A pesticide was something which deterred or killed pests. A class of pesticide, insecticide, was used against insects.
In the early 1960s, Forester committed murder to cover up the far-reaching and dangerous side effects of DN6, the insecticide which he had developed. DN6 worked so effectively that, if used, it would have caused serious harm to Earth's ecology. (TV: Planet of Giants)
In the 1970s, an improperly mixed pesticide spray caused insects to grow to giant sizes and start rampaging across the countryside. The Third Doctor discovered a formula which returned the insects to their normal size, ending the threat. (COMIC: Insect)
In 2172, a pesticide called GK50 was used on wasps. In an alternative timeline, the Daleks which had succumbed to the Mutant Phase were killed on Earth by consuming traces of this pesticide. (AUDIO: The Mutant Phase)