Hi there! Sometimes bees build comb outside of their frame space. The comb that they build in this areas is often referred to as burr comb. Bees build burr comb to fill in empty space or to reduce ventilation in their hive. If they are building comb on your bottom board they may be trying to reduce ventilation or there may be excess spacing below the frame. Besides making it a little more challenging to remove frames for inspection burr comb is not bad for the hive. As your hive matures more burr comb will be made. You can always remove it if it is impeding your ability to check the frames, the bees may or may not build it back.
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