>
> >I'm also wondering, though, if I picked up a more virulent strain of
> varroa recently.


Not sure if "virulent" is the right term, but not interested in splitting
hairs...  I sounds to me that you are asking whether you could have a mite
strain with a higher rate of reproductive success.

>My varroa levels are ten times what they were over a month later last fall.

10x is a big difference!  Mites have been shown to have 94% reproductive
success at rearing at least one daughter in nonresistant bee stocks.  It
would be hard to increase that rate substantially.  At that rate, the mite
population roughly doubles each month is broodrearing (including drones) is
taking place.  As Bob says, varroa is all about the math.

As Chapleau showed in the pages that you posted, resistant bees can
suppress that rate of increase.

To have a 10x higher level, would suggest that either your hives started
with higher levels, or that the bee stock this year was less successful at
suppressing varroa reproduction.

In my area, in dry years, varroa does not reproduce as well. I don't know
the weather for your area.

I'm assuming that you have no "outside" colonies within flight range from
which your bees could be picking up mites.

But hard to imagine that you have a mite that can reproduce at a higher
rate.

-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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