It looks like a Yellow Bloon with 2 wide black stripes. It first appears in the ninth level of the game, Bees Knees. Like darts, they cannot pop Metal Rubber Blocks. If they try to pop a block that needs 2 hits to pop, 2 bees will be used. Also, if you pop Bee Bloons with bees, 3 bees are added to the swarm. You can only have limited amounts of bees out at a time. This isn't shown as much in the normal mode, but if you place a whole load of Bee Bloons and pop them all, you'll see that no bees are added after.