Crop dusting is not the easiest field to break into. (Un, no pun intended..
)
It is really easy to hit wires and crack up the plane. (I flew an AG-Truck that had wire-cutters on front of the canopy..)
I would think ya need a bunch of time before ya apply. Make it tail wheel time and turbine
time and round engine time etc.
A good back ground would probably be bush-pilot in Alaska.
There is some schools that will "train" ya as an Ag-pilot, but I still think high time would be an insurance requirement.
Hope this helps, althougt I never flew ag, just know some guys that did and flew the planes in other applications. (Fish-haul)
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Men, this is no drill...