> I have observed that bees appear to love old dark combs

Sure, many of us use them to bait swarm traps, but there are basic sanitary
considerations if one produces honey, nucs, or any item for sale to others.
One cannot control that which they do not measure.

> I have also observed healthy colonies of bees
> on old dark combs, some with considerable
> residues (by analysis).

"Apparently healthy" would be a more accurate description. The residues
will, at some point, start to foster robust miticide-resistant mites,
implying bees that would not be so healthy over the long term.

> However, I'm hardly recommending that one
> run old dark combs.  Rather, use your eyes.
> If there is poor brood survival on an old dark
> comb, I'd certainly rotate it out.

I'd not want anyone to wait until brood started actually dying in obvious
numbers to change out brood combs!  A strict rotation schedule is the only
way to assure combs with, at most, trace levels of residue, rather than the
levels that could create miticide resistance.  This implies marking frames.

Someone recently said:

> But to return to the subject of old dark combs
> and sublethal residues, it certainly seems plausible
> that they could contribute to the development of
> acaricide resistance in varroa.

It's much more than plausible.  We've been warned for over 5 years.  Here's
just one warning from Reed Johnson dating back to 2009:
http://www.beeccdcap.uga.edu/documents/CAPArticle2.html
or
http://tinyurl.com/pk4c9e4

And someone recently said:

> Judging from comb analyses that I've seen,
> coumaphos and fluvalinate appear to have
> very long half lives in beeswax.  Some comb
> analyses showed surprisingly high residues
> of these products.

> On the other hand, commercial beekeepers
> have used amitraz continually for quite a few
> years.  Residues in their combs of DMPF
> currently run about 100 ppb...  The above
> results suggest that with repeated application,
> DMPF residues could indeed build up.

Its worse than that. According to MaryAnn, the mean residues of DMPF were
229 ppb over the 23 states sampled in 2009, but those residues would be even
higher by now, unless combs were tracked and recycled on a strict schedule:
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0009754
or
http://tinyurl.com/yflogn7

I can say for sure that I have zero brood combs from 2009 left in my hives
by now, and my only investment has been a few boxes of colored metal thumb
tacks, and a few sheets of foundation per hive each year.

While DMPF may or may not seem a significant worry at present, I would not
want to ignore history.  Not so long ago, we were all assured that we were
avoiding miticide resistance by alternating miticides, and look what a mess
that turned out to be.  Reed's article was published in both ABJ and Bee
Culture, and posted online, and we still have not grasped what guinea pigs
our bees are.

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