Battling the bugs

Today's exciting science question? How do insecticides work and why are they selective? I thought, correctly as it turns out, that most of the retro-toxins, parathion etc, were ACE inhibitors and had this feeling that pyrethroids, permethrin etc, were calcium channel blockers (they're not — they actually act on sodium). And the selectivity thing? It basically boils down to the insecticide being only slightly more toxic to bugs than they are to humans...