Glyphosat
https://www.beeculture.com/catch-the-buzz-its-not-the-glypho...
> States with the most significant dip in bee numbers have the largest increase in the use of pesticides like neonicotinoids, insecticides, and fungicides, per Live Science.
There are several studies pointing to glyphosphat (this one just an example, use Google). Actually there are so many studies that Europe ends the use in December 2022.