How a Lying Plant Steals from a Thieving Fly

Most plants are upstanding citizens, fairly trading nectar for pollination service from insects and other animals. Some of them, though—an estimated 4 to 6 percent of all flowering plants—don’t hold up their end of the transaction. They lure pollinators in with the promise of food, shelter or even sex, but don’t deliver on it. 

The smearwort (Aristolochia rotunda), an herbaceous plant found around the Mediterranean, is one of these liars. It dupes fruit flies into entering its flowers and then traps them there, getting pollinated without offering any reward. But the plant puts a twist on this deception by cheating another cheater. The species of flies that the smearwort tricks are thieves themselves, kleptoparasites that seek out and steal from the kills of insect predators like spiders and praying mantises. 

How does the smearwort catch a thief? Scientists used to think that its flowers imitated the flies’ egg-laying sites, but a new study suggests that the truth is a little stranger. The plant actually appears to mimic the chemical cues that attract the flies to their ill-gotten meals.

When some insects are attacked, they release chemicals from their scent glands that repel predators and signal danger to other members of their species. These chemicals are what the flies follow to the kill sites that they rob, and as German biologist Birgit Oelschlägel discovered, they also share some components with the scents released by the smearwort’s flowers. When Oelschlägel and her team isolated some of these compounds, they found that same ones drew the flies to both freshly-killed bugs and the smearwort. In other words, the plant promises a corpse that isn’t really there in order to exploit a fly that showed up so it could steal food from a spider. Ain’t nature grand?