The thread on screened bottom boards is very interesting.  The majority of
contributors (7 out of 8) felt screened bottom boards provide a benefit to
controlling varroa, although not a large effect.  Both Mark Payton and Mike
Tooley felt the effects of screened bottom boards on varroa control would
increase when combined with other 'soft' mite control treatments.  I think
the opinions speak well to the general faith among BEE-L subscribers that
many 'small' varroa management methods can add up and provide significant
control.  Is there research to support this hope for combining
less-effective control methods?

The published research on the issue of combining screened bottom boards with
other soft treatments, such as thymol fumigation and hygienic queens,
suggests combination provides a benefit.  Consider two studies widely
available to beekeepers:

Efficacy of a bottom screen device, Apistan, and Apilife VAR, in controlling
Varroa destructor.
By J. D. Ellis, K. S. Delaplane and W. M. Hood
Am Bee J. Nov 2001 p. 813-816

IPM for Varroa Control: Exploring an IPM approach for varroa control.
By: Nathan D. Rice and Mark L. Winston
http://www.honeycouncil.ca/chc-ccm/winric00.html
(Aside: Visit the Canadian Honey Council WWW for great online summaries of
Canadian bee research.  Consider also subscribing to CHC's excellent glossy
and information packed magazine HiveLights).

In the first study (Ellis) the researchers set up varroa infested colonies
with or without screened bottom boards (SBB) and combined them with either
no miticide treatment, treatment with Apistan or treatment with Apilife VAR,
a thymol miticide that is somewhat less effective than Apistan.  The
experiment was conducted in two sites: one in Georgia where the mites were
resistant to Apistan and the other in S. Carolina where Apistan was
effective.  Although SBB provided modest Varroa control on their own (15%
fewer varroa after 50 days compared to colonies with no SBB and no
miticide), they had two important psoitive effects: 1) SBB reduced the
variability in varroa control among the thymol treatments and 2) SBB
provided 44% control of varroa in Apistan resistant colonies treated with
Apistan (compared to 0% among Apistan treated colonies without SBB).  The
results should provide hope to beekeepers facing an uncertain future with
synthetic miticides:
    1)  SBB may buy a beekeeper time when Apistan resistance first shows up
by providing sizable control of Varroa when strips are in place,
    2)  SBB may provide more consistent control with more variable 'soft'
alternatives such as thymol or organic acids, making it possible to use
these treatments for more consecutive seasons before needing to use a
synthetic miticide

The second study (Rice) compared an IPM approach (SBB + thymol + hygienic
queens) to treatments only with thymol or only with Apistan.  The experiment
ran for 3 consecutive years.  The experiments showed that over time,
colonies in the IPM program had lower levels of varroa compared to colonies
treated with thymol, although not different than those treated with Apistan.


*    *     *    *

One final comment about statistics.  When measurements among colonies vary
considerably, the ability to gather evidence to 'see' a difference is
eroded.  The erosion of the 'power' of an experiment can be buffered by
replicating the treatment many many times (ie an experiment with 100
colonies can detect smaller differences than an experiment with only 5
colonies).  A valid question for a beekeeper to ask of any study, therefore,
is: 'how large a difference would your experiment be able to detect'.  If an
experiment cannot find the difference between 50 and 80% varroa control, it
does not necessarily mean that the treatment efficacy is much too variable
to be useful for a beekeeper, but may mean that the study was not replicated
among enough colonies to find anything but the most oblique differences.
Something to keep in mind.

Regards,
Adony