Tom,

Part of the trouble with determining levels for treatment is the influence
of the various viruses which vector with Varroa destructor. Norman Carreck
stated, in last years lectures at Gormanstown, that their experience was
that some colonies with low levels of infestation died; whilst others with
heavy infestation survived. Further discussion at Gormanstown ( I think the
previous year stated that a natural mite drop of more than 10 per day would
warrant immediate treatment.

The calculator gives only a rough idea of population and development and
cannot be taken as gospel. It would give you an idea as to whether you could
wait until the crop is removed or not before treatment.

> Investigations carried out in the UK showed, that if there are more than
> 2500 mites in the colony that immediate treatment is required. But I have
> data from the Swiss Bee Research to say that if more than 30 mites fall
per
> day that the colony is heading for trouble. This would mean that if the
> above table be used that 900 mites would be the threshold in June.

Monitoring the daily mite drop each week/ month would determine whether mite
counts were increasing or not. Unless you are using Queen trapping I would
expect with most of the physical methods that mite counts are going to
increase. They are a holding operation to increase the interval between
chemical  (whether hard or soft) treatments.
> It would appear essential that if non acaricide treatments are used that
> some reliable data must be available to determine if the colony is headed
> for the threshold to find out if the treatments are working.

Ruary