Haven't I read somewhere, sometime in the past, that if you report bee
kills to the county agricultural agent that the beekeeper runs a chance
of getting recompense ($$) for the kill?  Of course, that doesn't help
with the current loss.  Also, if you keep your agent abreast of where
your bee yards are, you might get notification for the agency of upcoming
spray schedules so that you can put screens on the entrances of your
hives and keep your bees in on the day of the proposed spray.  It also
helps if you communicate, and possibly give a jar of honey in the fall,
with surrounding farmers and ask them to keep you updated with their
spray schedule so as to prevent further bee kills.  A lot of work, I
know.  But that might help prevent future drastic bee kills in you yards.