I feel like anyone who's discussing "toxins" or "poisons" or "chemicals" should be required to learn what things like "LD50" means, and the entire "dosage-based toxicity" paradigm that surrounds dangerous substances.
And this of course is another huge consideration. (What follows is stuff Reb knows, so this is for others.)
There are some pesticides which do affect both humans and insect pests. My favorite example is rotenone, because it is made naturally by plants, and since it is "natural" it can be used by organic farms and gardens and the crops may still be labeled "organic". (Many people think organic produce has no pesticides on them, but they are often grown with natural pesticides.) Although rotenone use is becoming rarer due to its effects on humans, it was used on home gardens for a long time and may still be in some parts of the world.
So imagine rotenone is sprayed on a vegetable garden. The LD50 for rats is about 400 mg/kg, and this is about the same value for humans as well (in estimate of course). I don't know what it might be for insects, they may well be more susceptible, but lets presume the same dosage.
A gardener sprays his garden with the stuff, heavily diluted in water. The mist is blown by the air, some of it settles on the plant. With each watering of the garden and each rain, some of the rotenone is washed away. Eventually, and insect larva takes a bite out of the leaf, and even though there's just a tiny bit of residue on there, it's a larva, so it only weighs 0.00001 kg, meaning it only has to consume about 4 micrograms to die (a microgram is 1/1000th of a gram, which is a very small unit of measure).
So, dead bugs. Anyway, the garden continues to grow. Watering happens. Rain happens. Pesticides are usually applied when plants are first growing, not close to the harvest. Then the vegetables are picked and washed, and eaten by a human. By this time it is hard to believe much of the rotenone is left at all except in such trace amounts that even the insect larva wouldn't die, but certainly not in the amounts needed to kill a human. After all, most humans are about 50 kg, and so would require 20 g for the lethal dose. (This would be a pile of chemical you could see with the naked eye, filling more than a tablespoon, totally undiluted.)
Of course this is an oversimplification and there are other factors, but in many cases even pesticides that are potentially dangerous to humans are simply not dangerous to humans under normal eating conditions.
Now - rotenone on crops is still a horrible idea. Many better non-natural (synthetic) alternatives exist and should be used. The fact that organic farms will use the stuff just to keep the organic label despite the increased odds of harm is just one reason I hate the organic label. But I don't fear eating organic produce for this reason. I do fear for the farm workers who spray rotenone, though, as they may be exposed to higher concentrations in more-dangerous routes of administration (inhalation can often be worse than oral consumption).