Way back, when I was brand new beekeeper in the early 90s, I sent a letter
to the editor of Gleanings that said we were breeding for Varroa because of
cell size. It got little response.

When I finally got on the internet, I searched for research that proved my
supposition. There was on study done and it disproved it, but not
conclusively. If my memory is correct, it was done in Sweden by a grad
student. So there is at least one study and it is in accord with Mike
Allsopp's comments on cell size and its lack of effect on varroa.

I did try smaller size foundation on my hives about six years ago and had
excellent success until last winter. But two years ago I added different
plastic foundation to my three hives to trial the differences between them.
As editor of our state newsletter, my experiments are a good source of
articles, especially on what not to do :)

So, did the larger plastic foundation cell size cause my hive loss or was
it apistan resistant varroa or were my naturally mated queens now more
susceptible to varroa or was it really tracheal or was it bad honey or was
it robbing from a new beekeeper's hives in the area or was it....

Any of those things could have caused the loss of two of my colonies. But,
if I were an earnest proponent of small cell size, then obviously the
introduction of larger plastic foundation cell size is the reason. I would
disregard all else.

There are too many variables out there to say it works. We have different
Varroa. Many of us allow natural requeening so we select for our area as
well as mite tolerance. So we have different bees as well as different
varroa. We may be in an area that Varroa is absent because all the feral
colonies were killed off. The bees may forage on nectar or pollen that
naturally controls varroa. Our own beekeeping practices, like smoking the
hives or not, and with what kind of fuel can influence varroa numbers. Do
we use screened bottom boards. Pollen traps.
I think you get the picture. With the number of variables out there,
without controlled tests,  we are no better than my letter to the editor, a
reasoned guess. The guess may be correct, but I originally thought FGMO was
too.

And in view of Mike Allsopp's comments, we may be looking at the wrong
reason that some are enjoying success with small cell size.

Bill Truesdell
Bath, ME