A single person cannot guard 15 acres of crops. The arrogance is the assumption that the farmers are so stupid that they didn't even try this "simple solution" that would never work.
But that is the question: why can’t a single person guard the crop?
The answer to that question would reveal more about the shape of the problem.
Is it because the single guard won’t even see and hear when the illegal harvesters get to work? (That is to say harvesting is so quick and silent that the guard would be none the wiser?)
Is it because the guard would see the harvest but would be overwhelmed by the illegal harvesters?