laying columbine leaves underneath an artificial sticky mesh where they were visually hidden, with the idea being that if insects were attracted to the mesh it would indicate the presence of some "volatile cue" (a scent, basically). Indeed, the columbine leaves attracted significantly more bugs than the non-columbine control leaves. They couldn't see the leaves, but they knew the leaves were there.
LoPresti calls it the "siren song" of the columbine.
The experiments were devised somewhat on the spot, as LoPresti was already in the field working on a different experiment that wasn't going so well. In the process, he came across a sticky columbine covered in dead bugs and had a flash of an idea. "Having petri dishes, plastic mesh and tanglefoot in my field kit," he writes. "I made little sticky traps, with a sticky mesh top and a petri dish bottom and I put either columbine stems and leaves 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."
The second part of the experiment consisted of removing some of the carrion from columbine test subjects and then later checking up to see how the carrion-poor plants were doing relative to the carrion-rich plants. The carrion-poor plants were 121 percent more likely to have incurred damage to their reproductive structures than otherwise. So, the death-sheen adaptive strategy is clearly successful.
Still, it's unlikely that the columbine is entirely unique. As the researchers note, while examples of carrion-based defenses are being reported at a very fast rate, "the great majority of insect-entrapping plants have not been examined for this indirect resistance."
The siren song seemed like a natural metaphor, LoPresti recalls: "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?"