Every scribble tells a story

Glen Paul: G’day and welcome to CSIROpod, I’m Glen Paul.

If you’ve ever taken a walk through the bushland of South Eastern Australia you’ll have very likely noticed the unusual scribbles on the smooth barked Eucalyptus trees.  At first glance they appear almost as if someone has scrawled them into the tree but they are, in fact, the work of caterpillars from what was previously thought to be a single species of moth called the Australian Scribbly Gum Moth.  A team of CSIRO Honorary Fellowes have now uncovered at least 11 new species of moths responsible for the iconic bush graffiti.

Dr Marianne Horak, a retired moth expert working in an honorary capacity at CSIRO’s Australian National Insect Collection was one of the team to make the discovery and joins me on the phone.

Firstly, Marianne, you’re part of a group that can be described as scientific heavy-hitters, all technically retired but still working and advancing scientific knowledge.  Tell us a bit about the team and why you continue to be involved in research.

Dr Horak: Well, I continued to be involved in the research because I enjoy it and because there still is so much to be done.  Of the Australian moths only half have been described and named.  The team is across several disciplines and that makes its strength.  The driving force was Dr Max Day who many, many years ago was head of CSIRO Forestry.  He is interested in insect physiology and obviously the forestry part.  The second important person was Celia Barlow from CSIRO Plant Industry, she is the botanist.  She did all the work, figuring out what actually happened in the insect/plant interaction.  Ted Edwards and I are both from the Australian National Insect Collection, we both work on moths.

Glen Paul: I don’t normally give peoples ages away but Max Day, I understand, is 96 years old…

Dr Horak: … yes…

Glen Paul: … which is just amazing.  We can all take a page out of Max’s book.

Dr Horak: Yes.

Glen Paul: What brought you all together to work on this project?

Dr Horak: Well, it was really Max who, five years ago, walked into my office and said “It is appalling that we still do not really know how the Scribbly Moths do their scribbles. They are very intricate, there is clearly something elaborate involved.”  We took it from there.

Glen Paul: So, from the research, how do the moths put these scribbly marks into the gum trees?

Dr Horak: Well, very briefly, small caterpillar sits along the level in the bark where next year the cork cambium will grow the cork.  It makes a long zigzag until it finds a good place and then it makes a double track where it returns along the same track.  At the end of this it turns round again and that is exactly the time when the cork cambium starts growing to produce the cork that helps throw off the outer bark.  Where the caterpillar track is, instead of producing cork it grows a soft nutritious scar tissue, fills that tunnel with this highly nutritious tissue.  The caterpillar moults into the last, turns round and feeds on this tissue in its tunnel.

Glen Paul: Does the tree actually benefit from this in any way?

Dr Horak: No.  I think the tree doesn’t suffer, doesn’t benefit because this outer bark is going to be thrown off anyway.  The caterpillar somehow tricks the tree into producing a bit of nutritious food in that tunnel.  For the tree it’s a very small loss.

Glen Paul: How was it determined then that there were a number of species involved in the creation of these tunnels?

Dr Horak: Well, some years ago a schoolgirl, Julia Cook, actually did a project with Ted Edwards.  She just measured the scribbles and realised there were different kinds of scribbles on the trees, even in Canberra alone.  We knew from the collection that there were different looking moth species that we knew were producing those scribbles.

Glen Paul: How did you separate which moths were creating which scribbles and just how closely related are they?

Dr Horak: There are actually three groups of Scribbly Moths.  We only know the biology of the one that makes the bark scribbles, the ones we see on the smooth barked Eucalypts.  There are two other groups in the same genus, we do not know the biology.  We think they feed in the rough barked Eucalypts and we never see their scribbles but we have not tried to prove that yet.  There still are more things to come.

Glen Paul: I understand these moths have a link with the ancient super-continent, Gondwana.  What’s the connection there?

Dr Horak: When we finally studied the larvae we found characters in the larvae that link them to other moths, one of them another genus in Australia and the third a genus in South Africa and the fourth a worldwide genus.  Now the Australian and the South African ones are much more closely related than the worldwide distributed one.  It is clear that they all go back to a common ancestor on the old Gondwana continent.  They all also feed on plants restricted to the Gondwana continent like eucalypts.

Glen Paul: Do the African ones get involved in scribbles of any kind?

Dr Horak: No, the African one produces galls.  A gall is a structure produced by the plant in response to the feeding of the caterpillar, in which the caterpillar feeds.

Glen Paul: OK, and the research has been published in the journal Invertebrate Systematics.  Is that the only place where people can find it if they’d like to have a read?

Dr Horak: Well, CSIRO Publishing was extremely generous and has actually made that article available free on the internet for everybody.  Obviously you can just Google it.

Glen Paul: Alright.  Thank you very much for talking to us today about that, Marianne, and also thanks immensely to you and the team for your continued involvement in science.

Dr Horak: It’s a pleasure, both of it.

Glen Paul: Dr Marianne Horak.  And to find out more about the research or to follow us on other social media just visit www.csiro.au.