We've documented at least one toxic corn planting dust incident through our own experiments.  

We've also found that even when near corn fields, where you set the hives makes a difference.  Our preliminary data suggests that it may not be so much a case of bees foraging on a bare field, unless the field has weeds in bloom, as it's drift onto the hives.  Put the hives beside the fence next to the field being planted, you'll see a higher exposure.  Set them a bit away, with some shrubs, etc. between them and the field, problem is reduced or is eliminated.

Corn planting in corn belt areas generally happens over a 10 day to two week period, maximum.  That's not an impossible scenario for the beekeeper to take steps to avoid.  

If its a few hives next to one field - talk to the grower.  Hold the bees in, cover hives, sprinkle with garden hose - anything to keep bees in for the day.

I'd much rather be beside a field planted with treated seed, than the other option - spraying the foliage multiple times during a growing season.

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