|MadSci Network: Agricultural Sciences|
Hi.. This is a rather broad question and to answer it in depth would take a few years.. Each pesticide and herbicide has its own method of action by which life of the target is inhibited usually to the point of termination. Most pesticides and herbicide generally limit the plant or insects ability to reproduce cells or metabolize energy and I emphasize generally. There are many products on the market where the mode of action is not really understood; they just know it works but not why. If you would like to pull some specifics about various chemicals try this link.
There's a lot of info out there on the net. It is by nescesity usually rather specific since the action of these products need to be selective (specific). A herbicide that kills the crop the farmer is trying to grow isn't going to be very popular. The ideal pesticide or herbicide of course would only kill or inhibit 1 specific insect or plant. We have not achieved this yet but it illustrates the way research is directed in these areas. Sorry I can't give you one neat answer. You might like to investigate the acetylcholinesterase inhibiting affect of the organophosphate compounds and what this means to animals (that includes us humans).
Search and learn...... Mark
Try the links in the MadSci Library for more information on Agricultural Sciences.