While Randy knows more about mites than I ever want to, let me chime in
with my answer:

Varroa is the bug-a-BO of beekeeping.  Before you do any treating you
ought to know your mite levels if for no other reason than to test
afterwards to see if your treatment was effective.

We've had a bunch of really humid weather here lately, so my preferred
method for testing continues to be the alcohol wash - too much clumping
with the powdered sugar.  If you are opposed to testing, treat anyways.
Good IPM requires that you measure your mite loads before treating

Your goal ought to making sure that your winter bees are healthy and
will live as long as they need to in order to get you started next
spring.  (Of course they also need nectar/pollen.)  It is good that
you've treated now.  You are correct that Oxalic does not penetrate the
cell cappings and so is only getting the phoretic mites.  The drops you
are getting are significant.  When brood is present the usual
recommendation is to treat I think it is 4 times, a week to 10 days
apart, to get the mites as they emerge and are out of the brood cells.
You are also correct that only one treatment is required when the colony
is broodless.  Forgive me for my vagueness but Oxalic has not yet been
approved for use in Maine - at least I have seen nothing on it yet - and
so I don't have first hand experience with it.

A follow up treatment is often done for those colonies subject to
re-infestation from nearby collapsing colonies.

             ***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software.  For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html