In message <[log in to unmask]>, Bob
Harrison <[log in to unmask]> writes
>Until a way is found to calculate the amount of varroa in brood cells I find
>the above method crude. I will however continue to use the method until a
>better method comes along. In my area a hive with a natural mite fall in
>June of 100 would be dead by end of August or infested so bad the hive could
>not be saved. 100 mites would be at least 3000 mites and the figure could be
>as high as 6000 in my opinion. There is no research so all you hear is
>mostly theory.
Steve did predictions based on his research and there are graphs in our
CSL/MAFF (now DEFRA) publication on the subject. They show how to
predict the increase of mite numbers from any estimate at any time of
the year to any other time. 3000 in June would be ripe for treatment and
as you say, the increase after June would take it too high for survival.
According to the graph, 300 in June (beginning?) would be 2500 in 90
days so presumably 3000 in June would be 25000 in September, way above
any of our colonies' ability to cope. The publication also gives  the
maximum number of mites in any one month not requiring treatment that
same year (without reinfestation). Without reinfestation, an autumn
figure of 200 would not need treatment in the following year.
>All depends on the amount of varroa in brood cells, number of frames of
>brood and  population of the hive. Our hives are getting close to 60,000
>population on 15 frames of brood in June. 900 mites would not be a serious
>problem in such a hive.
>A hive with 4 or 5 frames of brood and say 30,000 bees might need treatment.
>The more you look at threshold the more puzzeled you become.
I am sure you are right in leading us to % bees with mites. Some posts
do use this and 10-15% has been mentioned as (I think) a threshold
level. We do have the problem that if we treat too soon, we may not see
the bees sudden response to high levels by "chewing out" larvae and
grooming off the adults. Sorry I forget who posted the observation
recently about mite falls climbing to 70 when they suddenly dropped due
to this phenomenon.

In any case, the specific threshold for a colony will depend on its
resistance to viruses. Given that substances such as thymol help the
bees immune system perhaps this allows a higher threshold?

I am lurching towards an IPM system with selection and breeding from
bees that snip mite legs. I left 4 colonies untreated last winter. I
added bees to 2 which had few bees and good queens from a strong colony
also untreated. The first shake seemed fine. A few days later, a second
shake on to a board for them to climb up to the second colony, taken
from the centre of the brood nest showed 1 bee with deformed wings and a
dozen or so smaller, grey looking, *very* slow moving bees (virtually
still) (say under 2%) contrasting strongly with the black active bees. I
did not notice them in my inspection of this hive. This tells me that
they also have one of the paralysis viruses. Needless to say, all
untreated colonies are now treated. I did treat a bit late last year and
noticed some colonies with dwv. This year, the range in strength of the
colonies is enormous. I have split 3 into 2 boxes - 2 acting like it was
summer, packing in nectar and drawing wax in the supers. Others are
advantaged by larger brood frames and I have several strong ones on 8
frames with brood, much more advanced than previous years. My hypothesis
is that these colonies are better placed regarding varroa and viruses.
More detailed observations with mesh floors and a trawl of my records
will hopefully demonstrate this.
--
James Kilty