[goodcanadian]

Over a given field, once or twice per year at most. Most farms probably do not use aircraft for spraying at all. They use ground equipment: tractors pulling sprayers. Granted that my experience is a different part of the world, but crop dusters are really rather rare.

[bkjelden]

I lived in the Dakotas for 20 years and crop dusters were present, but not terribly common. Ground spraying equipment was way more common.

As an aside, the City of Fargo uses crop dusters to spray for mosquitoes in the summer - which is either super cool or super annoying, depending on your opinion of airplanes :)

[protomyth]

Not in ND, they are fairly common. [edit: I'm in the NE to N central]