Photo by Neville Young
Dear Readers, every so often someone spots something unusual in Coldfall Wood, and at the weekend my friend Neville sent me some snaps of what looks like sawdust distributed all over the ivy at the bottom of this dead tree. Well, I was flummoxed, but my friend Linda D suggested that I have a look at ambrosia beetles as the potential culprits. And what a revelation it was!
Oak Pinhole Borer (Platypus cylindrus)Illustration by John Curtis (1791–1862) from https://commons.wikimedia.org/wiki/File:Britishentomologyvolume2Plate51.jpg
The Oak Pinhole Borer is a UK example of an ambrosia beetle. These beetles have a symbiotic relationship with a fungus – the fungus is essential for the development of the beetle larvae, but the fungus itself only lives in the tunnels that are excavated by the beetle. Such a specialised relationship! So specialised, in fact, that the beetle has special indentations in its carapace where the fungus is collected when the beetle changes from a larva to an adult beetle inside the tree. This fungus is then transferred to a new tree.
Ambrosia beetles favour stressed, sick or dead trees, and the process starts when a male beetle excavates a tunnel a few centimetres deep. This attracts a female, and they mate on the bark. The female then enters the tunnel and starts to excavate a series of radial tunnels, inoculating the tunnel walls with the spores of the fungus as she goes. The male follows behind and kicks out the ‘sawdust’ or frass, which is what Neville saw.
The female lays her first batch of eggs after about a month of tunnelling, and will then continue to lay further batches over her lifetime, which can be two or three years. The tunnels can extend for up to two metres. The larvae only eat the fungus, but they may also extend the tunnels as they grow: later in their development the larvae have strong, powerful jaws. It takes them up to two years to achieve full adult size (which is only about 8 mm long), and when they are ready to pupate they excavate a chamber inside the tree. Sometimes several generations of beetle will occupy the same tunnel system.
The adult females produce a very fine, powdery frass, while that of the larvae is a bit coarser – a skilled entomologist can tell the difference.
By John Curtis – https://www.biodiversitylibrary.org/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10785198
The Oak Pinhole Borer is a native species, but it became much more common after the Great Storm of 1987, when no doubt all those fallen oak trees felt like a bonanza for the beetle. As the beetle mainly feeds in trees that are dead or diseased already it is not thought to do a lot of damage, unlike some other members of the family.
The relationship between the beetle and the fungus is one of those things that makes me wonder how on earth it all got started. Whatever the origin, it’s an ancient symbiosis, and one that often goes unnoticed. It’s always worth keeping your eyes peeled when you go to the woods, or indeed whenever you’re out and about. It’s surprising what you can spot in even the tiniest patches of habitat.
Fascinating! Thank you so much for researching this and writing it up. I was truly baffled by the sawdusty leaves. And Peggy Dog says thank you for using her photograph! xx
Peggy Dog probably should have a blogpost of her own!
I’ll talk to her agent!
I went last night to check it was all still in situ – it was – and to see if I could find any more affected trees.
I got very excited over a dead log with quantities of sawdust but I was a little suspicious that the nature of the sawdust was quite different from the known site. Also I couldn’t find any insect boreholes, whereas the initial site is covered in them.
Fortunately, before making an utter fool of myself sending you photos etc, I realized that it was REAL sawdust from an actual saw! The log had clearly been used as a sawhorse by volunteers cutting posts for a dead hedge. Silly Neville.
In the meantime, no I didn’t find any more sites. I guess my initial site is particularly visible because of the contrast of the white dust on the green leaves below the boreholes. Without this it might be much more difficult to detect a site, with the dust just falling camouflaged on the woodland floor. Ho hum. I’ll go on looking.