I posed this question to California State Apiarist, Dr. Eric Mussen.
Quick question, how long does an adult Varroa Mite feast on an adult
 bee before returning to the brood to start birthing?

His reponse:
    The answer depends upon which mite you are watching.  A mite that
has been laying in a cell, already, is going to stay exposed only until
she can find another suitable cell and move in - probably less than a
day or two.

    A new, daughter mite that has not been actively laying tends to stay
out of the brood cells for a number of days (6ish ?).  This is the time
that the mites are phoretic and get moved from hive to hive.
Eventually, they find their way to the brood nest and go into the cells
to reproduce.  They aren't exposed a whole lot after that.

    The timing of our more recent control practices probably allows many
of the older, established, laying mites to return to their business.
However, a number of phoretic mites, which are supposed to be the next
generation, should be affected by the treatments.

    Another aspect to this question is how long the treatment remains at
a lethal level.  Sucrocide has to contact the mites in an adequate
amount to injure them.  The bees seem to clean it up in a day or two.
So, we really don't have many long-term treatments available: Apistan
and CheckMite+ don't seem to be very effective and I am really concerned
about the safety of handling Mite Away II, which is a 21 day treatment.

    I hope this information helps a bit.

    Eric.

-- Visit www.honeybeeworld.com/bee-l for rules, FAQ and  other info ---