Walter Weller said, "One hive which had been crawling with bees all over the
outside of it three days ago, had not a single living bee in it this
morning -- not even a raider, nor even a wax moth.  The screened
bottom-board was covered with dead bees.  There had not yet been time for
wax-worms to get started.  There was not a drop of honey in the hive.  I
suppose they were raided out -- I can think of no other explanation for this
sudden and total collapse, but a strong colony like that ??"

If the hive in question was robbed out, there should have been signs such as ragged edges on cells from being opened, cell cappings on the screened bottom board, etc.

Actually, having bees from strong hives disappear has not been uncommon for me in years past...I have not yet had the experience this year.  The hives with the most bees will also have the most mites; that is just a truism.  If that number of mites reaches a certain threshold (I don't know where that threshold is) the hive will collapse in a matter of days.  Some say that the surviving bees abscond, something I have never observed.  After collapse, the robbing begins and bees can easily rob 60-80 pounds in a day or two.

Are you certain your Apistan works?  To determine, ether roll a few hives that have been treated for a full 42 days.  If you see ONE mite, the mites have become resistant to Apistan and you need to treat with CheckMite now or almost certainly suffer a huge loss this coming winter.

Hope I have helped.

Lloyd
Lloyd Spear, Owner of Ross Rounds, Inc.
Manufacturer of round comb honey equipment and Sundance pollen traps