I've lived in the middle of farm land just west of Phoenix for 32 years. I used to see both planes and helicopters crop dusting all the time. Now I never see it. I heard now they use GPS and do it on the ground. Not sure how it works, anyone know?

I recently read an article talking about robotic application of bot pesticides and herbicides.
Depending on crop and location there are different methods.
First indoors are spray heads on tracks.
In fields there are both drones and wheeled vehicles. Some even have cameras to identify weeds that get a shot of herbicide.
Google search should yield some articles, or images.