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
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