Hello Barry S. & all
Sounds like you are asking questions in this post Barry. I will give my
opinion for what it is worth.

> Is anyone in the US jurisdiction using Africanized honeybees (AHBs)
> in managed beehives?

In Missouri we passed a state order preventing the keeping of A. scutellata
and allowing our state inspector to destroy those hives. The order was
passed by all members of the Missouri State Beekeepers assn. without one no
vote.
They simply possess too many bad traits for us.

> 1. AHBs in South and Central America are said to be resistant to
> varroa.

We know quite a bit more now about varroa than we did when all known
information was put in "The Varroa Handbook" in I believe 1989.  It seems
feral colonies of AHB are more tolerant of varroa than are our U.S. European
bees.  I believe part of the feral hives success lies in the swarming and
abscounding traits of the scuts.  If you take a heavily infested European
hive and shake down the hive onto foundation you lose all the varroa in
cells and greatly reduce mite load.  The new swarm can then start the season
with a mite load it can handle.  Which is why" in my  opinion"  Barrys
swarms survive until death at the hands of capensis.
 U.S. AHB trap lines show AHB swarms have a harder time with tracheal mites
than varroa at first.  I get the information off the net from a study of 244
AHB colonies trapped on the border and closely looked at.  Abscounding and
swarming does NOT reduce high levels of tracheal mites.

 However, it is said that these jurisdictions carry the less
> virulent form of the mite.

Easy to tell *round versus oval*. Why not check for V. destructor?

> 2. Over the past year there has been zero difference in hives I have
> treated (Bayvarol) and not treated for varroa. Both treated and
> untreated colonies HAVE varroa mites, but there is no visible damage
> caused by the mites, whether in the bees or brood of any age.

No surprises here. In my opinion you are most likely wasting money on
Bayvarol on new swarms as two years is the time period for varroa to cause
damage in the situation you describe. MANY packages are sold in the U.S.
every year with a high level of varroa. Most do OK and die only if untreated
the next winter.

> Evidently I must conclude that these scutellata do NOT need any
> treatment for varroa. And it is said that South Africa carries the more
> virulent form of varroa (since 1997).

Varroa will build up to a threshold level but you have allready made a honey
crop and capensis has taken over. I suspect if you were to check the varroa
load on a hive with low productivity at around 8 months you would find a
high varroa load. Those with high productivity a lower mite load. Bees
raisng a huge amount of brood can stay ahead of varroa until the brood
rearing slows . Then the colony crosses the mite threshold and total
callapse is coming. Those on the Bee-L list saying they are not *seeing*
varroa need to check their best hives FIRST as those are the hives you will
most likely find varroa the easiest.

> 3. All my colonies are started on strip foundation. I have just measured
> worker brood cells near the bottom of brood frames, that is, cells
> furthest from the foundation strip. These "natural" cells range between
> 4.878 and 4.762mm.

As I said in a previous post when you gave the above figures that these are
smaller than 4.9mm and closer to cerana size than 49ers use. It is too bad
you can not keep capensis out long enough to do proper testing with controls
to see if scuts can survive varroa when kept as our European bees are. Many
say the scuts constant swarming is a control in itself as part of the varroa
mite load leaves with each swarm if they were kept in standard hives.  We
have got quite a bit of learning yet to do with scuts. Any American
researchers want to study Scuts?  None I am aware of!.  I wonder why?

> It just seems too convenient and easy to conclude that scutellata (in
> my very narrow case) are resistant to varroa. But it must mean
> something that the wild scutellata are building cells at 4.9mm or less.

The small cell could be a factor as is that only scuts with the SMR gene
have survived. I personally believe the reason is simply the swarming and
absconding nature of the scut combined with the above.
Sincerely,
Bob Harrison
Odessa, Missouri
Ps. Bees in warm areas tend to keep a smaller brood nest than bees coming
out of our Midwest winters. We trick our bees into raising huge brood nests
in the Midwest so we can gather huge crops of honey on the one major honey
flow. When I kept bees in Florida and bees raised brood most of the year the
bees were hard to trick and simply kept brood nests at a certain size.
Varroa is kept in check when a hive does not need to maintain say 10 frames
of brood and only four or five. Hmmm.
Long posts are necessary to explain complicated subjects. I used to think as
a kid that all that was known about a subject was in the encyclopedia. Boy
was I wrong!