It would seem (sorry for lack of a reference here) that certain species of Acacia contain betacarbolines as well as tryptamines and are thus orally active. I would guess that these would be the trees in which to look for the witchetty grubs.
But this is merely my boreal speculation, for all we know the insects could metabolically produce MAO inhibitors, considering we don't even really know what 2-methyltetrahydrobetacarboline does pharmacologically.
“There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli