Hi
Screened bottom boards are THE method of ease of observation and good
housekeeping.I elaborated on the original ( as mentioned in "Apis" letter by
Dr Tom Sanford in Jan 1999) by placing a removable / adjustable board ( 1/4
inch) beneath the screen serving as a "sticky " Board and adjustable
ventilation feature.( fabricated  July / August 1999 )
This design was improved upon (H."Eric"Garz ,US & Jack Percival, Australia
, September 1999 at Apimondia / Vancouver / Bc/ Canada)  by making the
Screen also removable to  clean larger debris accumulating onto the screen,
such as dead bees. All other material including falling mites goes right
through the material. The mesh used is so called 1/8 th galvanised "Hardware
cloth". This design proved to be  the best for convenience and least
disturbing when inspecting the bottom board feature. As the case may be this
design can be fabricated either in line or in right angles to the
combs.Furthermore some beekeepers used the rear of the hivebox for
manipulating the "catchtray".The movable screen/bottom board incorporates
the flightdeck. the space between top and bottom affords plenty ventilation
and can be closed if so desired. This feature makes it possible to reduce
the entry for the workers to a minimum to keep pests out and ease of
defense. The bees have NO trouble negotiating ( walking on) the mesh.
The maximum ventilation feature ( Complete open bottom) Has no adverse
effect noted to this day.( Open since February, Pacific Northwest). At this
time one live varroa was detected, no chalk brood , but I did have Nosema
and TM infestation last fall.Lost 4 colonies out of 14.But this had no
bearing relative to the screened boards since they were installed in late
August/ early September.Note: this Bottom board does not affect the
propagation of Varroa but curbs or radically erases the mite finding their
return to the bees once they fell off them , It is an INDIRECT "control" of
the mite population.
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Catfish