Carlos Aparicio questioned why a colony would collapse from Varroa when it
has been existing with populations of Varroa for some time.
I think we have all experienced this.  I live in a temperate climate and,
because I raise only comb honey, I keep good records on every colony.  Twice
in past years I have had my highest producing colony collapse just after the
supers were pulled.
Dr. Nick Calderone from Cornell University thinks this is due to such an
explosion of Varroa that both a high percentage of nurse bees are infected
and a high percentage of worker larvae are infected.  The net effect is that
very few worker larvae mature into bees, and the existing worker bees have a
very short life.  In these conditions, a colony can collapse in just 3 weeks
as there are no new worker bees to replace those that die.  Dr. Calderone
thinks that tracheal mites do not cause a sudden collapse, but a longer-term
degeneration.
Carlos, this applies to hives in temperate climates where summer/early fall
collapse is seen.  If bees are raising brood throughout your winter it might
also apply in that case.
Lloyd Spear
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