"Bill Truesdell"  "Varroa is the problem. Otherwise we would monitor for
the virus."

With respect, I doubt that.  Whilst it is child's play to monitor the size
of the varroa population (since varroa are easliy separted from their host
and are then visible to the naked eye), detecting viruses requires
laboratory techniques (explained in Bailey and Ball, Honey Bee
Pathology,1991 ).  Anyway , what would be the point?  There is no known
treatment for virus infections, they have to take their course, all we can
do is to avoid any increase in the natural transmission rate and then
colonies can cope.  So, we control varroa since it is the mites that spread
the viruses.
(Apparently tracheal mites do too, the killer combination is v + t
together). But controlling varroa as the vector is not the same as
controlling the cause of bee death directly.

The analogy is dirt on a child.  The dirt does minimal damage directly to
the child but provides a breeding site for germs that then make the child
sick.  The dirt is removed immediately by scribbing but the sickness drags
on until overcome by the child's natural defences (immune system).

IMHO it is this linkage that makes the concept of an 'economic threshold'
for pest treatment inappropriate for beekeeping.  That concept comes from
agriculture where say beetles eat a field crop.  By monitoring the number of
beetles at a certain time you can work out whether the total damage to the
crop before harvest justifies the cost of applying pesticide to kill the
beetles.  But I have never heard of a way to estimate the reduction in honey
crop due to a moderate varroa infestation, starting from just an estimate of
the mite
population on a particular date.  We make only a crude estimate of whether
the colony will end up completely dead if not treated within time.  And we
cannot even know for certain whether or not a particular level of mites will
lead to death  - since we do not know the present or predicted level of
virus
in that colony.

The rule of thumb in UK is that colonies are safe from collapse if the mites
never exceed 2,500 - but we know colonies without virus have survived mites
of
10,000 plus - and those with both varroa and tracheal can collapse with only
1,000.  (I am quoting from memory of Prof Mark Winston at lectures - no
publication of his reaseach work is known to me).  This is therefore only a
crude guide - it sets a theshold for 'effective treatment (ie one that works
more often than not) but it
is not a level set by any valid economic argument.  I myself monitor varroa
just on a scale of 3 - a few (=up to 10/day) , no problem; some (=to 30/day,
think of treatment) ; a lot (=c100/day) treat urgently. As has been said,
each beekeeper needs to start with some assumption on a safety level in his
area,  as measured by whatever means the beekeeper finds most convenient,
(natural drop, ether roll, drone forking) and then see if it can be raised
or lowered without apparent excessive damage to the colony.

Any such damage, as said, is caused by increase in virus,  not the mites
directly.  Bees emerging from cells where varroa have bred are underweight
but recover after eating pollen on emergence - but longevity is reduced to
some extent.  Virus damage can be visible when it has reached high levels -
bees with K-wings (similar to tracheal mite damage), bees with shrivelled
wings which will never fly, bees that quiver on the comb with perhaps piles
of dead on the floor (Acute Paralysis). So far as I know, other viruses do
not produce such visible symptoms.   The final test of course is 'did the
colony survive winter at full strength?'  Virus affected bees die early - so
an over-wintering colony can
just dwindle.

So far all UK beekeepers have had to when the danger level has been reached
is insert Apistan, UK gov scientists do not object to treatment with supers
on, so anytime.  Easy - but resistant mites are now appearing across UK. We
will have to use a variety of ways thru the season to successively curb mite
growth.  Much more difficult.    So we watch this list with great interest -
and gratitude to those willing to share their experience .

Robin

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