Fifteen years ago there were two good ole boys from Mississippi or Louisiana can't recall which but, they were farmers using a chemical called methyl parathion, http://www.pan-uk.org/pestnews/Actives/methylpa.htm
In theory this chemical breaks down rapidly when exposed to the sun. Problem is they decided to use it indoors for the control of roaches among other things.
Doesn't break down indoors and they got into a load of serious trouble as they actually had quite a sideline business going which included at least one school that I can remember, in Louisiana and Mississippi and perhaps Alabama.
Long story short, there were several buildings including this school that had to be "cleaned" from top to bottom, Silkwood style, http://www.imdb.com/title/tt0086312/
The main use of methyl parathion was for cotton.
As others mentioned, find out what and when the farmer will be spraying, and its effect on your bees.
How many hives are we talking?