The disappearance is most often not a mystery, especially with what you
    described.  You noted that the hive that was the weakest all season is
    now the strongest, while the others are gone.  It sounds to me there is
    a healthy, thriving Varroa mite population (perhaps in addition to
    tracheal mites).
 
    Here in the sw BC we have been hearing beekeepers telling the same
    story.  When there is relentless pressure of parasitism on the beebrood
    during the summer season, the rate of bee population replacement slows
    down, and the adults seem to be induced to seek other colonies.  This
    exodus seems exponential over time and within a few the bee population
    collapses.  The brood left behind has deteriorated rapidly and with some
    spotty appearance, one may be tempted to think of a bacterial disease.
    But, try to remove some of cappings and you may have the mites 'jumping'
    at you.  A dissecting microscope is most helpful but a good magnifying
    glass will do the same.
 
    It appears that during the population collapse, adult bees use cues in
    selecting certain colonies, while others do not nearly receive
    equivalent numbers of bees.  What these cues are, is not known.  The
    sudden increase in population size most often causes the beekeeper to
    become complacent by thinking his/her colony is doing wonderful.  Of
    course, a substantial number of the adults are infested with mites and
    the bee brood is being increasingly parasitized.   Ultimately, these
    populated colonies will collapse also.
 
    We advise beekeepers not to use simple visual inspection of the bees as
    the only method to determine mite population.  Frequent monitoring is
    key to effective control.  These controls ought to be applied only after
    the level of mite infestation has been determined.  We recommend an
    Apistan strip(s) for 24 hours and a sticky board as the most accurate
    detection method.
 
    I suggest you do the same and I suspect you may be startled at the
    number of mites present in you apparently healthy colony.  Also, when
    you do find mites, you may wish to leave the strips in for the
    recommended 40+ days to bring the population down.  In your part of
    Washington, where winter conditions are so mild that bees retain brood
    well into November and start up as early as mid/late January, the mites
    will proliferate along with the brood.  So, make sure to bring the
    population down before winter.
 
    Good luck,
 
    Paul van Westendorp                         [log in to unmask]
    Provincial Apiculturist
    British Columbia