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Scribbles in Time

Scribbly gum

Australian scribbly gum moths might already have been associated with Eucalyptus when Australia broke free from Gondwana. Credit: You Ning Su, CSIRO Ecosystem Sciences

By Marianne Horak

Scribbles on eucalypts are the marks of a unique interaction with caterpillars that may date back to the Gondwana supercontinent.

The “scribbles” on smooth-barked Eucalyptus species are quintessentially Australian. They have long featured in children’s stories like Snugglepot and Cuddlepie, in Australian poetry and even in the ABC TV’s banner for Australian Story.

The author of the scribbles remained a mystery until 1934, when a small moth was reared from scribbles on snow gum. But it was not until recently that the complex life history of the moth was decoded from the scribbles by a dedicated multidisciplinary team.

The results of this new research are exciting and indicative of how much is still to be discovered about the Australian fauna. The study describes 11 new species of scribbly gum moths, with many more still to be named.

In a previously unknown insect/plant interaction, the tree is induced to fill the tunnel made by the moth larva with nutritious tissue otherwise not present in the bark. Adaptations of the caterpillar to this complex life history have provided clues to the position of the scribbly gum moths in the Tree of Life, revealing an ancestry dating back to the Gondwana supercontinent.

The study was set in motion by a school project’s conclusion that scribbles in the Canberra region involved three different scribbly gum moth species. The puzzle took a team of botanists, moth taxonomists and molecular scientists several years to solve because the action is hidden beneath the bark.

The purpose of the bark is to provide protection for the tree trunk and its vital sap-transporting region. While it is a very poor source of nutrition, the scribbly gum moths have found a way to modify this inhospitable environment to deliver high quality food for their caterpillars.

Each autumn the adult moth lays its eggs singly into depressions and crevices in the bark, which will be shed in the next year. The emerging caterpillar bores through the underside of the egg directly into the bark to the level where next year’s cork cambium will be formed. As a growth layer, the cork cambium eventually produces the friable cork tissue that allows the outer bark to be shed.

The entire meandering track chewed by the caterpillar during the next 9 months runs through the bark exactly at the level of this future cork growth layer. Hence the scribbles become neatly exposed when the outer bark falls off.

In the first months of its life the caterpillar bores a thin, irregular and often widely arcing track that widens only slightly as the caterpillar grows. After an often particularly long loop, the pattern changes to a section of more regular, tighter zig-zags. The track now is wider, and the set of zig-zags ends in a small turning loop after which the track returns either parallel or adjacent to the earlier tunnel.

Up to this point the caterpillar is a typical borer, feeding at the head of its tunnel on the bark tissue it excavates. Long and slender and without any legs, it is perfectly adapted to this task.

At the exact moment when the cork cambium becomes active, the caterpillar moults into its last instar with legs and a functional silk gland. In the meantime, the tunnel of the doubled zig-zag track becomes filled with succulent wound tissue produced by the cork cambium as a result of the caterpillar’s feeding. Elsewhere, the cork cambium produces cork that allows the outer bark to eventually fall off.

The last instar caterpillar now turns back into its old tunnel, exclusively feeding on the nutritious wound tissue and growing very quickly to full size before boring out of the tree and pupating at its base. After about a month, the moth emerges from its pupa and the life cycle starts again. The outer bark is shed shortly after the caterpillars have left their tunnel, so when the scribbles become exposed they are already empty.

This unique interaction between the caterpillar and the tree, which produces the necessary nutrients otherwise lacking in bark, is far from understood. How does the caterpillar detect the exact layer that will turn into the cork cambium in the following season? What induces the cork cambium to produce wound tissue instead of cork in the caterpillar’s doubled-up track? How does the caterpillar time its moult to the final instar with legs – and a functional silk gland to eventually spin its cocoon – to coincide with the growth of the wound tissue? Much more research is clearly required.

Unravelling the life history of the scribbly gum moths was only the beginning of the story. Molecular analysis of another unusual caterpillar, extracted from deep in a eucalypt in the growth region between sapwood and bark, identified it as a relative of the scribbly gum moths. Coincidentally, a DNA study done in Finland on some enigmatic Australian moths pointed to the same genus as this single caterpillar.

The upshot was that this second genus of larger moth, closely related to the scribbly gum moths, produces the so-called “ghost scribbles” on eucalypts. These sinuous raised welts are the scars of tracks laid down long ago in the growth layer between sapwood and bark. They only become visible after years of bark-shedding brings their impression to the surface.

Closer investigation confirmed a very similar life history to the scribbly gum moths, with the caterpillar re-entering the last part of its tunnel to feed on wound tissue produced by the growth layer. Up until now we had no idea about the biology of these ghost scribbly moths, nor did we know where they belonged taxonomically.

The shape of the cocoon and details of the caterpillar’s structure for both of these genera provided the clues to fit together the next pieces of the puzzle. They linked the Australian scribbly gum and ghost scribbly moths as well as a single South African species in its own genus to a very large worldwide group of tiny moths whose relationship had always been enigmatic. Finally enough information was available to understand this moth family and to fit it into an evolutionary scheme.

Furthermore, the highly specialised male genitalia of the Australian and South African genera left no doubt that they were each other’s closest relatives, and quite distant from the highly specialised remainder of the family. The morphology of both caterpillars and adults in this Southern Hemisphere group is very rich in ancestral characters, suggesting that they have undergone relatively little change over millions of years.

The final chapter to our story is the shared history with eucalypts of both scribbly gum and ghost scribbly moths. Their complex life history, intimately linked with the growth cycle of Eucalyptus, points to a very long association between the moths and their host.

Fossils recently found in Argentina demonstrate that

Eucalyptus was present on the Gondwana supercontinent before it separated into fragments, and all the evidence, including the fact that they are found on no other plants, suggests that the Australian scribbly gum moths might already have been associated with Eucalyptus when Australia broke free.

A truly Australian icon indeed!

Marianne Horak is an Honorary Fellow studying moths at the Australian National Insect Collection at CSIRO Ecosystem Sciences. This article is adapted from a paper published in Invertebrate Systematics co-authored with Max Day, Celia Barlow, Ted Edwards, You Ning Su and Stephen Cameron.