In message <[log in to unmask]>, Peter
Edwards <[log in to unmask]> writes
>My understanding is that the damage to colonies is not caused by directly by
>varroa, but by the fact that they act as vectors for viruses.  Of these, the
>most significant are Deformed Wing Virus (DWV) and Acute Paralysis Virus
>(APV).  So a large infestation of varroa may not be a problem if DFW and APV
>are absent, whereas a small infestation can cause severe problems where
>these viruses are present.
This is a rather belated reply.

In the UK, the probability of colony demise increases above about 2500
mites. I believe the work did not differentiate amongst colony
variables. I have not heard of severe problems with small infestations.
Do you have a reference please.
>Should we, therefore, be monitoring the presence of the viruses (how do we
>do that?)
I think we have to presume that once seen we have confirmed they are
endemic. So the issue is susceptibility. But since the mechanism of
transfer (injection during feeding) means that more varroa means more is
transferred, we are back to numbers.
> rather than getting obsessed about the actual number of varroa
>present?
If we find ways to assist the bees keep the numbers down to a
sustainable level, then we have a solution. SMR, hygiene, and leg-biting
have been demonstrated to work in different ways, though there may be a
correlation between the last two, at least in the UK. IMHO we can select
for and breed in our own bees these characters. I think this should be
done for each successful race or strain in each country, or else you
lose the best of your own strains by importing others' work as the
"magic bullet". Monitoring numbers is part of this process.

I took a risk last autumn and selected 4 colonies with OMF's to leave
without treatment. 1 of these has few bees left, the other is weak and
both now show DWV. I can rescue them with treatment and a few bees from
stronger colonies as they were a good strain otherwise.

I have started a co-operative project, with a group which we are calling
the Cornwall Bee Improvement and Bee Breeders Group, affiliated to
BIBBA, which we call the Varroa Tolerance Project. We have fitted mesh
floors to about 120 hives to date with the aid of a grant and hope to
increase this considerably with further grants. I believe it can be done
by careful selection, breeding and culling, with drone flooding from the
best colonies.
--
James Kilty