My understanding is that the spores of chalkbrood are in virtually every hive
and usually it persists at a low rate without being a problem.  However it is
when the colony is stressed that Chalkbrood gains the upper hand..  It can be
regarded by the beekeeper, not as a problem to worry about on its own account,
but as a useful indicator that something else is wrong.
 
Often Chalkbrood is noticed when there is an imbalance between the brood and
the nurse bees, possibly following swarming or manipulations by the beekeeper.
Other causes may be varroa or other infestations/ diseases; medication to
treat them; damp hives; poor foraging conditions, especially for pollen; or
even disturbance by the beekeeper.
 
Like David Eyre, I am not convinced that requeening is the answer.  I have not
tried it myself as my bees have never been affected seriously enough for me to
contemplate interfering.  The problem has always gone away.  Since starting
this mail it has occurred to me that it might be an interesting experiment
(and save the cost of a new queen) if, in a badly affected hive, the resident
queen were caged for a few days and then released.
 
This is late night armchair beekeeping and others may well debate  my
hypothesis but I think the break in egg deposition (she may continue to lay
eggs but not in cells) will enable bees to emerge and get to the nursing stage
and then be under worked rather than over worked.  This will enable them to
give the larvae the very best attention and diet which will enable the larvae
to cope successfully with the spores they will inevitably ingest.
 
If there is ANY chalkbrood in a hive the spores will be everywhere.  The
question is not why do so many bees die of chalkbrood but why do so many
survive unscathed?  I suggest the survivors are better fed.
 
I should be interested to learn from anybody willing to try the experiment in
a real hive rather than an armchair whether the theory works in practice.  The
next question which can be answered at the same time is "Is it worth the
effort?".  Caging the queen will result in a lack of bees later.  How many
bees depends what her laying rate was and how many would have died of
chalkbrood.  If the experimentor (I am getting quite enthusiastic about this
and might even do it myself if my bees had much chalkbrood), having first
found and caged the queen, shakes the bees off every frame with brood and
photographs both sides he will be able, by using the fingers of both hands or
a calculator, to determine the approximate laying rate and the degree of
chalkbrood infestation.  The latter will be a rule of thumb benchmark as it
will only show the dead larvae that have been decapped prior to removal by the
undertakers.
 
To reduce the chance of the running of the experiment itself producing a
biassed result I suggest doing in in the best of weather (whatever that is)
and having somebody else take the photographs.
 
Chris Slade