An insecticide was a type of pesticide, or poison, specifically against insects.
One insecticide was DN6, which was created by Forester, who was also concerned about its effects and impact on the environment, including being deadly to bees, if it went into production. (TV: Planet of Giants)
Piperine, which was an active ingredient in pepper, was often used as an insecticide. (TV: The Unicorn and the Wasp)
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)