Dave is right in his comments on spraying at night or late evening as being
fairly safe to bees.
The reason is that, as he says, most sprayed insecticides have a half life
such that they will be fairly, but not completely, benign the next day.
Also, since they are sprayed, their primary kill mechanism is contact. Bees
gathering nectar will be minimally affected the next day as they will have
little contact with what is left of the insecticide.
The same for hives sprayed during the day. Foraging bees will be killed but
the hive bees will be relatively safe, depending on the spray.
But, if powder, and even worse encapsulated powder, is used, then it does
not matter when the insecticide is applied. You will have bee kill. The
reason is the bees do not die in the field but bring the encapsulated
powder back to the hive as if it were pollen. Then the colony either dies
or comes close to it. Sevin is often applied by backyard gardeners as a
powder and is an efficient bee killer. But mix it with water and spray it
and it loses most of its wallop since it sticks to leaves and is not
easily, if at all, picked up by the bees. I like to keep my neighbors
informed about Sevin and how it affects bees. I do not tell them not to use
it, only how to use it, especially since I use it myself to combat cucumber
beetles.
I would be careful on imposing too stringent requirements on a grower. We
ask them to read the label. So should we.
Bill Truesdell
Bath, ME