The following is conjecture, but I'm guessing that insects of the last century mainly survived off of crops from farmland, either directly or indirectly.

So, if true, that means that it was hardly natural beforehand either. It may be we are just using better pesticides, so we moved from one artificial environment to a different one. Both are out-of-whack compared to what would naturally be there. How do you know the previous conditions, where there was lots of farmland, but no good pesticides was the better one?