Sticky Plants Protect Themselves with a Deadly 'Siren Song'

In the forests of Northern California, a plant called Van Houtte’s columbine always draws a crowd. It’s a beautiful plant, though not quite as charismatic as a towering redwood or ancient bristlecone pine. But that’s fine, because its visitors aren’t people looking for photo ops, but hummingbirds and bees coming for nectar and pollen, and plenty of insects in search of a meal.

Some of these bugs are a problem for the plant, like the moth caterpillars that munch on its buds and flowers. Others are helpful, scavengers and predators like stilt bugs and assassin bugs that eat the caterpillars and other herbivores. And then there are the insects that show up to the plant seemingly for no reason at all. They don’t live on the columbine and don’t come to eat the plant or other insects, and all they get for their visit is a slow death. These visitors, or “tourists,” as entomologist Eric LoPresti calls them, come by only to get stuck and die in the fine, sticky “hairs” called trichomes that cover the plant.

The plants are literal tourist traps, and LoPresti shows in a new study published recently in the journal Ecology that their victims don’t just show up and get caught by accident, but are lured in so that the columbines can attract all those predatory bugs that protect them from caterpillars. 

Quiet as they seem to us, many plants are chemical chatterboxes, communicating with each other and with insects through chemical signals. Some signals warn other plants of danger and prompt them to mount a defense. Others are used recruit helpful insects that act as bodyguards. LoPresti suspected that’s what the columbine might be doing, but in a more roundabout way. Instead of calling directly to predators for help, he thought, they lure innocent tourists to their deaths with a chemical “siren song” and then use their bodies as bait to entice predators to hang around.

COLLECTING HUNDREDS OF TOURIST CORPSES

To test that idea, LoPresti ran two experiments at a nature reserve where Van Houtte’s columbine is plentiful. First, he and his team wanted to see whether the dead tourist bugs did what they thought they did and attracted predators that helped the plants. Last July, they found an isolated group of columbines in a forest and pulled all of the dead bugs off of half of them every few days. The other half they left alone, letting them collect hundreds to thousands of tourist corpses. They did this for three months, keeping track of the number of trapped tourists, predators and caterpillar-damaged parts on each plant as they went along.

They also tested whether the tourists just had bad luck and showed up on their own or if they columbine was really luring them. In a meadow where the columbines grow, they clipped some leaves and other pieces from the plants and put them in petri dishes covered in a plastic mesh. They laid these petri dishes out along the side of the meadow, alternating them with empty ones. A day later, they returned to see how many bugs got caught in the mesh.

With both of their predictions, LoPresti and his team were right. In the forest, the plants with the trapped bugs on them had 74 percent more assassin bugs and other predators crawling on them than the ones that had their tourists removed. These bugs ate the caterpillars or scared them off, and the plants that hosted them had much less damage to their parts than the tourist-free ones. Meanwhile, the petri dishes that had columbine bits in them had 21 percent more bugs trapped in their mesh than the empty controls. Because the mesh hid the plant parts from view, LoPresti says, the attraction is very likely due to volatile chemicals the plants release.

The researchers say their results demonstrate a “‘siren song’ indirect defense” that relies on drawing tourists that then draw predators. While direct attraction of bodyguards is a common plant defense, the team writes, a plant using a middleman like this is a first.

LoPresti thinks that other plants could be using the same defense as Van Houtte’s columbine, but haven’t been reported by scientists. Looking through other studies, they identified sticky insect-trapping plants in 49 different plant families, most of them non-carnivores that wouldn’t benefit directly from attracting bugs and being covered in their corpses. The researchers want to do similar experiments with some of these other plants to see if they’re also defending themselves indirectly by inviting tourists to hang around, but never letting them leave alive.