The serpentine columbine, Aquilegia eximia, has a fascinating and macabre strategy for defending itself from pesky plant-eaters. Rather than emit toxins or mustard oils, this plant lures innocent insects to their death with sticky little hairs. These corpses attract predators that come over and prey on the plant-eating caterpillars before they get the chance to prey on the columbine. The findings were published in Ecology this week.
In June, you might find hundreds of hapless insects trapped on a single stem of serpentine columbine, also aptly known as sticky columbine. Their little hairs, called trichomes, have glands that secrete gooey droplets at the tips. But unlike carnivorous sundews, the sticky columbine has no intention of eating and digesting the ensnared insects. Rather, as the plant becomes more and more coated with corpses, predators like spiders start making their way over. As they feast on the dead bodies, the spiders end up eating the larvae and eggs of Heliothis phloxiphaga moths. These caterpillars like to munch on the columbine’s buds, flowers, and fruits – all the tasty reproductive parts.
But are these plants actively attracting and killing insect passersby (dubbed “tourists”) in order to feed predatory insects and spiders who do them this huge favor? To investigate, a trio of researchers led by Eric LoPresti from the University of California, Davis, removed all the carrion from about two dozen sticky columbine plants. This resulted in a decrease of predator numbers and an increase of plant-eating caterpillars and subsequent plant damage.
Heliothis phloxiphaga eating a flower bud of Aquilegia eximia, the serpentine or sticky columbine in Lake County, CA. Eric LoPresti
In another experiment, LoPresti created little sticky traps using petri dishes, mesh, and a tiny amount of columbine stems and leaves. After 24 hours out in the field, those petri dishes contained far more insects than empty, unbaited dishes covered with sticky mesh. These were the same flies, wasps, and beetles as the ones found on the plants themselves. With the plastic mesh, the insects couldn’t see the plant parts inside, which suggests that the columbine sends out chemical signals that attract tourist passersby.
Carrion entrapment, they conclude, is an active process, and this is the first time we’ve seen an indirect defense against predators in plants. Though the team suspects there are plenty more examples out there. When they surveyed the literature on insect-trapping sticky plants, they were able to compile 110 different genera in 49 families, including both carnivorous and non-carnivorous plants. “The coolest part about the commonness is that it is not just one evolutionary lineage,” LoPresti tells Inkfish. Flypaper plants seem to have evolved more than 100 separate times.