Two-millimeter long caterpillar defends its tiny territory by buzzing

By Stephen Beech

Baby warty birch caterpillars defend the world’s smallest territory, according to new research.

At just one to two millimeters long, the insects are minute.

But the study shows that they still manage to defend the world’s smallest territory: the tip of a birch leaf.

Scientists found that the minuscule insects scrape and thump their leaf-tip homes to produce vibrations that warn off intruders.

But confrontations over their tiny territories never come to blows, protecting both protagonists from injury.

Study leader Professor Jayne Yack, of Carleton University in Canada, discovered that new-born warty birch caterpillars exhibit "complex" territorial behaviors despite their tiny size.

The ground-breaking research, published in the Journal of Experimental Biology, highlights the sophisticated communication methods of minute organisms.

Yack, who first encountered the tiny creatures in 2008, said: "We had noticed that tiny warty birch caterpillars produced vibrations."

She also noted that they reside in solitude on birch leaves, leading her to speculate whether they are warning off intruders with the vibrations they transmit through their leaves from their leaf-tip residences.

The team found that the caterpillars are territorial, warning off intruders with vibrations produced by scraping their rear ends along the leaf, in addition to beating the leaf tip with their thorax.

The researchers collected female two-lined hook-tip moths, ready to lay eggs on leaves and pieces of paper in the lab, to collect the caterpillar youngsters and transfer them to the center of a single birch leaf as soon as they emerged from the eggs, to find out which region of the leaf they prefer to inhabit.

A day later, almost 90% of the youngsters had wriggled to the tip of the leaf to set up home.

Having determined that the youngsters establish a territory, the team filmed the tiny caterpillars as they scouted their minute home range, rested and fed, rarely straying more than a few body lengths from the tip.

And from time to time the youngsters beat the leaf with their head and thorax, producing a percussive tone, in addition to dragging oar-shaped hairs on their rear along the leaf, producing a buzzing scraping sound, while simultaneously striking the leaf with the head end of their bodies.

Yack said: "The sounds produced by these caterpillars are not audible humans, so we had to use specialized equipment to pick up the vibrations."

Once she was certain that the teeny creatures were territorial, Yack and her colleagues wondered how
the youngsters would react when an intruder came calling.

Placing an interloper near the center of an occupied leaf, the team sat back and watched each encounter unfold.

Yack said: "Sometimes the intruder turned tail and headed off in search of another leaf after approaching the resident, which had been frantically scraping at the leaf sending urgent warnings as the trespasser advanced.

"However, on other occasions, the occupant secured themselves to the leaf with a silken thread before flinging themselves over the edge – after a protracted period of leaf scraping – as the threat approached, leaving themselves dangling safely beneath the leaf.

"Yet the caterpillars never came to physical blows, biting or thrashing each other.

"The outcome of each contest depended entirely on the warning vibrations transmitted through the leaf."

Yack suspects that the youngsters might prefer this more diplomatic form of conflict resolution to ensure they both survive.

She believes the tiny caterpillars defend the leaf tip as it may be more succulent than other parts of the
leaf, offering the resident better nutrition, in addition to providing the occupant with a springboard for
escape, if necessary.

Yack also suggests that intruders are unable to approach residents ensconced at the tip from behind, limiting their opportunities for attack, and could even allow the caterpillars to appear larger than they are as the leaf might amplify the resident’s warning signals.

She added: "Whatever the reason, tiny new-born warty birch caterpillars could tip the scales as the tiniest kings in their leaf-tip castles."