In the olden days (when the world was young) beekeepers in Alberta
routinely killed ALL their bees in the fall and started over each spring
with new package bees from the United States. The method used was to put a
tablespoon of Cyanogas (powdered calcium cyanide) on a sheet of cardboard
which was then slid inside the hive. The moisture in the air reacts to
form HCN gas which kills the bees almost instantaneously. The gas
dissipates and leaves the combs undamaged.

Best regards,

Donald Aitken
Edmonton Alberta Canada