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Planet Earth

Plants Murder Bugs to Pay Their Bodyguards

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It's not only carnivorous plants that bugs have to watch out for. Sure, if an ant tumbles into a pitcher plant or a spider stands in the open maw of a Venus flytrap, we know what's coming next. But certain innocent-looking plants—perhaps very many of them, even including ones in your own yard—murder hosts of insects that they have no plans to eat. They lure passing bugs into a slow death, then exchange their corpses with other insects for protection. One of these plants is the serpentine columbine, or Aquilegia eximia. Native to California, it grows downward-hanging blossoms in fiery hues. The plant is covered all over in fine hairs, or trichomes, each with a gluey droplet at its tip. Like flypaper, these sticky plants ensnare many of the tiny creatures that are unfortunate enough to land there. By June, a single stem of serpentine columbine may hold the corpses of hundreds of bugs, write UC Davis graduate student Eric LoPresti and his coauthors. These bugs have been called "tourists" because they don't live on the plant or need to be there; they're just making an ill-fated visit. (This makes the sticky plant, the authors note, an actual "tourist trap." Ahem.) These adhesive death traps are common in the plant world. Scientists have noticed that the corpse-covered plants can attract other, predatory insects that are looking for food. And while they're in the neighborhood, these predators might eat some of the plant's pests—critters that haven't gotten caught in the flypaper but are happily munching on the their host's leaves and flowers. One study of a sticky daisy found that these predators were helpful to the plant. But have the plants engineered the whole thing? Or is bedazzling themselves with bug bodies just a happy accident? LoPresti studied columbine plants growing in a California reserve to learn more. Researchers marked 50 plants, then used delicate forceps to remove every trapped bug from half the plants. They returned several times throughout the summer to pick off new corpses and to note whether the plants had been damaged by caterpillars. One caterpillar called Heliothis phloxiphaga is fond of chewing up the columbine's buds, flowers and fruit. When the researchers checked on the plants, they found 74 percent more predatory bugs (such as spiders, assassin bugs, and stilt bugs) on corpse-covered plants than on clean ones. These predators had presumably stopped by for the free meal, but were also killing caterpillars or eating their eggs. Plants with their corpses removed were more than twice as likely to have damage from pests. The predatory bugs were helping to protect columbines. But it still wasn't clear whether the columbine was as passive in all this as a piece of flypaper. So in a second experiment, the researchers placed petri dishes along a meadow. Half the dishes held pieces of columbine stem and leaf, and the other half were empty. Mesh covered all the petri dishes, making their contents invisible. Bugs that visited got trapped in the mesh, which let researchers count them later. After just 24 hours, dishes hiding columbine clippings had trapped 21 percent more bugs in their mesh than the empty dishes. This was "pretty unexpected," LoPresti says, "though it makes perfect sense." All kinds of plants are known to send out chemical signals. For example, leaves that are being chewed by pests may emit alarm cues that attract predatory insects, or that tell neighboring leaves to crank up their own chemical defenses. So it's not unheard of for plants to summon insects by sending scents onto the breeze. But it is news to scientists that plants like the columbine will lure innocent bystanders just to die on their stems. LoPresti calls the columbine's apparent chemical signal a "siren song." Come visit, it calls to passing bugs. There's no food for you here but it'll be nice anyway. You definitely won't be glued down and eventually eaten. Surveying the literature, LoPresti and his coauthors found more than 110 types of plants that trap bugs on sticky surfaces. A few of these plants are outright carnivores; they absorb nutrients from these bug corpses for their own use. But most, like the columbine, seem innocent at first glance. LoPresti wants to test more of these plants and find out whether they also use trapped corpses to indirectly defend themselves from pests. It's also possible that the stickiness is an accident in certain plants, he says, evolved for some purpose other than murder. "The coolest part about the commonness is that it is not just one evolutionary lineage," LoPresti says. The sticky plants aren't all relatives. Rather, flypaper plants seem to have evolved more than 100 separate times. They may all make different gluey compounds. But whatever siren songs they're sending out, bugs would be better off ignoring them. Image: Courtesy of Eric LoPresti

LoPresti, E., Pearse, I., & Charles, G. (2015). The siren song of a sticky plant: columbines provision mutualist arthropods by attracting and killing passerby insects. Ecology DOI: 10.1890/15-0342.1

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