What is the adaptation called insecticide resistance a result of

It's called pesticide resistance xD. The adaptation called pesticide resistance is a result of natural selection. A pesticide that was once highly effective in controlling and maintaining a pest is no longer working well because ...

The best and strongest bugs/pests/you get what I mean survived this pesticide and passed the strong genes onto their children! Over time, all the weak pests who could not survive the pesticide died out, and now only the strong ones are left.