Treat the problem just as if it were a pesticide kill:

1)  Sample the dead bees, get them in a ziplock and frozen.  
     A hundred from each hive would be ideal, one bag per hive.

2)  Ditto for the whitish dead-looking grass.  Photos of the
     grass (to show that only grass in front of entrances was
     bleached) would also be a big help.  Lots of things can
     kill bees, but only a very few things can bleach grass,
     so I'd take a lot of photos and samples of the grass.

3)  Jot down the lot number on the Mite-Away II container,
     and perhaps post it here.  Your description of bleached
     grass tends to support a massive "overdose" of formic,
     so the problem may be as simple as a "bad batch" of the
     stuff, where the Formic Acid was simply not diluted 
     enough or not mixed as thoroughly enough.

4)  Contact NOD Apiary Products, as they have taken pains to
     explain how easy and fool-proof their products are, and 
     should be given the opportunity to address your complaint.
     Sounds to me like they owe you at least an explanation.

Your temps certainly were not too hot, and you've used the
product before, so my wild guess is that there might be a 
"bad batch" of product out there, where mixing was 
insufficient, and some pads got too much formic.  (This 
implies that other pads in that same batch got not enough 
formic).

Please post the batch number.  

Others might want to hold off on using it until NOD has a chance
to review the situation.

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