and we are usually aware of them

Actually no, we mostly *aren't* aware of them. Food scientists might be, or agricultural specialists, but you and I almost certainly aren't. Even with poisonous species/varieties (or the poisonous parts of those plants), time hasn't always been spent on finding out what the poison is, unless there's some benefit to be had from doing that. Eventually people probably will get round to doing this, but the sheer number of plant species/varieties in existence means it's not going to happen in your lifetime or mine. And remember that there are new discoveries all the time, so people might get all the way through the list and then find that they'd not checked for some particular feature which could be used as an additional ingredient for making the perfect souffle, say. :-)

And herbicides and pesticides are man-made chemicals which can be/are harmful to us.

...and nor is that the case either. There are naturally-occurring herbicides/pesticides which are harmful to us, naturally-ocurring herbicides/pesticides which are not harmful to us, and man-made herbicides/pesticides which are not harmful to us.

Also the quantity is relevant - for example, if you take care of your lawn, you'll know that lawn fertiliser will actually poison the grass if you put too much on. For us, vitamin C is a great example of that - don't eat fruit and you get scurvy, but eat too much and you get chronic diarrhoea.

Graham.