Bees are insects that appear in various Mario video games. Their first appearance in the series was Yoshi's Story. In this game, the bees would easily be distracted by the Yoshi that the player was controlling, and wouldn't allow him to pass. So, the player had to control Yoshi so that he would move very slowly when walking, which would cause the bees to not even notice him. Larger bees also appeared in the game, and would harm Yoshi if he approached them.
Bees would return to the series in Super Mario Sunshine for the GameCube. In the game they would appear in a variety of the title's stages, and would attack Mario if he used F.L.U.D.D. to knock their hive down. Using Yoshi to eat all the bees would net you a Blue Coin or 1-Up Mushroom if you've already done it.
Bees also appeared in Super Mario Galaxy for the Wii as a major species. They were under the jurisdiction of the Queen Bee and would be found in the Honeyhive Galaxy. They were friendly and would not attack Mario. A new suit in the game called the Bee Suit appeared on this stage and others; this suit granted Mario the ability to fly and climb on honeycombs, not to mention the queen herself.
Bees reappeared again in Super Mario Galaxy 2, where they could be found in a few galaxies. After the mission "The Secret Wall Jump" is completed in the Honeybloom Galaxy, one of them will appear on Starship Mario.
They also appear in the Honeyhop Galaxy, where their queen also makes a reappearance.