There is a good piece on this very subject on Flightaware.com
all about transponders and why some are not tractable with modern technology. I would be a good idea to look at this as it has been the root of a lot of threads on this site.

[link to flightaware.com]

It's in here somwhere.

Oh and just to let you know crop dusters do not have transponders on board.

Actually a lot of planes do not have them.