Fellow German here. I go out for mountain biking and gravel rides a lot. Every time I pass agriculture fields in summer, there is no "noise" anymore, it's totally quiet. No birds, no insects, nothing, it almost seems dead. As soon as I come to places where nature can be itself, there is an abundance of sounds, nature is thriving. Unfortunately the former prevails the latter several times.

I guess I don't need any studies of WHAT is killing these insects...