- You may be looking for the comic story.
On Earth, the population of insects was many times greater than humanity. It was mentioned that had Miracle Day, which caused death in humans to cease to exist, affected insects, they would have overrun the Earth in 48 hours. (TV: The New World)
On one planet, a crashed spacecraft spilled fuel which caused nearby insects and other life forms to mutate to giant sizes. These mutants plagued the native Meerags until the Fourth Doctor arrived and helped destroy them with fire. (COMIC: The Mutants)