Some chemicals irritate parasites and stimulate them. Sometimes you can feel them convulsing in their death throws. And some chemicals starve the parasites and they start moving because they want food. And some chemicals depress their nervous system until they are paralyzed.
Just like with humans, different chemicals cause different reactions.
It is ridiculous to decide that parasites only respond one way to ALL chemicals.
Clearly, it depends on what chemical you are using and what parasite you have.