Fungal farm to wormy wood: the ambrosia story

The original Wormy Chestnut was indeed a chestnut, scarred by worms. It was an American Chestnut, Castanea dentata, killed by chestnut blight then chewed by insects that left holes and discoloured the wood while they remained standing.

Because American Chestnut was almost wiped out by the early twentieth century blight, the wood became rare and valuable.

Australian Wormy Chestnut seems to be a mix of East Gippsland eucalypts, such as Eucalyptus obliqua, Eucalyptus sieberi and Eucalyptus fastigata. One of the companies selling it says it is "distressed by Mother Nature", which another company describes as the (attractive) ravages of "fire, ambrosia beetle, pin holes and squiggly worm marks".

The ambrosia beetle is kind of weevil. They are also called pin-hole borers, but not the ones responsible for the neatly drilled holes in old furniture (and the dust that they toss out of the hole). The ambrosia borers like 'green' wood, preferably freshly felled.

The beetle bores a hole into the wood to lay its eggs, and carries with it the spores of  the ambrosia fungus. The spores germinate, the fungus grows, the egg hatches, the larvae eat the fungus. The parent beetle often dies, conveniently, at the top of the tunnel, blocking it from predators and keeping the rain out.

By the time the wood is turned into flooring planks the beetle, it's larvae and the fungus are long gone. Or at least their remains are well dead. The fungus usually stains the wood, leaving a distinctive dark colouration, part of the attractiveness of the final wood product it seems.

representative image of taxa

The wonderfully named Austroplatypus incompertus (meaning something like the poorly-known southern flat-foot) likes trees burnt in bushfires, or similarly injured, and it produces 'extensive gallery systems in one plane only'. The plane, is horizontal, in a standing tree, meaning that the timber can easily fracture at an area of activity.

There is at least one other species of ambrosia beetle in these forests, the Mountain Pinhold Borer, Platypus subgranosus.  In case you are wondering, and I certainly was, the reason the platypus has the genus name Ornithorhynchus rather that Platypus is because this little fella, or at least one of its relatives, was named first.

But enough of beetles, this is a plant (and fungal, and algal...) blog! I bet you are wondering about the ambrosia fungus that colours up the Australian Wormy Chestnut wood so appealingly. There is a whole website devoted to the endearing relationship between beetle and fungus. And this (from the Ambrosia Symbiosis site) may, or may not be, the fungus...

The fungi associated with the ambrosia beetles are a mixed bag, and not all part of a single taxonomic group. This one is a Fusarium, a genus including lots of economically and environmentally important pathogens. There are also filament-forming yeasts and the helpfully named Ambrosiella.

Mostly the fungi can't survive without their ambrosia beetle overlords, but at least some of the Fusarium stock or crop (depending on your perspective, but do remember that fungi are more closely related to animals than to plants) seem to be able to fend for themselves if necessary.

Most or all of the fungi concerned have foregone sexual reproduction, apparently content with their life of coexistence with ambrosia beetle. Although it's not all peace and harmony. There is evidence of fungal rustling, with smaller beetles drilling holes near larger ones to save the bother of carrying around their own fungi.

So there you have it. A small glimpse into the murky world of the ambrosia symbiosis, as it's called. Beetles, fungi and dead trees combine to create an intriguing ecosystem that we slice up, polish and then walk on.

Image: The top picture is Oscar, a dog, doing tricks on the newly laid Australian Wormy Chestnut floor in the house of my friends Mal and Sandy. The annotated wood panel is from Triton International Woods and the image of the Austroplatypus is from Atlas of Living Australia (the species incompertus was included originally in the genus Platypus)