I went for some observing. I saw activity on one end of the "landing board". When I came closer, I saw that there were a few ants trying to get onto the board and perhaps into the hive. But I also saw two or three "guards" on that side, doing nothing but walking back and forth.
  This is a fascinating experience. There are bees with clear duties. And the three I am watching for a while do nothing other than guarding against the ants that try to come up. When there is one that actually makes it to the landing board, it is picked up and dumped back on the ground--see below.

The guard bees were watching, here one is looking down at an ant making it up toward the ramp.

Sometimes it looks like a gamne of hide-and-seek, here the bee is looking the wrong way. But not for too long.

Here there is a bee heading toward an ant that has made it onto the ramp.

And then she just pounces on the ant, picks it up, and drops it to the ground after rapidly lifting it and flying enough so that she is clear off the board.