Bookworm

Bookworms were bug-like creatures which lived on the library planet Catalog. Apparently, they were, as the Seventh Doctor put it, "agents of chaos". They were only supposed to be living at the beginning and end of the universe. Because Catalog's new filing system had stretched backwards and forwards through time, however, it became a significant temporal event. The bugs noticed it and crawled around creation on the Catalog-generated timelines. If they kept advancing, they'd undo the fabric of the Web of Time. The threat was warded off by the Doctor by depriving them of their food and then forcing them to shrink back to the Big Bang and the collapse of the universe. (IHP: Hunger from the Ends of Time)

OVERVIEW
The Foreign Hazard Duty and the Seventh Doctor fought tiny bookworms who infested the library world of Catalog. (IHP: Hunger from the Ends of Time)

Four bookworms were affected by Barbara Cartland's alien parchment. One of them ate Wilkie Collins' writing. (BFBS: The Diet of Worms)