Plant’s “Call for Help” Brings Enemies, Too

By Kate Andries

When a caterpillar sits down to a nice lunch of leaves and other greenery, the last thing on its mind is probably the complicated interspecies interaction that bite of cabbage leaf has spurred.

The cabbage leaf, a bit miffed at being munched on, emits volatile compounds into the air with the intention of attracting some nearby parasitoids—usually a species of wasp named Cotesia glomerata—that swoop in and lay eggs in the offending caterpillar, said Andre Kessler, professor of ecology and evolutionary biology at Cornell University.

A parasitic wasp attacks a caterpillar. Photograph courtesy Hans Smid,


The caterpillar is essentially used as an incubator for the parasitoid’s eggs, and as the larvae hatch, is eventually eaten from the inside out. In their final trick, the larvae tunnel out from their host and convince the half-dead caterpillar to construct them a protective silk web.

(Also see “Pictures: Wasps Turn Ladybugs Into Flailing ‘Zombies.'”)

But what about any residual compounds floating around the air? Wouldn’t other insects be attracted to them, too?

Dutch researchers say yes. Those insects—called hyperparasitoids—turn the tables on parasitic wasps, turning them into veritable Russian nesting dolls by making the parasite into the host.

On the Lookout for Parasites

In a study published Tuesday in the journal PLOS Biology, scientists looked into how hyperparasitoid wasps locate their hosts, a phenomenon that study leader Erik Poelman said is quite understudied.

“These herbivores are small and don’t smell a lot. They’re difficult to find,” said Poelman, of Wagenigen University in the Netherlands. “We had an idea that these wasps might use something that was already detectable.”

To test this, the team collected samples of air from plants damaged by uninfected caterpillars, plants damaged by caterpillars infected with wasp larvae, as well as plants completely undamaged by caterpillars. They then studied which scents attracted hyperparasitoid wasps.

(Learn more about insects.)

Surprisingly, the herbivores themselves aren’t what’s attracting the hyperparasitoids. “It’s purely the plant odor,” Poelman said—suggesting that a plant’s ‘call for help’ actually brings more guests to the table than expected.

“If the plant is putting out a volatile cue,” said Cornell’s Kessler, who wasn’t part of the study, “this cue can be interpreted by whoever is able to read it. It’s not a private channel.”

In that case, maybe plants should heed the oft-repeated parental advice: Watch what you say!