<I would like your considered opinions please concerning the number of colonies to test per apiary.>

As an amateur hobbyist in a fairly rural location, my considered opinion would be to monitor EVERY colony.  Outliers can easily crop up (and often do) which can fly under the radar and easily get out hand.  (In more urban areas, I'm sure it's even worse).  Every colony gets a wash coming out of winter, once mid-season prior to winter bee production, and once later while winter preps are being made.  Additional ongoing surveillance with 24 or 72 hour sticky board counts are about as easy as you can get.  Many variables play into the decisions of monitoring frequency, methodology,  and treatment thresholds but candidly, I would say it's mandatory to have a handle on EVERY colony.

Obviously, the pros with hundreds of colonies have their own systems, but I'm speaking for the smaller yards - (which quite frankly is where a lot of the problem exists).  

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