Oleaster's bitter taste

In Metamorphoses 14:514 ff, Turnus's ally Venulus, after his disappointing visit to Diomedes, comes across a cave where nymphs had been mocked by a rustic:

The shepherd mocked them, leaping wildly in imitation, and adding foul language, with coarse abuse. Nor was his mouth silent till tree-bark imprisoned his throat: he is indeed a tree: you may know its character, by the taste of its fruit that bears the mark of his speech in the wild olives’ bitterness.The sharpness of his words has entered them.

Improbat has pastor, saltuque imitatus agresti 
addidit obscenis convicia rustica dictis, 
nec prius os tacuit, quam guttura condidit arbor: 
arbore enim est, sucoque licet cognoscere mores; 
quippe notam linguae bacis oleaster amaris 
exhibet: asperitas verborum cessit in illas.

The passage is enigmatic, one of many in the poem that seem to gratuitously present something that has no obvious relationship to its context. Of course we'd seen a depiction of Athena's gift of the domestic olive in book 6 -- on the image the goddess weaves in competition with Arachne. And we'll soon meet Pomona, gifted in the art of grafting.

The wild olive -- Nature's olea oleaster, not Ovid's -- did receive commentary:

In the fourth century BCE Theophrastus, the most prominent pupil of Aristotle, wrote an Enquiry into Plants that stands at the head of the literary tradition of botany.

Theophrastus noted the kinship of wild-olive with the cultivated olive, but his correspondents informed him that no amount of pruning and transplanting could transform kotinos into olea. Through lack of cultivation, he knew, some cultivated forms of olive, pear or fig might run wild, but in the "rare" case where wild-olive was spontaneously transformed to a fruit-bearing one, it was to be classed among portents.