I would go further: we shouldn't use the word "nature" in this sense at all. Because what
is nature? The woolly meaning you are using, which I presume is the common one we hear in this context, is at best so utterly banal, that it looses all meaning. At worst, it is incoherent. What is "natural" vs. "unnatural" according to such a definition? Is nature all that wasn't produced by human beings? Since everything human beings produce is produced from preexisting things (a process ancient philosophers might call "mutation" --
mutatio and
poiesis), does that mean all such things are unnatural? Is rainwater natural, but water synthesized from hydrogen and oxygen unnatural? Is corn unnatural, given it is the result of selective breeding carried out by human beings? Is a blueberry natural, but a blueberry danish unnatural? Are human tears unnatural?
And this contemporary use of the word isn't really in accord with the traditional meaning, if we can say that, certainly not the traditional philosophical view. Nature concerns the nature of the thing. Thus, the nature of water is to be wet and transparent in its liquid state, but equally, the nature of certain plastics is to be flexible within certain conditions. And we can extend this to organisms: the nature of human beings is to be rational; the nature of lions is to hunt prey and consume their flesh; the nature of synthetic water is identical to the nature of water found in a puddle; the nature of corn is to grow to produce the fruit that we happen to eat.
Now, in a traditional philosophical context, we may speak of creation, that is, all that is, metaphysically speaking, contingent, that which did not need to exist, or that which cannot account for its own existence, not just in time, but here and now. This draws the distinction between creation and creator, that is to say, that cause which causes things to be and which itself is not a thing, not contingent, and does not require a cause, as it is that cause. But this has nothing to do with distinguishing between things we already find in the world, and things made by human beings, like pesticides or artifacts or synthetic objects. Everything that is is creation in this sense, man-made or otherwise.
Also according to this traditional metaphysical view is how human beings differ from other embodied living things, and that is the capacity to reason and the capacity to choose (either in accord or against reason, effectively). And these faculties of intellect and will, dependent as they are on the physical to function, but inexplicable as physical phenomenon as such, mean human beings are a unity of the physical and immaterial. It is not my intent to prove this claim here (it has to do with the act of abstraction and matter as the principle of individuation, if you're interested; you might find this video of use [0]). The point is to show a sound basis for making certain distinctions, none of which conform to the woolly, contemporary notion of "nature".
What we might say is that the pesticide in question, because of its nature, is harmful to human beings, because of our nature.
[0] https://www.youtube.com/watch?v=w6GmCyKylTw