Disclaimer: Where would we be without the great Tolkien?
AN: Haruuum. It must be hard being a tree.
Of all the creatures that dwelled in his forest and under his protection, Treebeard hated only a very select few. Those few species that lived in and amongst the trees, yet could not distinguish them from Ents. They were both irritating and dangerous, often inflicting grievous wounds on their Entish victim before their escapades could be arrested.
In the past, Treebeard saw to it that all organisms harmful to him and his arboreal fellows were banished from the Fangorn Forest. His actions granted Entkind almost an age of unconcerned peace in the wild woods, but all that soon changed as Saruman's armies drew closer.
The vermin had returned.
Several days ago, Treebeard happened upon one of his brothers among the wood's many twisted, untamed ways. Poor Beechbone told him a tale of depression, feelings of emptiness, and a hollow existence. With an ear to his old friend's bark, Treebeard quickly deduced what was ailing him so. The scratching, crawling noises and faint heat betrayed termites. A colony devouring Beechbone from the inside.
Not long after, Quickbeam sought out Treebeard, complaining of an excruciatingly intense headache and desperate for a cure. A brief inspection revealed a series of large holes, big enough to fit a finger-branch into, drilled into the back of his head. Though he had no idea where the beast might have come from, or how Quickbeam failed to notice it gouging its home into the back of his head, it was painfully clear to Treebeard what the holes were. Woodpecker nests.
Then, Treebeard himself had an encounter with the unwanted beasts. Waking up from a meditative slumber, he found a minute red squirrel sitting squarely on his nose, quite absorbed in its efforts to open an acorn. Once it noticed Treebeard's large, yellow, owlish eyes on it, the squirrel chittered an fled, raking tiny claw marks into the bark under it. With a long-suffering sigh, Treebeard shook his head and was dismayed to find the action sent nuts flying everywhere. He had noticed that things seemed slightly off when he awoke. The sounds of the forest seemed dulled, the smells not as rich as he remembered, and there was a funny taste in his mouth. Now he knew why.
His ears, his nose, his mouth... Every crevice on his craggy body that a squirrel could reach was stuffed. Completely packed with acorns, hazelnuts, beech nuts, and an assortment of seeds. Overnight he had been transformed into a squirrel's pantry. And here he had almost forgotten why he banished the vermin from his forest all those centuries ago.
There was no question in his mind, it was time to do it again.
AN: Yep, it sure must be hard being a tree... Just for the record, I like squirrels and woodpeckers. I've fed the latter out of my hand before (yes, I still have my hand intact) and would love the former as a pet. Termites are on their own, even though they are master architects. However, if I was an Ent, I should think I'd hate those little buggers with all my heart.
And, as for why the Ents' pest animals would return after being banished, it's simple ecology. They were kicked out of the Fangorn forest, but lived in the surrounding wooded areas for an age. Then, when Saruman's Orks and Uruk-hai started tearing down those areas, the creatures lost their habitat and fled back to the Fangorn in search of food and shelter. Yay.