(20 Jan 2017) “The gum-tree stands by the spring, I peeled its splitting bark, And found the written track, Of a life I could not read.”
Judith Wright’s poem of 1955 shows the Australian fascination with one of the most well know national icons: the scribbles on gum trees. It was not until 2010 that CSIRO revealed that the cause of the scribbles was finally proven to be the burrowing larva of tiny scribbly gum moth.
‘Reading’ the scribbles is tough, but a few ‘essentials’ help to get a grasp of it. Early-instar larvae, 2 mm in size, bore wide, arcing tracks; the result looks like a sort of chain track. Later instars create zig zag lines; the second-last instar turns and bores either closely parallel to the initial mine or doubles its width, along the zig-zag-shaped mine, because it feeds on the very nutritious scar tissue the tree has formed in the original zig zag mine. This creates the dark, zig zag patterns.
All of this happens under the bark; the scars of mines only become ‘visible scribbles’ following the shedding of the outer bark, exposing them to the outside world.
When the larvae have reached their maximum size they emerge and crawl out from between the bark and into the litter, where they pupate in November/December. Adult moths emerge in autumn. Because of a small wing span (3 mm) they are hardly ever seen; they lay eggs in the bark again, to complete the cycle.
Nothing special. Standard photography.