Look in moist forests along the coast of California and you might find Aquilegia eximia, or Van Houtte’s columbine, a pretty, reddish-orange flower. “The columbine is a big beautiful plant that grows in nice little streams; there are always hummingbirds buzzing around and lots of greenery in these little seeps during the dry, golden California summer,” says Eric LoPresti, whostudies the interactions between plants and insects at the University of California, Davis.
LoPresti is interested in A. eximia not because of its beauty, but because of a not-so-beautiful aspect to the plant: It’s covered in carrion. The plant is a “sticky columbine,” and it traps hundreds of tiny arthropods on its stem, leaves, flower and other parts. The plants are also host to a variety of omnivores, scavengers and predators — bugs and other insects that eat the dead arthropods trapped on the plant’s sticky surface.
LoPresti and his colleagues suspected that there was more to the relationship, and that the plants might be getting a big benefit from covering themselves with dead arthropods. The columbines, they hypothesized, might be luring the tiny arthropods to their death — like sirens in classical mythology — to attract bigger critters that keep away or eat the plant’s main herbivorous predator, the Heliothis phloxiphaga caterpillar.
The researchers began by testing whether the plant could actually lure arthropods to it. They took pieces of the A. eximia, a bit of stem and a leaf, and placed them in a Petri dish covered in a plastic mesh and a sticky substance. Similar dishes were set up with no bits of plant but the same covering as a control. The researchers left the dishes in a meadow near where the columbine grows. Dishes with plant bits trapped about 21 percent more arthropods than the empty ones. The plants do appear to be luring in the arthropods, possibly with a scent or other volatile chemical, LoPresti says.
The team then removed carrion from some columbines and in the weeks afterward, compared them with plants that hadn’t had any arthropods removed. Plants that still had dead arthropods had about 74 percent more predatory insects, and they were much less likely to be damaged by the caterpillars. When there was no carrion, plants were 121 percent more likely to have damaged reproductive structures. The siren song has definite benefits for the plant, the researchers conclude in the November Ecology.
Insect entrapment is a common feature in the plant world. A few species are carnivorous and eat what they kill, but most are not. LoPresti and his colleagues suspect that many of those may be like A. eximia, using their sticky nature as a defense. “I'm not sure whether other plants rely on it as much as [the] columbine,” LoPresti says, “but I suspect many derive some benefit from scavenging predators.”