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. e-mail; [log in to unmask] Catfish