The Story of the Scribbly Gum Moth

Scribbly gums are a distinctive group of Australian eucalypts that get their name from the graffiti-like ‘scribble lines’ on their smooth bark. These wandering tracks are tunnels made by larvae of the Scribbly Gum Moth and tell a story of the insect’s life cycle. Despite the tracks being highly visible, the tiny moth larva is elusive and was almost unknown until recently. It had been assumed there was only one species

Photo – Young larvae tracks taken by Sue Siwinski at the North Coast Regional Botanic Garden March 2024.

Then in 1999 a Canberra school student, Julia Cooke, investigated the scribbles with the help of Dr Ted Edwards (CSIRO entomologist) for her entry into the BHP Science prize. Julia’s investigation suggested that the scribbles made on different species of eucalypts were made by different moth species. In 2005, a team of curious retired CSIRO scientists worked to confirm two different species of moth in the Canberra region each with a distinctive larval track pattern.

Photo – decorticating (shedding of bark) rids the tree of larval parasites. Taken by Sue Siwinski at the North Coast Regional Botanic Garden March 2024.

Further research has found 14 moth species that make feeding tunnels through the bark of a variety of scribbly gums. When the parasitic larvae first hatch from eggs laid by the parent moth, they tunnel in long meandering single loops. They feed from the tree’s cambium bark cells, grow and moult, the tunnels increasing in diameter until the larvae reaches about 10mm length. In this final stage they retrace their tunnel to feed on nutrient-rich scar tissue produced by the tree to repair the earlier damage. Then each scribble has a thin and thick feeding line, usually with a loop at the end where the larvae turned back. The full-grown larvae crawl out and form cocoons, often dropping to the ground until hatching into tiny grey moths.

Photo of more decorticating taken by Sue Siwinski at the North Coast Regional Botanic Garden March 2024.

The scribbly gum trees have evolved a decorticating defence mechanism where they shed their bark regularly to get rid of the parasite load. This is when the calligraphy of the moth larvae is most visible.

Taxonomists now recognise two groups of tunnelling moths in Australia, related to a group in South Africa, with a common ancestor from Gondwana time that fed on plants of the southern supercontinent.

As you can imagine gaining a photograph of these moths is extremely difficult. This photo is courtesy of the CSIRO and you can read more about their work in this area here.

Blog post written by Sue Siwinski

Cover photo of mature larvae tracks by Zoe Milinski