Disunited

I still don't know what went wrong here.  I previously wrote a post about a colony that swarmed leaving no brood or queen cells behind.  It was a split from the mother colony below and had been in air and pheromone contact since the split through the mesh in a two-queen board.  As it is queenless I decided to unite it with the colony below.  Although they have had contact I used newspaper anyway, pinning it in place of the mesh and leaving the queen excluder on the other side of the board.  We have had pretty wet weather recently and next day it rained all day.  When I ventured out in the evening to have a look there was a pile of dead bees outside the main front entrance, looking like the scene after a gruesome mediaeval battle.

Yesterday morning it was worse and bodies were regularly being dumped outside or carried away by struggling undertaker bees.  This continued all day.

It was the saddest day of my beekeeping and I couldn't think of anything useful to do.  In all there must have been thousands of dead bees.  The strange thing is that they are all sisters - all offspring of the same queen in the lower colony as there hasn't been a queen in the upper half - and they have always been in air contact.  I even erred on the side of caution and used newspaper as well but it still didn't work.  Dave Cushman's website says the method is usually successful but will fail occasionally.  I have used newspaper to unite other colonies successfully but this failure is puzzling.  Even though the bees in this hive are very good tempered (to me at least) they certainly behave strangely.