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Species: birch caterpillar (Drepana arcuate)

Habitat: Deciduous woodland with birch or alder in North America

Bees buzz, cicadas sing, but caterpillars are the real musical maestros of the insect world. It turns out they use different parts of their body to get the attention of other caterpillars.

The tiny birch caterpillar makes special vibrations, inaudible to human ears, using their mouths, body and anal parts. These appear to send out information about food and shelter to other caterpillars nearby.

Within a couple of hours, a small group of some 2-6 individuals forms around the drummer – a behaviour that may provide safety from predators or bad weather.

“These tiny caterpillars produce a complex diversity of signals – they shake their bodies, drum and scrape their mouthparts, and drag specialised anal ‘oars’ against the leaf surface to create bizarre signals,” says evolutionary biologist Jayne Yack at Carleton University in Ottawa, Canada, who led the new study.

“I’ve been studying insect sounds for more than 30 years, and I’ve never seen one insect species produce such a diversity of signal types.”

The study is the first to provide evidence for the use of vibratory signals for complex acoustic communication in caterpillars, Yack says.

But why does this tiny caterpillar need such a complex repertoire of signals? This is still not clear, says Yack. “But probably they are using different signals to gauge distance or different food quality, or to help others localise the source,” she adds.

In fact, the vibrations continue even after the group has formed. “They keep communicating with each other,” says Yack. “Maybe they are saying things like ‘hey, we need to fix this big hole in the shelter’ or ‘Hey guys, I’m over here! I found a really good feeding spot!’ or perhaps ‘Move over! this is MY spot!’

Conflict resolution

Until recently caterpillars were thought to rely primarily on chemical signals such as pheromones to communicate – unlike insects such as wasps, bees and ants, which use both vibratory and chemical signals to communicate information about food or safety.

In an earlier study, Yack’s team discovered the vibratory signals in the late stages of these caterpillars. They found that the signals were used to solve territorial disputes – the anal scraping, for example, was thought to have evolved as a way to avoid one-to one confrontations.

The latest study reveals a whole new facet of this behaviour. Yack’s team recorded the vibrations made by the early stages of these caterpillars, as they formed their groups.

Analysis of the sounds showed that they produce four different types of vibratory signals associated with feeding and silk-making, which is used to build shelters. They used their mandibles and anal parts to scrape the surface of the leaf, shook their body to make a buzzing sound, and drummed with their mandibles.

The big difference between the vibrational signals sent by these young caterpillars compared with their older counterparts lies in the intentions, says Yack. These younger caterpillars only use their vibrations to tell other caterpillars about food and shelter, rather than to fight over a piece of leaf.

The finding could have implications for pest control, as many pests spent a significant part of their lives as caterpillars, and they likely have similar types of communication.

So, cracking the communication code could help researchers develop novel alternatives to pesticides. “Perhaps by jamming their signals or by monitoring the abundance of pest species on plants,” Yack says.

However, not everyone agrees that the caterpillars are using the vibrations to communicate. Tomer Czaczkes, from the University of Regensburg in Germany, says there might be another explanation.

“For me the smoking gun is missing: without playing back the vibrations to caterpillars, and seeing them approach the vibrations, we don’t actually know it’s the vibrations that are important. Maybe the caterpillars are releasing chemicals while doing this scraping behaviour?” he says.

Journal reference: Behavioral Ecology and Sociobiology, DOI: 10.1007/s00265-017-2280-x