Yum! Flies swarm to a flower that smells like wounded ants

A Japanese flower lures in pollinators with a strange perfume, new research finds.

The weird scent belongs to Vincetoxicum nakaianum. It’s one of the flower species known as Japanese dogsbane. Botanist Ko Mochizuki of the University of Tokyo studies the plant. And he noticed clouds of grass flies hovering around its flowers.

The plant’s scent, he found, matches signals released by injured ants. The smell dupes the flies — which feed on dead ants — into visiting the blooms. Once there, the flies pollinate the plants.

Mochizuki described his finding in the Oct. 20 Current Biology.

A strange sight

Mochizuki first noticed the grass flies at the Koishikawa Botanical Gardens. That’s in Tokyo, Japan. The flies might be species that don’t hunt their own prey, he realized. Instead, they might look to steal food from other predators.

Say, for instance, grass flies see honeybees or plant bugs injured from an attack. The flies might then swoop in and take the easy meal for themselves. This behavior is called kleptoparasitism (KLEP-toh-PAIR-uh-si-ti-zem).

Grass flies swarming around a flower is “really weird and rare,” Mochizuki notes. These flies “are known to feed on damaged insects,” not nectar.

He suspected the flowers had tricked these insects by smelling like injured prey. When the flies land on the flower, they expect to swipe a snack. Instead they’ll end up pollinating the plant.

A few plants are known to do this. Take the parachute plant (Ceropegia sandersonii). Its flowers smell like wounded honeybees. And smearworts (Aristolochia rotunda) mimic the odor of injured plant bugs.

Flowers and flies

Mochizuki tested his hypothesis a few ways. First, he confirmed that pollen-carrying grass flies visited dogsbane in the wild. That showed that in nature, the flies behaved the same way as they did in botanical gardens.

He also tested scented chemicals released by the flowers. They perfectly matched the mix released by injured ants. The grass flies also showed interest in these chemicals. That suggested they could be after ants, not the flower.

In a final experiment, Mochizuki put the flies in a maze with ants killed by spiders. The flies found the ants. This proved that they’re able to hunt by scent alone. Dogsbane flowers seem to be taking advantage of that skill.

Mochizuki now wants to study related plant species. He wonders how they might evolve “such weird mimicry systems.” Other plants that lure in kleptoparasites have trap-shaped flowers. These easily capture (and later release) the pollinating flies. In contrast, dogsbane blooms look rather plain.

It seems that flowers with weird shapes aren’t the only ones that can play tricks, he concludes.

Robert Raguso notes that other species experience the world much differently than we do. A biologist, he works at Cornell University in Ithaca, N.Y., and did not take part in the new study.

“We can barely imagine the sensory realities perceived by other organisms,” Raguso says. Dogsbane is “capable of conjuring the chemical essence of wounded ants. … It almost seems like a magic trick.”