I would add a few comments to Nancy's advice.  Each spray is different, and
some are far more toxic to bees than others.  The book "Hive and the
Honeybee" (and probably some others) lists the LD50 (lethal dose for 50% of
sampled bees) for most sprays and rates them for danger.
Common sense is important!  When the trees are flowering no spraying should
be done.  But after flowering most sprays will not affect the bees if
1.   The spray drift does not come in contact with the bees
2.   There are no flowers under the orchard that the bees might be visiting
that would be contaminated by spray.
 
Stan Sandler      Milk and Honey Farm