Screened Bottom Board
Lexikon
The screened bottom board is the lowest part of many modern box hives and consists of a fine-mesh screen instead of a closed wooden floor.
The screened bottom board is the lowest part of many modern box hives and consists of a fine-mesh screen instead of a closed wooden floor. Beneath the screen a removable insert, often called a varroa board, can be slid in, catching natural debris from the colony without the bees themselves being able to pass through.
The most important benefit of the screened bottom board lies in varroa diagnosis: dead varroa mites fall through the screen onto the varroa board and can be counted there, giving the beekeeper information about the natural mite drop and thus the colony's infestation level. In addition, the permanent opening underneath provides better ventilation, which counteracts mold formation from moisture inside the hive.
A common beginner's mistake is leaving the varroa board slid in permanently. For a meaningful count it should only stay in the floor for a set period of a few days and then be removed again, so that the colony benefits from the additional ventilation and no moisture collects on the insert, which would in turn favor mold. In winter, some beekeepers deliberately leave the varroa board out entirely to increase airflow.