I would recommend that you read a book published in 1962 by someone called Rachel Carson. It is called The Silent Spring. It is a good place to start your enquiries, even though it was written so long ago.
What she wrote then holds true now. The evidence is there for all to see but nobody wants to see because to see would mean to act and to act would mean to lose votes and to lose votes would mean to lose power. And one thing politicians love is power.
I am just old enough to remember farming before the introduction of cheap organophosphates and other chemical "improvers". Even though I grew up in a city that was engulfed in smog from coal mines, steel works and foundries, almost to the extent that we now see in China, the fields around the city were alive with insects when I was small.
Now I live in a National Park where one can hardly paint a fence without someone asking what is in the paint. Yet there are relatively few species of insects and relatively few of those in number.
My father was a biochemist and an amateur, though well-read, entomologist and I have always taken a keen interest in the subject.