What killed my mint?

I know the answer to this question, but first I will explain the scenario.  One of my potted mints had been looking increasingly ill of late and yesterday when I moved the pot the foliage simply separated from the potting mix.  Something had eaten its roots, all of them.  Mr 2 and I investigated and all through the pot we found these white curl grubs.  I think some people call them witchety grubs but that’s not really accurate.  The grubs are grey and white in appearance with an orange head and they are usually found curled in a foetal like position.  Hence the name white curl grubs I guess.

There were close to 50 of the things in a 30cm pot.  This was high density larvae.  No wonder all the roots got eaten.   Mr 2 was exceptionally pleased with them.   They are currently residing – complete with dirt- in the back of Mr 2’s ride on Scoop (Scoop is a character from Bob the Builder).   I’m not sure exactly what sort of beetle these particular ones would have metamorphosed into (from what I have read they are most likely to be scarab or cockchafer beetles) but I wish I had some chooks to feed them too.  They are, apparently, very high in protein.  I have noticed quite a few of these grubs when I’ve moved pots sitting on the lawn but I haven’t thought much about them.  My understanding is that they are only really destructive in pots or when in particularly high concentrations in lawns or garden beds.  I think I will spend the next few days inspecting my other pots – to lose some mint is one thing, a dwarf citrus would be something else entirely.