Bees are a type of insects, a species of which existed on Melissa Majoria. Earth also had its own species of bees, as well as a migrant population from Melissa Majoria.
By 2009, bees were disappearing on Earth, which Donna Noble noticed and mentioned to the Doctor on two separate occasions. He agreed that this was odd. (DW: Partners in Crime, Planet of the Ood)
Bees were also disappearing in the alternate reality where Donna had turned right instead of left. (DW: Turn Left)
When Earth was stolen by the New Dalek Empire, it became clear that this transport had for some time created a disturbance in the Tandoka Scale which the migrant bees used as carrier signals. The migrant Bees had sensed danger and gone home. (DW: The Stolen Earth)
