Here is a short timelapse (900x speed) of a Southern Pine Beetle chewing through some phloem and eventually burrowing into it and towards the outer bark.
This is actually not quite what I was hoping for. The idea was to put a beetle onto a piece of bark squeezed between two pieces of plexiglass (“phloem sandwich” – apparently a well established technique) and wait for the beetle to burrow through the bark to the phloem and make a gallery there for us to observe. Unfortunately, my phloem sandwich skills are currently poor, so this beetle found a way to get around my bark to the phloem. Fortunately, though, then it started chewing a bit and burrowing into the phloem so I could get a little bit of observational footage of the burrowing. Hopefully next time I can get the phloem sandwich set up well so we can see some more realistic gallery burrowing.