If you are seeing that many dead bees there is something wrong. Sounds like pesticide spraying. Ask the farm manager if they have sprayed anything at their farm, of course that doesn't mean the next farm over didn't spray.
If you know that insecticide spraying, your risking all your colonies being destroyed, and total loss of ANY honey crop coming from a contaminated source. You can limit your damages by moving your hives to a non spray area.
You will have to either absorb the losses, and maybe wait out the spraying, or move the hives to an area where no spraying ( if you can find a place like that ) is taking place, and hasn't. Another possible side effect of the spraying is your honey might be contaminated by the insecticide. Just food for thought.
a lot of dead bees is not very descriptive.. a picture certainly would have helped.
it appears to me via casual conversation with folks largely involved with pollination that even the sprays that are not designed to kill bugs (herbicides, fungicides that king of thing) will almost invariable interact with stuff that does kill bugs to kill them even faster. you will not find much 'science' written up in regards to this synergistic effect of all those sprays <mind you nothing evil is taking place here it is just a very very complex issue.
I have heard reports of 'normal losses' (in the setting you desribe) of about 50%.
I have had the dead bees in front of hive a few times. After investigating everyone was due to the spraying of cotton fields. Bad thing is around here cotton is getting to be grown everywhere and its getting hard to find a place that the spraying wont effect them